5 LVM configuration #

LVM enables flexible distribution of hard disk space over several physical volumes (hard disks, partitions, LUNs). It was developed because the need to change the segmentation of hard disk space might arise only after the initial partitioning has already been done during installation. Because it is difficult to modify partitions on a running system, LVM provides a virtual pool (volume group or VG) of storage space from which logical volumes (LVs) can be created as needed. The operating system accesses these LVs instead of the physical partitions. Volume groups can span more than one disk, so that several disks or parts of them can constitute one single VG. In this way, LVM provides a kind of abstraction from the physical disk space that allows its segmentation to be changed in a much easier and safer way than through physical repartitioning.

Figure 5.1, “Physical partitioning versus LVM” compares physical partitioning (left) with LVM segmentation (right). On the left side, one single disk has been divided into three physical partitions (PART), each with a mount point (MP) assigned so that the operating system can access them. On the right side, two disks have been divided into two and three physical partitions each. Two LVM volume groups (VG 1 and VG 2) have been defined. VG 1 contains two partitions from DISK 1 and one from DISK 2. VG 2 contains the remaining two partitions from DISK 2.

Figure 5.1: Physical partitioning versus LVM #

  

In LVM, the physical disk partitions that are incorporated in a volume group are called physical volumes (PVs). Within the volume groups in Figure 5.1, “Physical partitioning versus LVM”, four logical volumes (LV 1 through LV 4) have been defined, which can be used by the operating system via the associated mount points (MP). The border between different logical volumes need not be aligned with any partition border. See the border between LV 1 and LV 2 in this example.

LVM features:

Note: LVM and RAID

Even though LVM also supports RAID levels 0, 1, 4, 5 and 6, we recommend using mdraid (see Chapter 7, Software RAID configuration). However, LVM works fine with RAID 0 and 1, as RAID 0 is similar to common logical volume management (individual logical blocks are mapped onto blocks on the physical devices). LVM used on top of RAID 1 can keep track of mirror synchronization and is fully able to manage the synchronization process. With higher RAID levels you need a management daemon that monitors the states of attached disks and can inform administrators if there is a problem in the disk array. LVM includes such a daemon, but in exceptional situations such as a device failure, the daemon does not work properly.

Warning: IBM Z: LVM root file system

If you configure the system with a root file system on LVM or software RAID array, you must place /boot on a separate, non-LVM or non-RAID partition, otherwise the system will fail to boot. The recommended size for such a partition is 500 MB and the recommended file system is Ext4.

With these features, using LVM already makes sense for heavily-used home PCs or small servers. If you have a growing data stock, as in the case of databases, music archives, or user directories, LVM is especially useful. It allows file systems that are larger than the physical hard disk. However, keep in mind that working with LVM is different from working with conventional partitions.

You can manage new or existing LVM storage objects by using the YaST Partitioner. Instructions and further information about configuring LVM are available in the official LVM HOWTO.

An LVM volume group (VG) organizes the Linux LVM partitions into a logical pool of space. You can carve out logical volumes from the available space in the group. The Linux LVM partitions in a group can be on the same or different disks. You can add partitions or entire disks to expand the size of the group.

To use an entire disk, it must not contain any partitions. When using partitions, they must not be mounted. YaST will automatically change their partition type to 0x8E Linux LVM when adding them to a VG.

A logical volume provides a pool of space similar to what a hard disk does. To make this space usable, you need to define logical volumes. A logical volume is similar to a regular partition—you can format and mount it.

Use the YaST Partitioner to create logical volumes from an existing volume group. Assign at least one logical volume to each volume group. You can create new logical volumes as needed until all free space in the volume group has been exhausted. An LVM logical volume can optionally be thinly provisioned, allowing you to create logical volumes with sizes that overbook the available free space (see Section 5.3.1, “Thinly provisioned logical volumes” for more information).

5.3.1 Thinly provisioned logical volumes #

  

An LVM logical volume can optionally be thinly provisioned. Thin provisioning allows you to create logical volumes with sizes that overbook the available free space. You create a thin pool that contains unused space reserved for use with an arbitrary number of thin volumes. A thin volume is created as a sparse volume and space is allocated from a thin pool as needed. The thin pool can be expanded dynamically when needed for cost-effective allocation of storage space. Thinly provisioned volumes also support snapshots which can be managed with Snapper—see Chapter 10, System recovery and snapshot management with Snapper for more information.

