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SUSE Linux Enterprise Server 15 SP2

Virtualization Best Practices

SUSE Linux Enterprise Server 15 SP2

Publication Date: December 11, 2023

1 Virtualization Scenarios

Virtualization offers a lot of capabilities to your environment. It can be used in multiple scenarios. To get more details about it, refer to the Virtualization Guide and in particular, to the following sections:

This best practice guide will provide advice for making the right choice in your environment. It will recommend or discourage the usage of options depending on your workload. Fixing configuration issues and performing tuning tasks will increase the performance of VM Guest's near to bare metal.

2 Before You Apply Modifications

2.1 Back Up First

Changing the configuration of the VM Guest or the VM Host Server can lead to data loss or an unstable state. It is really important that you do backups of files, data, images, etc. before making any changes. Without backups you cannot restore the original state after a data loss or a misconfiguration. Do not perform tests or experiments on production systems.

2.2 Test Your Workloads

The efficiency of a virtualization environment depends on many factors. This guide provides a reference for helping to make good choices when configuring virtualization in a production environment. Nothing is carved in stone. Hardware, workloads, resource capacity, etc. should all be considered when planning, testing, and deploying your virtualization infrastructure. Testing your virtualized workloads is vital to a successful virtualization implementation.

3 Recommendations

3.1 Prefer the libvirt Framework

SUSE strongly recommends using the libvirt framework to configure, manage, and operate VM Host Servers and VM Guest. It offers a single interface (GUI and shell) for all supported virtualization technologies and therefore is easier to use than the hypervisor-specific tools.

We do not recommend using libvirt and hypervisor-specific tools at the same time, because changes done with the hypervisor-specific tools may not be recognized by the libvirt tool set. See Chapter 7, Starting and Stopping libvirtd for more information on libvirt.

3.2 qemu-system-i386 Compared to qemu-system-x86_64

Similar to real 64-bit PC hardware, qemu-system-x86_64 supports VM Guests running a 32-bit or a 64-bit operating system. Because qemu-system-x86_64 usually also provides better performance for 32-bit guests, SUSE generally recommends using qemu-system-x86_64 for both 32-bit and 64-bit VM Guests on KVM. Scenarios where qemu-system-i386 is known to perform better are not supported by SUSE.

Xen also uses binaries from the qemu package but prefers qemu-system-i386, which can be used for both 32-bit and 64-bit Xen VM Guests. To maintain compatibility with the upstream Xen Community, SUSE encourages using qemu-system-i386 for Xen VM Guests.

4 VM Host Server Configuration and Resource Allocation

Allocation of resources for VM Guests is a crucial point when administrating virtual machines. When assigning resources to VM Guests, be aware that overcommitting resources may affect the performance of the VM Host Server and the VM Guests. If all VM Guests request all their resources simultaneously, the host needs to be able to provide all of them. If not, the host's performance will be negatively affected and this will in turn also have negative effects on the VM Guest's performance.

4.1 Memory

Linux manages memory in units called pages. On most systems the default page size is 4 KB. Linux and the CPU need to know which pages belong to which process. That information is stored in a page table. If a lot of processes are running, it takes more time to find where the memory is mapped, because of the time required to search the page table. To speed up the search, the TLB (Translation Lookaside Buffer) was invented. But on a system with a lot of memory, the TLB is not enough. To avoid any fallback to normal page table (resulting in a cache miss, which is time consuming), huge pages can be used. Using huge pages will reduce TLB overhead and TLB misses (pagewalk). A host with 32 GB (32*1014*1024 = 33,554,432 KB) of memory and a 4 KB page size has a TLB with 33,554,432/4 = 8,388,608 entries. Using a 2 MB (2048 KB) page size, the TLB only has 33554432/2048 = 16384 entries, considerably reducing TLB misses.

4.1.1 Configuring the VM Host Server and the VM Guest to use Huge Pages

The AMD64/Intel 64 CPU architecture supports larger pages than 4 KB: huge pages. To determine the size of huge pages available on your system (could be 2 MB or 1 GB), check the flags line in the output of /proc/cpuinfo for occurrences of pse and/or pdpe1gb.

Table 1: Determine the Available Huge Pages Size

CPU flag

Huge pages size available

Empty string

No huge pages available


2 MB


1 GB

Using huge pages improves performance of VM Guests and reduces host memory consumption.

By default the system uses THP. To make huge pages available on your system, activate it at boot time with hugepages=1, and—optionally—add the huge pages size with, for example, hugepagesz=2MB.

Note: 1 GB huge pages

1 GB pages can only be allocated at boot time and cannot be freed afterward.

To allocate and use the huge page table (HugeTlbPage) you need to mount hugetlbfs with correct permissions.

Note: Restrictions of Huge Pages

Even if huge pages provide the best performance, they do come with some drawbacks. You lose features such as Memory ballooning (see Section 6.1.3, “virtio balloon”), KSM (see Section 4.1.4, “KSM and Page Sharing”), and huge pages cannot be swapped.

Procedure 2: Configuring the use of huge pages
  1. Mount hugetlbfs to /dev/hugepages:

    > sudo mount -t hugetlbfs hugetlbfs /dev/hugepages
  2. To reserve memory for huge pages use the sysctl command. If your system has a huge page size of 2 MB (2048 KB), and you want to reserve 1 GB (1,048,576 KB) for your VM Guest, you need 1,048,576/2048=512 pages in the pool:

    > sudo sysctl vm.nr_hugepages=512

    The value is written to /proc/sys/vm/nr_hugepages and represents the current number of persistent huge pages in the kernel's huge page pool. Persistent huge pages will be returned to the huge page pool when freed by a task.

  3. Add the memoryBacking element in the VM Guest configuration file (by running virsh edit CONFIGURATION).

  4. Start your VM Guest and check on the host whether it uses hugepages:

    > cat /proc/meminfo | grep HugePages_
    HugePages_Total:1     512
    HugePages_Free:2       92
    HugePages_Rsvd:3        0
    HugePages_Surp:4        0


    Size of the pool of huge pages


    Number of huge pages in the pool that are not yet allocated


    Number of huge pages for which a commitment to allocate from the pool has been made, but no allocation has yet been made


    Number of huge pages in the pool above the value in /proc/sys/vm/nr_hugepages. The maximum number of surplus huge pages is controlled by /proc/sys/vm/nr_overcommit_hugepages

4.1.2 Transparent Huge Pages

Transparent huge pages (THP) provide a way to dynamically allocate huge pages with the khugepaged kernel thread, rather than manually managing their allocation and use. Workloads with contiguous memory access patterns can benefit greatly from THP. A 1000 fold decrease in page faults can be observed when running synthetic workloads with contiguous memory access patterns. Conversely, workloads with sparse memory access patterns (like databases) may perform poorly with THP. In such cases it may be preferable to disable THP by adding the kernel parameter transparent_hugepage=never, rebuild your grub2 configuration, and reboot. Verify if THP is disabled with:

> cat /sys/kernel/mm/transparent_hugepage/enabled
always madvise [never]

If disabled, the value never is shown in square brackets like in the example above.

Note: Xen

THP is not available under Xen.

4.1.3 Xen-specific Memory Notes Managing Domain-0 Memory

In previous versions of SUSE Linux Enterprise Server, the default memory allocation scheme of a Xen host was to allocate all host physical memory to Dom0 and enable auto-ballooning. Memory was automatically ballooned from Dom0 when additional domains were started. This behavior has always been error prone and disabling it was strongly encouraged. Starting in SUSE Linux Enterprise Server 15 SP1, auto-ballooning has been disabled by default and Dom0 is given 10% of host physical memory + 1GB. For example, on a host with 32GB of physical memory, 4.2GB of memory is allocated for Dom0.

The use of dom0_mem Xen command line option in /etc/default/grub is still supported and encouraged (see Section 7.5, “Change Kernel Parameters at Boot Time” for more information). You can restore the old behavior by setting dom0_mem to the host physical memory size and enabling the autoballoon setting in /etc/xen/xl.conf.

4.1.4 KSM and Page Sharing

Kernel Samepage Merging is a kernel feature that allows for lesser memory consumption on the VM Host Server by sharing data VM Guests have in common. The KSM daemon ksmd periodically scans user memory looking for pages of identical content which can be replaced by a single write-protected page. To enable KSM, run:

#  echo 1 > /sys/kernel/mm/ksm/run

One advantage of using KSM from a VM Guest's perspective is that all guest memory is backed by host anonymous memory. You can share pagecache, tmpfs or any kind of memory allocated in the guest.

