21 Block Devices in Xen #

The disk(s) specification for a Xen domain in the domain configuration file is as straightforward as the following example:

disk = [ 'format=raw,vdev=hdc,access=ro,devtype=cdrom,target=/root/image.iso' ]

It defines a disk block device based on the /root/image.iso disk image file. The disk will be seen as hdc by the guest, with read-only (ro) access. The type of the device is cdrom with raw format.

The following example defines an identical device, but using simplified positional syntax:

disk = [ '/root/image.iso,raw,hdc,ro,cdrom' ]

You can include more disk definitions in the same line, each one separated by a comma. If a parameter is not specified, then its default value is taken:

disk = [ '/root/image.iso,raw,hdc,ro,cdrom','/dev/vg/guest-volume,,hda','...' ]

For more information about specifying virtual disks, see /usr/share/doc/packages/xen/misc/xl-disk-configuration.txt.

Similar to mapping a local disk image (see Section 21.1, “Mapping Physical Storage to Virtual Disks”), you can map a network disk as a virtual disk as well.

The following example shows mapping of an RBD (RADOS Block Device) disk with multiple Ceph monitors and cephx authentication enabled:

disk = [ 'vdev=hdc, backendtype=qdisk, \
target=rbd:libvirt-pool/new-libvirt-image:\
id=libvirt:key=AQDsPWtW8JoXJBAAyLPQe7MhCC+JPkI3QuhaAw==:auth_supported=cephx;none:\
mon_host=137.65.135.205\\:6789;137.65.135.206\\:6789;137.65.135.207\\:6789' ]

Following is an example of an NBD (Network Block Device) disk mapping:

disk = [ 'vdev=hdc, backendtype=qdisk, target=nbd:151.155.144.82:5555' ]

When a virtual machine is running, each of its file-backed virtual disks consumes a loopback device on the host. By default, the host allows up to 64 loopback devices to be consumed.

To simultaneously run more file-backed virtual disks on a host, you can increase the number of available loopback devices by adding the following option to the host’s /etc/modprobe.conf.local file.

options loop max_loop=x

where x is the maximum number of loopback devices to create.

Changes take effect after the module is reloaded.

Tip

Enter rmmod loop and modprobe loop to unload and reload the module. In case rmmod does not work, unmount all existing loop devices or reboot the computer.

While it is always possible to add new block devices to a VM Guest system, it is sometimes more desirable to increase the size of an existing block device. In case such a system modification is already planned during deployment of the VM Guest, some basic considerations should be done:

When resizing an LVM device that is assigned to a VM Guest, the new size is automatically known to the VM Guest. No further action is needed to inform the VM Guest about the new size of the block device.

When using file system images, a loop device is used to attach the image file to the guest. For more information about resizing that image and refreshing the size information for the VM Guest, see Section 23.2, “Sparse Image Files and Disk Space”.

There are scripts that can help with managing advanced storage scenarios such as disk environments provided by dmmd (“device mapper—multi disk”) including LVM environments built upon a software RAID set, or a software RAID set built upon an LVM environment. These scripts are part of the xen-tools package. After installation, they can be found in /etc/xen/scripts:

The scripts allow for external commands to perform some action, or series of actions of the block devices prior to serving them up to a guest.

These scripts could formerly only be used with xl or libxl using the disk configuration syntax script=. They can now be used with libvirt by specifying the base name of the block script in the <source> element of the disk. For example:

<source dev='dmmd:md;/dev/md0;lvm;/dev/vgxen/lv-vm01'/>