To set up a thinly provisioned logical volume, proceed as described in Procedure 5.1, “Setting up a logical volume”. When it comes to choosing the volume type, do not choose Normal Volume, but rather Thin Volume or Thin Pool.

Thin pool

The logical volume is a pool of space that is reserved for use with thin volumes. The thin volumes can allocate their needed space from it on demand.

Thin volume

The volume is created as a sparse volume. The volume allocates needed space on demand from a thin pool.

Important: Thinly provisioned volumes in a cluster

To use thinly provisioned volumes in a cluster, the thin pool and the thin volumes that use it must be managed in a single cluster resource. This allows the thin volumes and thin pool to always be mounted exclusively on the same node.

> sudo lvcreate -L 500G -m 2 -n lv1 vg1

> sudo lvcreate --type raid1 -m 1 -L 1G -n lv1 vg1

Activation behavior for non-root LVM volume groups is controlled in the /etc/lvm/lvm.conf file and by the auto_activation_volume_list parameter. By default, the parameter is empty and all volumes are activated. To activate only some volume groups, add the names in quotes and separate them with commas, for example:

auto_activation_volume_list = [ "vg1", "vg2/lvol1", "@tag1", "@*" ]

If you have defined a list in the auto_activation_volume_list parameter, the following will happen:

By default, non-root LVM volume groups are automatically activated on system restart by dracut. This parameter allows you to activate all volume groups on system restart, or to activate only specified non-root LVM volume groups.

The space provided by a volume group can be expanded at any time in the running system without service interruption by adding physical volumes. This will allow you to add logical volumes to the group or to expand the size of existing volumes as described in Section 5.6, “Resizing a logical volume”.

It is also possible to reduce the size of the volume group by removing physical volumes. YaST only allows to remove physical volumes that are currently unused. To find out which physical volumes are currently in use, run the following command. The partitions (physical volumes) listed in the PE Ranges column are the ones in use:

> sudo pvs -o vg_name,lv_name,pv_name,seg_pe_ranges
root's password:
  VG   LV    PV         PE Ranges
             /dev/sda1
  DATA DEVEL /dev/sda5  /dev/sda5:0-3839
  DATA       /dev/sda5
  DATA LOCAL /dev/sda6  /dev/sda6:0-2559
  DATA       /dev/sda7
  DATA       /dev/sdb1
  DATA       /dev/sdc1

In case there is unused free space available in the volume group, you can enlarge a logical volume to provide more usable space. You may also reduce the size of a volume to free space in the volume group that can be used by other logical volumes.

Note: “Online” resizing

When reducing the size of a volume, YaST automatically resizes its file system, too. Whether a volume that is currently mounted can be resized “online” (that is while being mounted), depends on its file system. Growing the file system online is supported by Btrfs, XFS, Ext3, and Ext4.

Shrinking the file system online is only supported by Btrfs. To shrink the Ext2/3/4 file systems, you need to unmount them. Shrinking volumes formatted with XFS is not possible, since XFS does not support file system shrinking.

If there is an error on the LVM storage, the scanning of LVM volumes may prevent the emergency/rescue shell from being entered. This makes further problem diagnosis impossible. To disable this scanning in case of an LVM storage failure, you can pass the nolvm option on the kernel command line.

For information about using LVM commands, see the man pages for the commands described in the following table. All commands need to be executed with root privileges. Either use sudo COMMAND (recommended), or execute them directly as root.

Tip: Bypassing udev on volume creation

In case you want to manage LV device nodes and symbolic links by using LVM instead of by using udev rules, you can achieve this by disabling notifications from udev with one of the following methods:

• Configure activation/udev_rules = 0 and activation/udev_sync = 0 in /etc/lvm/lvm.conf.

  Note that specifying --nodevsync with the lvcreate command has the same effect as activation/udev_sync = 0; setting activation/udev_rules = 0 is still required.