KSM is controlled by sysfs. You can check KSM's values in /sys/kernel/mm/ksm/:

  • pages_shared: The number of shared pages that are being used (read-only).

  • pages_sharing: The number of sites sharing the pages (read-only).

  • pages_unshared: The number of pages that are unique and repeatedly checked for merging (read-only).

  • pages_volatile: The number of pages that are changing too fast to be considered for merging (read-only).

  • full_scans: The number of times all mergeable areas have been scanned (read-only).

  • sleep_millisecs: The number of milliseconds ksmd should sleep before the next scan. A low value will overuse the CPU, consuming CPU time that could be used for other tasks. We recommend a value greater than 1000.

  • pages_to_scan: The number of present pages to scan before ksmd goes to sleep. A high value will overuse the CPU. We recommend to start with a value of 1000, and then adjust as necessary based on the KSM results observed while testing your deployment.

  • merge_across_nodes: By default the system merges pages across NUMA nodes. Set this option to 0 to disable this behavior.

Note: Use Cases

KSM is a good technique to over-commit host memory when running multiple instances of the same application or VM Guest. When applications and VM Guest are heterogeneous and do not share any common data, it is preferable to disable KSM. In order to do that, run:

> sudo systemctl disable --now ksm.service

Alternatively, it can also be disabled by running the command:

#  echo 0 > /sys/kernel/mm/ksm/run

In a mixed heterogeneous and homogeneous environment, KSM can be enabled on the host but disabled on a per VM Guest basis. Use virsh edit to disable page sharing of a VM Guest by adding the following to the guest's XML configuration:

Warning: Avoid Out-of-Memory Conditions

KSM can free up some memory on the host system, but the administrator should reserve enough swap to avoid out-of-memory conditions if that shareable memory decreases. If the amount of shareable memory decreases, the use of physical memory is increased.

Warning: Memory Access Latencies

By default, KSM will merge common pages across NUMA nodes. If the merged, common page is now located on a distant NUMA node (relative to the node running the VM Guest vCPUs), this may degrade VM Guest performance. If increased memory access latencies are noticed in the VM Guest, disable cross-node merging with the merge_across_nodes sysfs control:

#  echo 0 > /sys/kernel/mm/ksm/merge_across_nodes

4.1.5 VM Guest: Memory Hotplug

To optimize the usage of your host memory, it may be useful to hotplug more memory for a running VM Guest when required. To support memory hotplugging, you must first configure the <maxMemory> tag in the VM Guest's configuration file:

<maxMemory1 slots='16'2 unit='KiB'>209715203</maxMemory>
  <memory4 unit='KiB'>1048576</memory>
<currentMemory5 unit='KiB'>1048576</currentMemory>


Runtime maximum memory allocation of the guest.


Number of slots available for adding memory to the guest


Valid units are:

  • "KB" for kilobytes (1,000 bytes)

  • "k" or "KiB" for kibibytes (1,024 bytes)

  • "MB" for megabytes (1,000,000 bytes)

  • "M" or "MiB" for mebibytes (1,048,576 bytes)

  • "GB" for gigabytes (1,000,000,000 bytes)

  • "G" or "GiB" for gibibytes (1,073,741,824 bytes)

  • "TB" for terabytes (1,000,000,000,000 bytes)

  • "T" or "TiB" for tebibytes (1,099,511,627,776 bytes)


Maximum allocation of memory for the guest at boot time


Actual allocation of memory for the guest

To hotplug memory devices into the slots, create a file mem-dev.xml like the following:

<memory model='dimm'>
  <size unit='KiB'>524287</size>

And attach it with the following command:

> virsh attach-device vm-name mem-dev.xml

For memory device hotplug, the guest must have at least 1 NUMA cell defined (see Section, “VM Guest Virtual NUMA Topology”).

4.2 Swap

Swap is usually used by the system to store underused physical memory (low usage, or not accessed for a long time). To prevent the system running out of memory, setting up a minimum swap is highly recommended.

4.2.1 swappiness

The swappiness setting controls your system's swap behavior. It defines how memory pages are swapped to disk. A high value of swappiness results in a system that swaps more often. Available values range from 0 to 100. A value of 100 tells the system to find inactive pages and put them in swap. A value of 0 disables swapping.

To do some testing on a live system, change the value of /proc/sys/vm/swappiness on the fly and check the memory usage afterward:

#  echo 35 > /proc/sys/vm/swappiness
> free -h
total       used       free     shared    buffers     cached
Mem:      24616680    4991492   19625188     167056     144340    2152408
-/+ buffers/cache:    2694744   21921936
Swap:      6171644          0    6171644

To permanently set a swappiness value, add a line in /etc/systcl.conf, for example:

vm.swappiness = 35

You can also control the swap by using the swap_hard_limit element in the XML configuration of your VM Guest. Before setting this parameter and using it in a production environment, do some testing because the host can terminate the domain if the value is too low.

  <hard_limit unit='G'>1</hard_limit>2
  <soft_limit unit='M'>128</soft_limit>3
  <swap_hard_limit unit='G'>2</swap_hard_limit>4


This element provides memory tunable parameters for the domain. If this is omitted, it defaults to the defaults provided b the operating system.


Maximum memory the guest can use. To avoid any problems on the VM Guest it is strongly recommended not to use this parameter.


The memory limit to enforce during memory contention.


The maximum memory plus swap the VM Guest can use.

4.3 I/O

4.3.1 I/O Scheduler

The default I/O scheduler is Completely Fair Queuing (CFQ). The main aim of the CFQ scheduler is to provide a fair allocation of the disk I/O bandwidth for all processes that request an I/O operation. You can have different I/O schedulers for different devices.

To get better performance in host and VM Guest, use noop in the VM Guest (disable the I/O scheduler) and the deadline scheduler for a virtualization host.

Procedure 3: Checking and Changing the I/O Scheduler at Runtime
  1. To check your current I/O scheduler for your disk (replace sdX by the disk you want to check), run:

    > cat /sys/block/sdX/queue/scheduler
    noop deadline [cfq]

    The value in square brackets is the one currently selected (cfq in the example above).

  2. You can change the scheduler at runtime with the following command:

    > sudo echo deadline > /sys/block/sdX/queue/scheduler

To permanently set an I/O scheduler for all disks of a system, use the kernel parameter elevator. The respective values are elevator=deadline for the VM Host Server and elevator=noop for VM Guests. See Section 7.5, “Change Kernel Parameters at Boot Time” for further instructions.

If you need to specify different I/O schedulers for each disk, create the file /usr/lib/tmpfiles.d/IO_ioscheduler.conf with content similar to the following example. It defines the deadline scheduler for /dev/sda and the noop scheduler for /dev/sdb. This feature is available on SLE 12 and 15 only.

w /sys/block/sda/queue/scheduler - - - - deadline
w /sys/block/sdb/queue/scheduler - - - - noop

4.3.2 Asynchronous I/O

Many of the virtual disk back-ends use Linux Asynchronous I/O (aio) in their implementation. By default, the maximum number of aio contexts is set to 65536, which can be exceeded when running hundreds of VM Guests using virtual disks serviced by Linux Asynchronous I/O. When running large numbers of VM Guests on a VM Host Server, consider increasing /proc/sys/fs/aio-max-nr.

Procedure 4: Checking and Changing aio-max-nr at Runtime
  1. To check your current aio-max-nr setting run:

    > cat /proc/sys/fs/aio-max-nr
  2. You can change aio-max-nr at runtime with the following command:

    #  echo 131072 > /proc/sys/fs/aio-max-nr

To permanently set aio-max-nr, add an entry to a custom sysctl file. For example, include the following to /etc/sysctl.d/aio-max-nr.conf:

fs.aio-max-nr = 1048576

4.3.3 I/O Virtualization

SUSE products support various I/O virtualization technologies. The following table lists advantages and disadvantages of each technology. For more information about I/O in virtualization refer to the Section 1.5, “I/O Virtualization” chapter in the SUSE Linux Enterprise Server 15 SP2 Virtualization Guide.

