Application Notes Usage and best practices for Device Mapper Multipath with Xen enabled virtual machines for HP StorageWorks Disk Arrays (5697-7828, April 2009)

Recovery from all path failure condition

When all paths to the LUN which is used for installing virtual machine fails, the root file system will

be mounted with read-only permission. Recovery process depends on the type of storage used for

installing Xen virtual machine.

Physical disks or logical volumes for installing Xen virtual machine

Rebooting the virtual machine will remount the root file system with read/write permissions.

Xen disk Image files for installing Xen virtual machine

On all path failure condition, the mount point will be marked as read-only. To recover form this,

complete the following steps:

1. Shutdown the virtual machine.

2. Select the virtual machine from the list in the Virtual Machine Manager and click Details.

3. In the Hardware tab, select the disk which is used for installing the virtual machine and click Re-

move.

4. Unmount the device from the mount point:

#umount /home/Xenimage.

5. Mount the device with read/write permission:

#mount /dev/mapper/mpath8 /home/Xenimage.

6. Re-add the disk image file to the virtual machine. To achieve this, complete the following steps

provided in storage provisioning for virtual machine.

7. The root file system will be mounted with read/write permissions when the virtual machine is

booted.

Limitations

• Multipathing for FC and SAS devices are not supported on virtual machines.

• Storage devices in virtual machines are handled by virtual block device backend interface, storage

devices do not exhibit array specific information and properties, like vendor information, SCSI

persistent reservation feature. Thus cluster solutions which require this information do not function

correctly.

References

• http://wiki.xensource.com/

• http://www.hp.com/go/devicemapper

• http://www.cl.cam.ac.uk/research/srg/netos/papers/2006-xen-fosdem.ppt