• Setting the environment variable DM_DISABLE_UDEV:

  export DM_DISABLE_UDEV=1

  This will also disable notifications from udev. In addition, all udev related settings from /etc/lvm/lvm.conf will be ignored.

5.9.1 Resizing a logical volume with commands #

  

The lvresize, lvextend, and lvreduce commands are used to resize logical volumes. See the man pages for each of these commands for syntax and options information. To extend an LV there must be enough unallocated space available on the VG.

The recommended way to grow or shrink a logical volume is to use the YaST Partitioner. When using YaST, the size of the file system in the volume will automatically be adjusted, too.

LVs can be extended or shrunk manually while they are being used, but this may not be true for a file system on them. Extending or shrinking the LV does not automatically modify the size of file systems in the volume. You must use a different command to grow the file system afterward. For information about resizing file systems, see Chapter 2, Resizing file systems.

Ensure that you use the right sequence when manually resizing an LV:

• If you extend an LV, you must extend the LV before you attempt to grow the file system.

• If you shrink an LV, you must shrink the file system before you attempt to shrink the LV.

To extend the size of a logical volume:

1. Open a terminal.

2. If the logical volume contains an Ext2 or Ext4 file system, which do not support online growing, dismount it. In case it contains file systems that are hosted for a virtual machine (such as a Xen VM), shut down the VM first.

3. At the terminal prompt, enter the following command to grow the size of the logical volume:

   > sudo lvextend -L +SIZE /dev/VG_NAME/LV_NAME

   For SIZE, specify the amount of space you want to add to the logical volume, such as 10 GB. Replace /dev/VG_NAME/LV_NAME with the Linux path to the logical volume, such as /dev/LOCAL/DATA. For example:

   > sudo lvextend -L +10GB /dev/vg1/v1

4. Adjust the size of the file system. See Chapter 2, Resizing file systems for details.

5. In case you have dismounted the file system, mount it again.

For example, to extend an LV with a (mounted and active) Btrfs on it by 10 GB:

> sudo lvextend −L +10G /dev/LOCAL/DATA
> sudo btrfs filesystem resize +10G /dev/LOCAL/DATA

To shrink the size of a logical volume:

1. Open a terminal.

2. If the logical volume does not contain a Btrfs file system, dismount it. In case it contains file systems that are hosted for a virtual machine (such as a Xen VM), shut down the VM first. Note that volumes with the XFS file system cannot be reduced in size.

3. Adjust the size of the file system. See Chapter 2, Resizing file systems for details.

4. At the terminal prompt, enter the following command to shrink the size of the logical volume to the size of the file system:

   > sudo lvreduce /dev/VG_NAME/LV_NAME

5. In case you have unmounted the file system, mount it again.

For example, to shrink an LV with a Btrfs on it by 5 GB:

> sudo btrfs filesystem resize -size 5G /dev/LOCAL/DATA
sudo lvreduce /dev/LOCAL/DATA

Tip: Resizing the Volume and the File System with a Single Command

Starting with SUSE Linux Enterprise Server 12 SP1, lvextend, lvresize, and lvreduce support the option --resizefs which will not only change the size of the volume, but will also resize the file system. Therefore the examples for lvextend and lvreduce shown above can alternatively be run as follows:

> sudo lvextend --resizefs −L +10G /dev/LOCAL/DATA
> sudo lvreduce  --resizefs -L -5G /dev/LOCAL/DATA

Note that the --resizefs is supported for the following file systems: ext2/3/4, Btrfs, XFS. Resizing Btrfs with this option is currently only available on SUSE Linux Enterprise Server, since it is not yet accepted upstream.

A tag is an unordered keyword or term assigned to the metadata of a storage object. Tagging allows you to classify collections of LVM storage objects in ways that you find useful by attaching an unordered list of tags to their metadata.

tags {
   # Enable hostname tags
   hosttags = 1
}

tags {

   tag1 { }
      # Tag does not require a match to be set.

   tag2 {
      # If no exact match, tag is not set.
      host_list = [ "hostname1", "hostname2" ]
   }
}

  activation {
      volume_list = [ "vg1/lvol0", "@database" ]
  }