Table 2: I/O Virtualization Solutions




Device Assignment (pass-through)

Device accessed directly by the guest

No sharing among multiple guests

High performance

Live migration is complex

PCI device limit is 8 per guest

Limited number of slots on a server

Full virtualization (IDE, SATA, SCSI, e1000)

VM Guest compatibility

Bad performance

Easy for live migration

Emulated operation

Para-virtualization (virtio-blk, virtio-net, virtio-scsi)

Good performance

Modified guest (PV drivers)

Easy for live migration

Efficient host communication with VM Guest

4.4 Storage and File System

Storage space for VM Guests can either be a block device (for example, a partition on a physical disk), or an image file on the file system:

Table 3: Block Devices Compared to Disk Images




Block devices

  • Better performance

  • Use standard tools for administration/disk modification

  • Accessible from host (pro and con)

  • Device management

Image files

  • Easier system management

  • Easily move, clone, expand, back up domains

  • Comprehensive toolkit (guestfs) for image manipulation

  • Reduce overhead through sparse files

  • Fully allocate for best performance

  • Lower performance than block devices

For detailed information about image formats and maintaining images refer to Section 5, “VM Guest Images”.

If your image is stored on an NFS share, you should check some server and client parameters to improve access to the VM Guest image.

4.4.1 NFS Read/Write (Client)

Options rsize and wsize specify the size of the chunks of data that the client and server pass back and forth to each other. You should ensure NFS read/write sizes are sufficiently large, especially for large I/O. Change the rsize and wsize parameter in your /etc/fstab by increasing the value to 16 KB. This will ensure that all operations can be frozen if there is any instance of hanging.

nfs_server:/exported/vm_images1 /mnt/images2 nfs3 rw4,hard5,sync6, rsize=81927,wsize=81928 0 0


NFS server's host name and export path name.


Where to mount the NFS exported share.


This is an nfs mount point.


This mount point will be accessible in read/write.


Determines the recovery behavior of the NFS client after an NFS request times out. hard is the best option to avoid data corruption.


Any system call that writes data to files on that mount point causes that data to be flushed to the server before the system call returns control to user space.


Maximum number of bytes in each network READ request that the NFS client can receive when reading data from a file on an NFS server.


Maximum number of bytes per network WRITE request that the NFS client can send when writing data to a file on an NFS server.

4.4.2 NFS Threads (Server)

Your NFS server should have enough NFS threads to handle multi-threaded workloads. Use the nfsstat tool to get some RPC statistics on your server:

> sudo nfsstat -rc
Client rpc stats:
calls      retrans    authrefrsh
6401066    198          0          0

If the retrans is equal to 0, everything is fine. Otherwise, the client needs to retransmit, so increase the USE_KERNEL_NFSD_NUMBER variable in /etc/sysconfig/nfs, and adjust accordingly until retrans is equal to 0.

4.5 CPUs

Host CPU components will be translated to virtual CPUs in a VM Guest when being assigned. These components can either be:

  • CPU processor: this describes the main CPU unit, which usually has multiple cores and may support Hyper-Threading.

  • CPU core: a main CPU unit can provide more than one core, and the proximity of cores speeds up the computation process and reduces energy costs.

  • CPU Hyper-Threading: this implementation is used to improve parallelization of computations, but this is not as efficient as a dedicated core.

4.5.1 Assigning CPUs

CPU overcommit occurs when the cumulative number of virtual CPUs of all VM Guests becomes higher than the number of host CPUs. Best performance is likely to be achieved when there is no overcommit and each virtual CPU matches one hardware processor or core on the VM Host Server. In fact, VM Guests running on an overcommitted host will experience increased latency and a negative effect on per-VM Guest throughput is also likely to be observed. Therefore, you should try to avoid overcommitting CPUs.

Deciding whether to allow CPU overcommit or not requires good a-priori knowledge of workload as a whole. For instance, if you know that all the VM Guests virtual CPUs will not be loaded more than 50% then you can assume that overcommitting the host by a factor of 2 (which means having 128 virtual CPUs in total, on a host with 64 CPUs) will work well. On the other hand, if you know that all the virtual CPUs of the VM Guests will try to run at 100% for most of the time then even having one virtual CPU more than than the host has CPUs is already a misconfiguration.

Overcommitting to a point where the cumulative number of virtual CPUs is higher than 8 times the number of physical cores of the VM Host Server will most likely lead to a malfunctioning and unstable system and should hence be avoided.

Unless you know exactly how many virtual CPUs are required for a VM Guest, you should start with one. A good rule of thumb is to target a CPU workload of approximately 70% inside your VM (see Section 2.3, “Processes” for information on monitoring tools). If you allocate more processors than needed in the VM Guest, this will negatively affect the performance of host and guest. Cycle efficiency will be degraded, as the unused vCPU will still cause timer interrupts. In case you primarily run single threaded applications on a VM Guest, a single virtual CPU is the best choice.

A single VM Guest with more virtual CPUs than the VM Host Server has CPUs is always a misconfiguration.

4.5.2 VM Guest CPU Configuration

This section describes how to choose and configure a CPU type for a VM Guest. You will also learn how to pin virtual CPUs to physical CPUs on the host system. For more information about virtual CPU configuration and tuning parameters refer to the libvirt documentation at https://libvirt.org/formatdomain.html#elementsCPU. Virtual CPU Models and Features

The CPU model and topology can be specified individually for each VM Guest. Configuration options range from selecting specific CPU models to excluding certain CPU features. Predefined CPU models are listed in files in the directory /usr/share/libvirt/cpu_map/. A CPU model and topology that is similar to the host generally provides the best performance. The host system CPU model and topology can be displayed by running virsh capabilities.

Note that changing the default virtual CPU configuration will require a VM Guest shutdown when migrating it to a host with different hardware. More information on VM Guest migration is available at Section 9.7, “Migrating VM Guests”.

To specify a particular CPU model for a VM Guest, add a respective entry to the VM Guest configuration file. The following example configures a Broadwell CPU with the invariant TSC feature:

<cpu mode='custom' match='exact'>
  <feature name='invtsc'/>

For a virtual CPU that most closely resembles the host physical CPU, <cpu mode='host-passthrough'> can be used. Note that a host-passthrough CPU model may not exactly resemble the host physical CPU, since by default KVM will mask any non-migratable features. For example invtsc is not included in the virtual CPU feature set. Changing the default KVM behavior is not directly supported through libvirt, although it does allow arbitrary passthrough of KVM command line arguments. Continuing with the invtsc example, you can achieve passthrough of the host CPU (including invtsc) with the following command line passthrough in the VM Guest configuration file:

<domain type='kvm' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
     <qemu:arg value='-cpu'/>
     <qemu:arg value='host,migratable=off,+invtsc'/>
Note: The host-passthrough Mode

Since host-passthrough exposes the physical CPU details to the virtual CPU, migration to dissimilar hardware is not possible. See Section, “Virtual CPU Migration Considerations” for more information. Virtual CPU Pinning

Virtual CPU pinning is used to constrain virtual CPU threads to a set of physical CPUs. The vcpupin element specifies the physical host CPUs that a virtual CPU can use. If this element is not set and the attribute cpuset of the vcpu element is not specified, the virtual CPU is free to use any of the physical CPUs.

CPU intensive workloads can benefit from virtual CPU pinning by increasing the physical CPU cache hit ratio. To pin a virtual CPU to a specific physical CPU, run the following commands:

> virsh vcpupin DOMAIN_ID --vcpu vCPU_NUMBER
VCPU: CPU Affinity
0: 0-7
# virsh vcpupin SLE15 --vcpu 0 0 --config

The last command generates the following entry in the XML configuration:

   <vcpupin vcpu='0' cpuset='0'/>
Note: Virtual CPU Pinning on NUMA Nodes

To confine a VM Guest's CPUs and its memory to a NUMA node, you can use virtual CPU pinning and memory allocation policies on a NUMA system. See Section 4.6, “NUMA Tuning” for more information related to NUMA tuning.

Warning: Virtual CPU Pinning and Live Migration

Even though vcpupin can improve performance, it can complicate live migration. See Section, “Virtual CPU Migration Considerations” for more information on virtual CPU migration considerations. Virtual CPU Migration Considerations

Selecting a virtual CPU model containing all the latest features may improve performance of a VM Guest workload, but often at the expense of migratability. Unless all hosts in the cluster contain the latest CPU features, migration can fail when a destination host lacks the new features. If migratability of a virtual CPU is preferred over the latest CPU features, a normalized CPU model and feature set should be used. The virsh cpu-baseline command can help define a normalized virtual CPU that can be migrated across all hosts. The following command, when run on each host in the migration cluster, illustrates collection of all hosts' capabilities in all-hosts-caps.xml.

> sudo virsh capabilities >> all-hosts-cpu-caps.xml

With the capabilities from each host collected in all-hosts-caps.xml, use virsh cpu-baseline to create a virtual CPU definition that will be compatible across all hosts.

> sudo virsh cpu-baseline all-hosts-caps.xml

The resulting virtual CPU definition can be used as the cpu element in VM Guest configuration file.

At a logical level, virtual CPU pinning is a form of hardware passthrough. Pinning couples physical resources to virtual resources, and can also be problematic for migration. For example, the migration will fail if the requested physical resources are not available on the destination host, or if the source and destination hosts have different NUMA topologies. For more recommendations about Live Migration see Section 9.7.1, “Migration Requirements”.

4.6 NUMA Tuning

NUMA is an acronym for Non Uniform Memory Access. A NUMA system has multiple physical CPUs, each with local memory attached. Each CPU can also access other CPUs' memory, known as remote memory access, but it is much slower than accessing local memory. NUMA systems can negatively impact VM Guest performance if not tuned properly. Although ultimately tuning is workload dependent, this section describes controls that should be considered when deploying VM Guests on NUMA hosts. Always consider your host topology when configuring and deploying VMs.

SUSE Linux Enterprise Server contains a NUMA auto-balancer that strives to reduce remote memory access by placing memory on the same NUMA node as the CPU processing it. In addition, standard tools such as cgset and virtualization tools such as libvirt provide mechanisms to constrain VM Guest resources to physical resources.

numactl is used to check for host NUMA capabilities:

> sudo numactl --hardware
available: 4 nodes (0-3)
node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 72 73 74 75 76 77 78
79 80 81 82 83 84 85 86 87 88 89
node 0 size: 31975 MB
node 0 free: 31120 MB
node 1 cpus: 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 90 91 92 93
94 95 96 97 98 99 100 101 102 103 104 105 106 107
node 1 size: 32316 MB
node 1 free: 31673 MB
node 2 cpus: 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 108 109 110
111 112 113 114 115 116 117 118 119 120 121 122 123 124 125
node 2 size: 32316 MB
node 2 free: 31726 MB
node 3 cpus: 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 126 127 128
129 130 131 132 133 134 135 136 137 138 139 140 141 142 143
node 3 size: 32314 MB
node 3 free: 31387 MB
node distances:
node   0   1   2   3
0:  10  21  21  21
1:  21  10  21  21
2:  21  21  10  21
3:  21  21  21  10

The numactl output shows this is a NUMA system with 4 nodes or cells, each containing 36 CPUs and approximately 32G memory. virsh capabilities can also be used to examine the systems NUMA capabilities and CPU topology.

4.6.1 NUMA Balancing

On NUMA machines, there is a performance penalty if remote memory is accessed by a CPU. Automatic NUMA balancing scans a task's address space and unmaps pages. By doing so, it detects whether pages are properly placed or whether to migrate the data to a memory node local to where the task is running. In defined intervals (configured with numa_balancing_scan_delay_ms), the task scans the next scan size number of pages (configured with numa_balancing_scan_size_mb) in its address space. When the end of the address space is reached the scanner restarts from the beginning.

Higher scan rates cause higher system overhead as page faults must be trapped and data needs to be migrated. However, the higher the scan rate, the more quickly a task's memory is migrated to a local node when the workload pattern changes. This minimizes the performance impact caused by remote memory accesses. These sysctl directives control the thresholds for scan delays and the number of pages scanned:

> sudo sysctl -a | grep numa_balancing
kernel.numa_balancing = 11
kernel.numa_balancing_scan_delay_ms = 10002
kernel.numa_balancing_scan_period_max_ms = 600003
kernel.numa_balancing_scan_period_min_ms = 10004
kernel.numa_balancing_scan_size_mb = 2565


Enables/disables automatic page fault-based NUMA balancing


Starting scan delay used for a task when it initially forks


Maximum time in milliseconds to scan a task's virtual memory


Minimum time in milliseconds to scan a task's virtual memory


Size in megabytes' worth of pages to be scanned for a given scan

For more information see Chapter 11, Automatic Non-Uniform Memory Access (NUMA) Balancing.

The main goal of automatic NUMA balancing is either to reschedule tasks on the same node's memory (so the CPU follows the memory), or to copy the memory's pages to the same node (so the memory follows the CPU).

Warning: Task Placement

There are no rules to define the best place to run a task, because tasks could share memory with other tasks. For best performance, it is recommended to group tasks sharing memory on the same node. Check NUMA statistics with # cat /proc/vmstat | grep numa_.

4.6.2 Memory Allocation Control with the CPUset Controller

The cgroups cpuset controller can be used confine memory used by a process to a NUMA node. There are three cpuset memory policy modes available:

  • interleave: This is a memory placement policy which is also known as round-robin. This policy can provide substantial improvements for jobs that need to place thread local data on the corresponding node. When the interleave destination is not available, it will be moved to another node.

  • bind: This will place memory only on one node, which means in case of insufficient memory, the allocation will fail.

  • preferred: This policy will apply a preference to allocate memory to a node. If there is not enough space for memory on this node, it will fall back to another node.

You can change the memory policy mode with the cgset tool from the libcgroup-tools package:

> sudo cgset -r cpuset.mems=NODE sysdefault/libvirt/qemu/KVM_NAME/emulator

To migrate pages to a node, use the migratepages tool:

> migratepages PID FROM-NODE TO-NODE

To check everything is fine. use: cat /proc/PID/status | grep Cpus.

Note: Kernel NUMA/cpuset memory policy

For more information see Kernel NUMA memory policy and cpusets memory policy. Check also the Libvirt NUMA Tuning documentation.

4.6.3 VM Guest: NUMA Related Configuration

libvirt allows to set up virtual NUMA and memory access policies. Configuring these settings is not supported by virt-install or virt-manager and needs to be done manually by editing the VM Guest configuration file with virsh edit. VM Guest Virtual NUMA Topology

Creating a VM Guest virtual NUMA (vNUMA) policy that resembles the host NUMA topology can often increase performance of traditional large, scale-up workloads. VM Guest vNUMA topology can be specified using the numa element in the XML configuration:

    <cell1 id="0"2 cpus='0-1'3 memory='512000' unit='KiB'/>
    <cell id="1" cpus='2-3' memory='256000'4
    unit='KiB'5 memAccess='shared'6/>


Each cell element specifies a vNUMA cell or node


All cells should have an id attribute, allowing to reference the cell in other configuration blocks. Otherwise cells are assigned ids in ascending order starting from 0.


The CPU or range of CPUs that are part of the node


The node memory


Units in which node memory is specified


Optional attribute which can control whether the memory is to be mapped as shared or private. This is valid only for hugepages-backed memory.

To find where the VM Guest has allocated its pages. use: cat /proc/PID/numa_maps and cat /sys/fs/cgroup/memory/sysdefault/libvirt/qemu/KVM_NAME/memory.numa_stat.

Warning: NUMA specification

The libvirt VM Guest NUMA specification is currently only available for QEMU/KVM. Memory Allocation Control with libvirt

If the VM Guest has a vNUMA topology (see Section, “VM Guest Virtual NUMA Topology”), memory can be pinned to host NUMA nodes using the numatune element. This method is currently only available for QEMU/KVM guests. See Important: Non-vNUMA VM Guest for how to configure non-vNUMA VM Guests.

    <memory mode="strict"1 nodeset="1-4,^3"2/>
    <memnode3 cellid="0"4 mode="strict" nodeset="1"/>
    <memnode cellid="2" placement="strict"5 mode="preferred" nodeset="2"/>


Policies available are: interleave (round-robin like), strict (default) or preferred.


Specify the NUMA nodes.


Specify memory allocation policies for each guest NUMA node (if this element is not defined then this will fall back and use the memory element).


Addresses the guest NUMA node for which the settings are applied.


The placement attribute can be used to indicate the memory placement mode for a domain process, the value can be auto or strict.

Important: Non-vNUMA VM Guest

On a non-vNUMA VM Guest, pinning memory to host NUMA nodes is done like in the following example:

   <memory mode="strict" nodeset="0-1"/>

In this example, memory is allocated from the host nodes 0 and 1. In case these memory requirements cannot be fulfilled, starting the VM Guest will fail. virt-install also supports this configuration with the --numatune option.

Warning: Memory and CPU across NUMA Nodes

You should avoid allocating VM Guest memory across NUMA nodes, and prevent virtual CPUs from floating across NUMA nodes.

5 VM Guest Images

Images are virtual disks used to store the operating system and data of VM Guests. They can be created, maintained and queried with the qemu-img command. Refer to Section 31.2.2, “Creating, Converting and Checking Disk Images” for more information on the qemu-img tool and examples.

5.1 VM Guest Image Formats

Certain storage formats which QEMU recognizes have their origins in other virtualization technologies. By recognizing these formats, QEMU can leverage either data stores or entire guests that were originally targeted to run under these other virtualization technologies. Some formats are supported only in read-only mode. To use them in read/write mode, convert them to a fully supported QEMU storage format (using qemu-img). Otherwise they can only be used as read-only data store in a QEMU guest.

Use qemu-img info VMGUEST.IMG to get information about an existing image, such as: the format, the virtual size, the physical size, snapshots if available.

Note: Performance

It is recommended to convert the disk images to either raw or qcow2 to achieve good performance.

Warning: Encrypted Images Cannot Be Compressed

When you create an image, you cannot use compression (-c) in the output file together with the encryption option (-e).

5.1.1 Raw Format

  • This format is simple and easily exportable to all other emulators/hypervisors.

  • It provides best performance (least I/O overhead).

  • It occupies all allocated space on the file system.

  • The raw format allows to copy a VM Guest image to a physical device (dd if=VMGUEST.RAW of=/dev/sda).

  • It is byte-for-byte the same as what the VM Guest sees, so this wastes a lot of space.

5.1.2 qcow2 Format

  • Use this to have smaller images (useful if your file system does not supports holes).

  • It has optional AES encryption (now deprecated).

  • Zlib-based compression option.

  • Support of multiple VM snapshots (internal, external).

  • Improved performance and stability.

  • Supports changing the backing file.

  • Supports consistency checks.

  • Less performance than raw format.


qcow2 can provide the same performance for random read/write access as raw format, but it needs a well-sized cache size. By default cache size is set to 1 MB. This will give good performance up to a disk size of 8 GB. If you need a bigger disk size, you need to adjust the cache size. For a disk size of 64 GB (64*1024 = 65536), you need 65536 / 8192B = 8 MB of cache (-drive format=qcow2,l2-cache-size=8M).

Cluster Size

The qcow2 format offers the capability to change the cluster size. The value must be between 512 KB and 2 MB. Smaller cluster sizes can improve the image file size whereas larger cluster sizes generally provide better performance.


An image with preallocated metadata is initially larger but can improve performance when the image needs to grow.

Lazy Refcounts

Reference count updates are postponed with the goal of avoiding metadata I/O and improving performance. This is particularly beneficial with cache=writethrough. This option does not batch metadata updates, but if in case of host crash, the reference count tables must be rebuilt, this is done automatically at the next open with qemu-img check -r all. Note that this takes some time.

5.1.3 qed Format

qed is a follow-on qcow (QEMU Copy On Write) format. Because qcow2 provides all the benefits of qed and more, qed is now deprecated.

5.1.4 VMDK Format

VMware 3, 4, or 6 image format, for exchanging images with that product.

5.2 Overlay Disk Images

The qcow2 and qed formats provide a way to create a base image (also called backing file) and overlay images on top of the base image. A backing file is useful to be able to revert to a known state and discard the overlay. If you write to the image, the backing image will be untouched and all changes will be recorded in the overlay image file. The backing file will never be modified unless you use the commit monitor command (or qemu-img commit).

To create an overlay image:

# qemu-img create -o1backing_file=vmguest.raw2,backing_fmt=raw3\
     -f4 qcow2 vmguest.cow5


Use -o ? for an overview of available options.


The backing file name.


Specify the file format for the backing file.


Specify the image format for the VM Guest.


Image name of the VM Guest, it will only record the differences from the backing file.

Warning: Backing Image Path

You should not change the path to the backing image, otherwise you will need to adjust it. The path is stored in the overlay image file. To update the path, you should make a symbolic link from the original path to the new path and then use the qemu-img rebase option.

# ln -sf /var/lib/images/vmguest.raw  /var/lib/images/SLE15/vmguest.raw
# qemu-img rebase1-u2 -b3 /var/lib/images/vmguest.raw /var/lib/images/SLE15/vmguest.cow4

The rebase subcommand tells qemu-img to change the backing file image. The -u option activates the unsafe mode (see note below). The backing image to be used is specified with -b and the image path is the last argument of the command.

There are two different modes in which rebase can operate:

  • Safe: This is the default mode and performs a real rebase operation. The safe mode is a time-consuming operation.

  • Unsafe: The unsafe mode (-u) only changes the backing files name and the format of the file name without making any checks on the files contents. You should use this mode to rename or moving a backing file.

A common use is to initiate a new guest with the backing file. Let's assume we have a sle15_base.img VM Guest ready to be used (fresh installation without any modification). This will be our backing file. Now you need to test a new package, on an updated system and on a system with a different kernel. We can use sle15_base.img to instantiate the new SUSE Linux Enterprise VM Guest by creating a qcow2 overlay file pointing to this backing file (sle15_base.img).

In our example we will use sle15_updated.qcow2 for the updated system, and sle15_kernel.qcow2 for the system with a different kernel.

To create the two thin provisioned systems use the qemu-img command line with the -b option:

# qemu-img create -b /var/lib/libvirt/sle15_base.img -f qcow2 \
Formatting 'sle15_updated.qcow2', fmt=qcow2 size=17179869184
backing_file='sle15_base.img' encryption=off cluster_size=65536
lazy_refcounts=off nocow=off
# qemu-img create -b /var/lib/libvirt/sle15_base.img -f qcow2 \
Formatting 'sle15_kernel.qcow2', fmt=qcow2 size=17179869184
backing_file='vmguest-sle15_base.img' encryption=off cluster_size=65536
lazy_refcounts=off nocow=off

The images are now usable, and you can do your test without touching the initial sle15_base.img backing file. All changes will be stored in the new overlay images. Additionally, you can also use these new images as a backing file, and create a new overlay.

# qemu-img create -b sle15_kernel.qcow2 -f qcow2 sle15_kernel_TEST.qcow2

When using qemu-img info with the option --backing-chain, it will return all information about the entire backing chain recursively:

# qemu-img info --backing-chain
image: sle15_kernel_TEST.qcow2
file format: qcow2
virtual size: 16G (17179869184 bytes)
disk size: 196K
cluster_size: 65536
backing file: sle15_kernel.qcow2
Format specific information:
compat: 1.1
lazy refcounts: false

image: sle15_kernel.qcow2
file format: qcow2
virtual size: 16G (17179869184 bytes)
disk size: 196K
cluster_size: 65536
backing file: SLE15.qcow2
Format specific information:
compat: 1.1
lazy refcounts: false

image: sle15_base.img
file format: qcow2
virtual size: 16G (17179869184 bytes)
disk size: 16G
cluster_size: 65536
Format specific information:
compat: 1.1
lazy refcounts: true
Understanding Image Overlay
Figure 1: Understanding Image Overlay

5.3 Opening a VM Guest Image

To access the file system of an image, use the guestfs-tools. If you do not have this tool installed on your system you can mount an image with other Linux tools. Avoid accessing an untrusted or unknown VM Guest's image system because this can lead to security issues (for more information, read D. Berrangé's post).

5.3.1 Opening a Raw Image

Procedure 5: Mounting a Raw Image
  1. To be able to mount the image, find a free loop device. The following command displays the first unused loop device, /dev/loop1 in this example.

    # losetup -f
  2. Associate an image (SLE15.raw in this example) with the loop device:

    # losetup /dev/loop1 SLE15.raw
  3. Check whether the image has successfully been associated with the loop device by getting detailed information about the loop device:

    # losetup -l
    /dev/loop1         0      0         0  0    /var/lib/libvirt/images/SLE15.raw
  4. Check the image's partitions with kpartx:

    # kpartx -a1 -v2 /dev/loop1
    add map loop1p1 (254:1): 0 29358080 linear /dev/loop1 2048


    Add partition device mappings.


    Verbose mode.

  5. Now mount the image partition(s) (to /mnt/sle15mount in the following example):

    # mkdir /mnt/sle15mount
    # mount /dev/mapper/loop1p1 /mnt/sle15mount
Note: Raw image with LVM

If your raw image contains an LVM volume group you should use LVM tools to mount the partition. Refer to Section 5.3.3, “Opening Images Containing LVM”.

Procedure 6: Unmounting a Raw Image
  1. Unmount all mounted partitions of the image, for example:

    # umount /mnt/sle15mount
  2. Delete partition device mappings with kpartx:

    # kpartx -d /dev/loop1
  3. Detach the devices with losetup

    # losetup -d /dev/loop1

5.3.2 Opening a qcow2 Image

Procedure 7: Mounting a qcow2 Image
  1. First you need to load the nbd (network block devices) module. The following example loads it with support for 16 block devices (max_part=16). Check with dmesg whether the operation was successful:

    # modprobe nbd max_part=16
    # dmesg | grep nbd
    [89155.142425] nbd: registered device at major 43
  2. Connect the VM Guest image (for example SLE15.qcow2) to an NBD device (/debv/nbd0 in the following example) with the qemu-nbd command. Make sure to use a free NBD device:

    # qemu-nbd -c1 /dev/nbd02 SLE15.qcow23


    Connect SLE15.qcow2 to the local NBD device /dev/nbd0


    NBD device to use


    VM Guest image to use

    Tip: Checking for a free NBD Device

    To check whether an NBD device is free, run the following command:

    # lsof /dev/nbd0
    qemu-nbd 15149 root   10u   BLK   43,0      0t0 47347 /dev/nbd0

    If the command produces an output like in the example above, the device is busy (not free). This can also be confirmed by the presence of the /sys/devices/virtual/block/nbd0/pid file.

  3. Inform the operating system about partition table changes with partprobe:

    # partprobe /dev/nbd0 -s
    /dev/nbd0: msdos partitions 1 2
    # dmesg | grep nbd0 | tail -1
    [89699.082206]  nbd0: p1 p2
  4. In the example above, the SLE15.qcow2 contains two partitions: /dev/nbd0p1 and /dev/nbd0p2. Before mounting these partitions, use vgscan to check whether they belong to an LVM volume:

    # vgscan -v
        Wiping cache of LVM-capable devices
        Wiping internal VG cache
        Reading all physical volumes. This may take a while...
        Using volume group(s) on command line.
        No volume groups found.
  5. If no LVM volume has been found, you can mount the partition with mount:

    # mkdir /mnt/nbd0p2
    # mount /dev/nbd0p1 /mnt/nbd0p2

    Refer to Section 5.3.3, “Opening Images Containing LVM” for information on how to handle LVM volumes.

Procedure 8: Unmounting a qcow2 Image
  1. Unmount all mounted partitions of the image, for example:

    # umount /mnt/nbd0p2
  2. Disconnect the image from the /dev/nbd0 device.

    # qemu-nbd -d /dev/nbd0

5.3.3 Opening Images Containing LVM

Procedure 9: Mounting Images Containing LVM
  1. To check images for LVM groups, use vgscan -v. If an image contains LVM groups, the output of the command looks like the following:

    # vgscan -v
    Wiping cache of LVM-capable devices
    Wiping internal VG cache
    Reading all physical volumes.  This may take a while...
    Finding all volume groups
    Finding volume group "system"
    Found volume group "system" using metadata type lvm2
  2. The system LVM volume group has been found on the system. You can get more information about this volume with vgdisplay VOLUMEGROUPNAME (in our case VOLUMEGROUPNAME is system). You should activate this volume group to expose LVM partitions as devices so the system can mount them. Use vgchange:

    # vgchange -ay -v
    Finding all volume groups
    Finding volume group "system"
    Found volume group "system"
    activation/volume_list configuration setting not defined: Checking only
    host tags for system/home
    Creating system-home
    Loading system-home table (254:0)
    Resuming system-home (254:0)
    Found volume group "system"
    activation/volume_list configuration setting not defined: Checking only
    host tags for system/root
    Creating system-root
    Loading system-root table (254:1)
    Resuming system-root (254:1)
    Found volume group "system"
    activation/volume_list configuration setting not defined: Checking only
    host tags for system/swap
    Creating system-swap
    Loading system-swap table (254:2)
    Resuming system-swap (254:2)
    Activated 3 logical volumes in volume group system
        3 logical volume(s) in volume group "system" now active
  3. All partitions in the volume group will be listed in the /dev/mapper directory. You can simply mount them now.

    # ls /dev/mapper/system-*
    /dev/mapper/system-home  /dev/mapper/system-root /dev/mapper/system-swap
    # mkdir /mnt/system-root
    # mount  /dev/mapper/system-root /mnt/system-root
    # ls /mnt/system-root/
    bin   dev  home  lib64       mnt  proc        root  sbin     srv  tmp  var
    boot  etc  lib   lost+found  opt  read-write  run   selinux  sys  usr
Procedure 10: Unmounting Images Containing LVM
  1. Unmount all partitions (with umount)

    # umount /mnt/system-root
  2. Deactivate the LVM volume group (with vgchange -an VOLUMEGROUPNAME)

    # vgchange -an -v system
    Using volume group(s) on command line
    Finding volume group "system"
    Found volume group "system"
    Removing system-home (254:0)
    Found volume group "system"
    Removing system-root (254:1)
    Found volume group "system"
    Removing system-swap (254:2)
    Deactivated 3 logical volumes in volume group system
    0 logical volume(s) in volume group "system" now active
  3. Now you have two choices:

    • In case of a qcow2 image, proceed as described in Step 2 (qemu-nbd -d /dev/nbd0).

    • In case of a raw image, proceeds as described in Step 2 (kpartx -d /dev/loop1; losetup -d /dev/loop1).

    Important: Check for a Successful Unmount

    You should double-check that unmounting succeeded by using a system command like losetup, qemu-nbd, mount or vgscan. If this is not the case you may have trouble using the VM Guest because its system image is used in different places.

5.4 File System Sharing

You can access a host directory in the VM Guest using the filesystem element. In the following example we will share the /data/shared directory and mount it in the VM Guest. Note that the accessmode parameter only works with type='mount' for the QEMU/KVM driver (most other values for type are exclusively used for the LXC driver).

<filesystem type='mount'1 accessmode='mapped'2>
   <source dir='/data/shared'3>
   <target dir='shared'4/>


A host directory to mount VM Guest.


Access mode (the security mode) set to mapped will give access with the permissions of the hypervisor. Use passthrough to access this share with the permissions of the user inside the VM Guest.


Path to share with the VM Guest.


Name or label of the path for the mount command.

To mount the shared directory on the VM Guest, use the following commands: Under the VM Guest now you need to mount the target dir='shared':

# mkdir /opt/mnt_shared
# mount shared -t 9p /opt/mnt_shared -o trans=virtio

See libvirt File System and QEMU 9psetup for more information.

6 VM Guest Configuration

6.1 Virtio Driver

To increase VM Guest performance it is recommended to use paravirtualized drivers within the VM Guests. The virtualization standard for such drivers for KVM are the virtio drivers, which are designed for running in a virtual environment. Xen uses similar paravirtualized device drivers (like VMDP in a Windows* guest).

6.1.1 virtio blk

virtio_blk is the virtio block device for disk. To use the virtio blk driver for a block device, specify the bus='virtio' attribute in the disk definition:

<disk type='....' device='disk'>
    <target dev='vda' bus='virtio'/>
Important: Disk Device Names

virtio disk devices are named /dev/vd[a-z][1-9]. If you migrate a Linux guest from a non-virtio disk you need to adjust the root= parameter in the GRUB configuration, and regenerate the initrd file. Otherwise the system cannot boot. On VM Guests with other operating systems, the boot loader may need to be adjusted or reinstalled accordingly, too.

Important: Using virtio Disks with qemu-system-ARCH

When running qemu-system-ARCH, use the -drive option to add a disk to the VM Guest. See Section 31.1, “Basic Installation with qemu-system-ARCH for an example. The -hd[abcd] option will not work for virtio disks.

6.1.2 virtio net

virtio_net is the virtio network device. The kernel modules should be loaded automatically in the guest at boot time. You need to start the service to make the network available.

<interface type='network'>
    <model type='virtio' />

6.1.3 virtio balloon

The virtio balloon is used for host memory over-commits for guests. For Linux guests, the balloon driver runs in the guest kernel, whereas for Windows guests, the balloon driver is in the VMDP package. virtio_balloon is a PV driver to give or take memory from a VM Guest.

  • Inflate balloon: Return memory from guest to host kernel (for KVM) or to hypervisor (for Xen)

  • Deflate balloon: Guest will have more available memory

It is controlled by the currentMemory and memory options.

<memory unit='KiB'>16777216</memory>
    <currentMemory unit='KiB'>1048576</currentMemory>
        <memballoon model='virtio'/>

You can also use virsh to change it:

> virsh setmem DOMAIN_ID MEMORY in KB

6.1.4 Checking virtio Presence

You can check the virtio block PCI with:

> find /sys/devices/ -name virtio*

To find the block device associated with vdX:

> find /sys/devices/ -name virtio* -print  -exec ls {}/block 2>/dev/null \;

To get more information on the virtio block:

> udevadm info -p /sys/devices/pci0000:00/0000:00:08.0/virtio2
P: /devices/pci0000:00/0000:00:08.0/virtio2
E: DEVPATH=/devices/pci0000:00/0000:00:08.0/virtio2
E: DRIVER=virtio_blk
E: MODALIAS=virtio:d00000002v00001AF4

To check all virtio drivers being used:

> find /sys/devices/ -name virtio* -print  -exec ls -l {}/driver 2>/dev/null \;
lrwxrwxrwx 1 root root 0 Jun 17 15:48 /sys/devices/pci0000:00/0000:00:06.0/virtio0/driver -> ../../../../bus/virtio/drivers/virtio_console
lrwxrwxrwx 1 root root 0 Jun 17 15:47 /sys/devices/pci0000:00/0000:00:07.0/virtio1/driver -> ../../../../bus/virtio/drivers/virtio_balloon
lrwxrwxrwx 1 root root 0 Jun 17 14:35 /sys/devices/pci0000:00/0000:00:08.0/virtio2/driver -> ../../../../bus/virtio/drivers/virtio_blk

6.1.5 Find Device Driver Options

Virtio devices and other drivers have various options. To list all of them, use the help parameter of theqemu-system-ARCH command.

> qemu-system-x86_64 -device virtio-net,help

6.2 Cirrus Video Driver

To get 16-bit color, high compatibility and better performance it is recommended to use the cirrus video driver.

Note: libvirt

libvirt ignores the vram value because video size has been hardcoded in QEMU.

   <model type='cirrus' vram='9216' heads='1'/>

6.3 Better Entropy

Virtio RNG (random number generator) is a paravirtualized device that is exposed as a hardware RNG device to the guest. On the host side, it can be wired up to one of several sources of entropy (including a real hardware RNG device and the host's /dev/random) if hardware support does not exist. The Linux kernel contains the guest driver for the device from version 2.6.26 and higher.

The system entropy is collected from various non-deterministic hardware events and is mainly used by cryptographic applications. The virtual random number generator device (paravirtualized device) allows the host to pass through entropy to VM Guest operating systems. This results in a better entropy in the VM Guest.

To use Virtio RNG, add an RNG device in virt-manager or directly in the VM Guest's XML configuration:

   <rng model='virtio'>
       <backend model='random'>/dev/random</backend>

The host now should used /dev/random:

> lsof /dev/random
qemu-syst 4926 qemu    6r   CHR    1,8      0t0 8199 /dev/random

On the VM Guest, the source of entropy can be checked with:

> cat /sys/devices/virtual/misc/hw_random/rng_available

The current device used for entropy can be checked with:

> cat /sys/devices/virtual/misc/hw_random/rng_current

You should install the rng-tools package on the VM Guest, enable the service, and start it. Under SUSE Linux Enterprise Server 15, do the following:

# zypper in rng-tools
# systemctl enable rng-tools
# systemctl start rng-tools

6.4 Disable Unused Tools and Devices

Per host, use one virtualization technology only. For example, do not use KVM and Xen on the same host. Otherwise, you may find yourself with a reduced amount of available resources, increased security risk and a longer software update queue. Even when the amount of resources allocated to each of the technologies is configured carefully, the host may suffer from reduced overall availability and degraded performance.

Minimize the amount of software and services available on hosts. Most default installations of operating systems are not optimized for VM usage. Install what you really need and remove all other components in the VM Guest.

Windows* Guest:

  • Disable the screen saver

  • Remove all graphical effects

  • Disable indexing of hard disks if not necessary

  • Check the list of started services and disable the ones you do not need

  • Check and remove all unneeded devices

  • Disable system update if not needed, or configure it to avoid any delay while rebooting or shutting down the host

  • Check the Firewall rules

  • Schedule backups and anti-virus updates appropriately

  • Install the VMDP paravirtualized driver for best performance

  • Check the operating system recommendations, such as on the Microsoft Windows* 7 better performance Web page.

Linux Guest:

  • Remove or do not start the X Window System if not necessary

  • Check the list of started services and disable the ones you do not need

  • Check the OS recommendations for kernel parameters that enable better performance

  • Only install software that you really need

  • Optimize the scheduling of predictable tasks (system updates, hard disk checks, etc.)

6.5 Updating the Guest Machine Type

QEMU machine types define details of the architecture that are particularly relevant for migration and session management. As changes or improvements to QEMU are made, new machine types are added. Old machine types are still supported for compatibility reasons, but to take advantage of improvements, we recommend to always migrate to the latest machine type when upgrading.

Changing the guest's machine type for a Linux guest will mostly be transparent. For Windows* guests, we recommend to take a snapshot or backup of the guest—in case Windows* has issues with the changes it detects and subsequently the user decides to revert to the original machine type the guest was created with.

Note: Changing the Machine Type

Refer to Section 14.2, “Changing the Machine Type” for documentation.

7 VM Guest-Specific Configurations and Settings

7.1 ACPI Testing

The ability to change a VM Guest's state heavily depends on the operating system. It is very important to test this feature before any use of your VM Guests in production. For example, most Linux operating systems disable this capability by default, so this requires you to enable this operation (mostly through Polkit).

ACPI must be enabled in the guest for a graceful shutdown to work. To check if ACPI is enabled, run:

> virsh dumpxml VMNAME | grep acpi

If nothing is printed, ACPI is not enabled for your machine. Use virsh edit to add the following XML under <domain>:


If ACPI was enabled during a Windows Server* guest installation, it is not sufficient to turn it on in the VM Guest configuration only. For more information, see https://support.microsoft.com/en-us/kb/309283.

Regardless of the VM Guest's configuration, a graceful shutdown is always possible from within the guest operating system.

7.2 Keyboard Layout

Though it is possible to specify the keyboard layout from a qemu-system-ARCH command, it is recommended to configure it in the libvirt XML file. To change the keyboard layout while connecting to a remote VM Guest using vnc, you should edit the VM Guest XML configuration file. For example, to add an en-us keymap, add in the <devices> section:

<graphics type='vnc' port='-1' autoport='yes' keymap='en-us'/>

Check the vncdisplay configuration and connect to your VM Guest:

> virsh vncdisplay sles15

7.3 Spice default listen URL

If no network interface other than lo is assigned an IPv4 address on the host, the default address on which the spice server listens will not work. An error like the following one will occur:

> virsh start sles15
error: Failed to start domain sles15
error: internal error: process exited while connecting to monitor: ((null):26929): Spice-Warning **: reds.c:2330:reds_init_socket: getaddrinfo(,5900): Address family for hostname not supported
2015-08-12T11:21:14.221634Z qemu-system-x86_64: failed to initialize spice server

To fix this, you can change the default spice_listen value in /etc/libvirt/qemu.conf using the local IPv6 address ::1. The spice server listening address can also be changed on a per VM Guest basis, use virsh edit to add the listen XML attribute to the graphics type='spice' element:

<graphics type='spice' listen='::1' autoport='yes'/>>

7.4 XML to QEMU command line

Sometimes it could be useful to get the QEMU command line to launch the VM Guest from the XML file.

> virsh domxml-to-native1 qemu-argv2 SLE15.xml3


Convert the XML file in domain XML format to the native guest configuration


For the QEMU/KVM hypervisor, the format argument needs be qemu-argv


Domain XML file to use

> sudo virsh domxml-to-native qemu-argv /etc/libvirt/qemu/SLE15.xml
LC_ALL=C PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin \
   QEMU_AUDIO_DRV=none /usr/bin/qemu-system-x86_64 -name SLE15 -machine \
   pc-i440fx-2.3,accel=kvm,usb=off -cpu SandyBridge -m 4048 -realtime \
   mlock=off -smp 4,sockets=4,cores=1,threads=1 -uuid 8616d00f-5f05-4244-97cc-86aeaed8aea7 \
   -no-user-config -nodefaults -chardev socket,id=charmonitor,path=/var/lib/libvirt/qemu/SLE15.monitor,server,nowait \
   -mon chardev=charmonitor,id=monitor,mode=control -rtc base=utc,driftfix=slew \
   -global kvm-pit.lost_tick_policy=discard -no-hpet \
   -no-shutdown -global PIIX4_PM.disable_s3=1 -global PIIX4_PM.disable_s4=1 \
   -boot strict=on -device ich9-usb-ehci1,id=usb,bus=pci.0,addr=0x4.0x7 \
   -device ich9-usb-uhci1,masterbus=usb.0,firstport=0,bus=pci.0,multifunction=on,addr=0x4 \
   -device ich9-usb-uhci2,masterbus=usb.0,firstport=2,bus=pci.0,addr=0x4.0x1 \
   -device ich9-usb-uhci3,masterbus=usb.0,firstport=4,bus=pci.0,addr=0x4.0x2 \
   -drive file=/var/lib/libvirt/images/SLE15.qcow2,if=none,id=drive-virtio-disk0,format=qcow2,cache=none \
   -device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x6,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=2 \
   -drive if=none,id=drive-ide0-0-1,readonly=on,format=raw  \
   -device ide-cd,bus=ide.0,unit=1,drive=drive-ide0-0-1,id=ide0-0-1 -netdev tap,id=hostnet0  \
   -device virtio-net-pci,netdev=hostnet0,id=net0,mac=52:54:00:28:04:a9,bus=pci.0,addr=0x3,bootindex=1 \
   -chardev pty,id=charserial0 -device isa-serial,chardev=charserial0,id=serial0 \
   -vnc -device cirrus-vga,id=video0,bus=pci.0,addr=0x2 \
   -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x5 -msg timestamp=on

7.5 Change Kernel Parameters at Boot Time

7.5.1 SUSE Linux Enterprise 11

To change the value for SLE 11 products at boot time, you need to modify your /boot/grub/menu.lst file by adding the OPTION=parameter. Then reboot your system.

7.5.2 SUSE Linux Enterprise 12 and 15

To change the value for SLE 12 and 15 products at boot time, you need to modify your /etc/default/grub file. Find the variable starting with GRUB_CMDLINE_LINUX_DEFAULT and add at the end OPTION=parameter (or change it with the correct value if it is already available).

Now you need to regenerate your grub2 configuration:

# grub2-mkconfig -o /boot/grub2/grub.cfg

Then reboot your system.

7.6 Add a Device to an XML Configuration

To create a new VM Guest based on an XML file, you can specify the QEMU command line using the special tag qemu:commandline. For example, to add a virtio-balloon-pci, add this block at the end of the XML configuration file (before the </domain> tag):

    <qemu:arg value='-device'/>
    <qemu:arg value='virtio-balloon-pci,id=balloon0'/>

7.7 Adding and removing CPUs

Some virtualization environments allow adding or removing CPUs while the virtual machine is running.

For the safe removal of CPUs, deactivate them first by executing

# echo 0 > /sys/devices/system/cpu/cpuX/online

Replace X with the CPU number. To bring a CPU back online, execute

# echo 1 > /sys/devices/system/cpu/cpuX/online

8 Xen: Converting a Paravirtual (PV) Guest to a Fully Virtual (FV/HVM) Guest

This chapter explains how to convert a Xen paravirtual machine into a Xen fully virtualized machine.

Procedure 11: Guest Side

In order to start the guest in FV mode, you have to run the following steps inside the guest.

  1. Prior to converting the guest, apply all pending patches and reboot the guest.

  2. FV machines use the -default kernel. If this kernel is not already installed, install the kernel-default package (while running in PV mode).

  3. PV machines typically use disk names such as vda*. These names must be changed to the FV hd* syntax. This change must be done in the following files:

    • /etc/fstab

    • /boot/grub/menu.lst (SLES 11 only)

    • /boot/grub*/device.map

    • /etc/sysconfig/bootloader

    • /etc/default/grub (SLES 12 and 15 only)

    Note: Prefer UUIDs

    You should use UUIDs or logical volumes within your /etc/fstab. Using UUID simplifies using attached network storage, multipathing, and virtualization. To find the UUID of your disk use the command blkid.

  4. To avoid any error regenerating the initrd with the required modules you can create a symlink from /dev/hda2 to /dev/xvda2 etc. by using the ln:

    ln -sf /dev/xvda2 /dev/hda2
    ln -sf /dev/xvda1 /dev/hda1
  5. PV and FV machines use different disk and network driver modules. These FV modules must be added to the initrd manually. The expected modules are xen-vbd (for disk) and xen-vnif (for network). These are the only PV drivers for a fully virtualized VM Guest. All other modules, such as ata_piix, ata_generic and libata, should be added automatically.

    Tip: Adding Modules to the initrd
    • On SLES 11, you can add modules to the INITRD_MODULES line in the /etc/sysconfig/kernel file. For example:

      INITRD_MODULES="xen-vbd xen-vnif"

      Run mkinitrd to build a new initrd containing the modules.

    • On SLES 12 and 15, open or create /etc/dracut.conf.d/10-virt.conf and add the modules with force_drivers by adding a line as in the example below (mind the leading whitespace):

      force_drivers+=" xen-vbd xen-vnif"

      Run dracut -f --kver KERNEL_VERSION-default to build a new initrd (for the default version of the kernel) that contains the required modules.

      Note: Find Your Kernel Version

      Use the uname -r command to get the current version used on your system.

  6. Before shutting down the guest, set the default boot parameter to the -default kernel using yast bootloader.

  7. Under SUSE Linux Enterprise Server 11, if you have an X server running on your guest, you need to adjust the /etc/X11/xorg.conf file in order to adjust the X driver. Search for fbdev and change to cirrus.

    Section "Device"
              Driver       "cirrus"
    Note: SUSE Linux Enterprise Server 12/15 and Xorg

    Under SUSE Linux Enterprise Server 12/15, Xorg will automatically adjust the driver needed to be able to get a working X server.

  8. Shut down the guest.

Procedure 12: Host Side

The following steps explain the action you have to perform on the host.

  1. To start the guest in FV mode, the configuration of the VM must be modified to match an FV configuration. Editing the configuration of the VM can easily be done using virsh edit [DOMAIN]. The following changes are recommended:

    • Make sure the machine, the type and the loader entries in the OS section are changed from xenpv to xenfv. The updated OS section should look similar to:

                <type arch='x86_64' machine='xenfv'>hvm</type>
                <boot dev='hd'/>
    • In the OS section remove anything that is specific to PV guest:

      • <bootloader>pygrub</bootloader>
      • <kernel>/usr/lib/grub2/x86_64-xen/grub.xen</kernel>
      • <cmdline>xen-fbfront.video=4,1024,768</cmdline>
    • In the devices section, add the qemu emulator as:

    • Update the disk configuration so the target device and bus use the FV syntax. This requires replacing the xen disk bus with ide, and the vda target device with hda. The changes should look similar to:

      <target dev='hda' bus='ide'/>
    • Change the bus for the mouse and keyboard from xen to ps2. Also add a new USB tablet device:

      <input type='mouse' bus='ps2'/>
                <input type='keyboard' bus='ps2'/>
      <input type='tablet' bus='usb'/>
    • Change the console target type from xen to serial:

      <console type='pty'>
                <target type='serial' port='0'/>
    • Change the video configuration from xen to cirrus, with 8 MB of VRAM:

                <model type='cirrus' vram='8192' heads='1' primary='yes'/>
    • If desired, add acpi and apic to the features of the VM:

  2. Start the guest (using virsh or virt-manager). If the guest is running kernel-default (as verified through uname -a), the machine is running in Fully Virtual mode.

Note: guestfs-tools

To script this process, or work on disk images directly, you can use the guestfs-tools suite (see Section 18.3, “Guestfs Tools” for more information). Numerous tools exist there to help modify disk images.

9 External References