Archive:Online backup of virtual XEN machines
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THE CONTENT OF THIS PAGE IS OUTDATED |
| SEP AG has discontinued support for obsolete SEP sesam versions. Instructions are still available for these SEP sesam products, however, SEP AG accepts no responsibility or liability for any errors or inaccuracies in the instructions or for the incorrect operation of obsolete SEP sesam software. It is strongly recommended that you update your SEP sesam software to the latest version. For the latest version of SEP sesam documentation, see documentation home. |
Introduction
Currently XEN does not have a backup API. That means, that Xen virtual machines can not be backed up online with using the existing XEN tools. An effective way to perform a useful backup is to use LVM (Logical Volume Manager) as an intermediate layer between physical disk and the file system. Using the LVM presents the opportunity to back up a virtual XEN machine with an LVM snapshot. This LVM snapshot can be mounted into the file system and then written to disk or tape with SEP sesam as a path-backup.
Note, you should also backup the VM configuration files in the directory /etc/xen. On Linux systems it makes generally sense to backup the complete /etc directory.
| Attention |
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Please note that neither Xen nor LVM are part of SEP sesam software. Further information can be found in the literary references at the end of the document or generally on the Internet. |
| Better Solution in Sesam version 4.2: |
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From SEP sesam version 4.2 on, there is a possibility to define a LVM snapshot based backup. For that, the checkbox Backup with LVM has to be activated in the backup task properties. If a backup source points onto a point in the filesystem which is inside a LVM volume, SEP sesam creates automatically a LVM snapshot, backup this snapshot source and release the snapshot after backup. |
System Requirements
- A working XEN environment on LVM (Logical Volume Manager)
- An installed SEP sesam client on the XEN host system
- The XEN host systems must be added as a client to the SEP sesam server
Basic approach
Backup
- Create an LVM snapshot of the logical volume where the VM is stored
- Mount this snapshot into the filesystem
- Perform a pathbackup with SEP sesam from the mountpoint of the LVM snapshot
- Dismount the snapshot from the filesystem after backup
- Delete the snapshot
Recovery
- Shut down the XEN VM to be recovered
- Restore the savesets of the VM backup to the original destination of the VM with option overwrite
- Start the VM
Procedure
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This scenario presents one example. The dimensioning of the size of the Logical Volumes (LVs)depends on the virtual machine and the applications that run on this system environment. |
In this example the virtual machine VM1 on the XEN host astronomix is stored on the logical volume (LV) /dev/system/xenvm. This partition is mounted to /srv/xenvm. The files of the VM vm1 are stored in the directory /srv/xenvm/vm1.
Backup of the XEN VM
Create an LVM snapshot
A snapshot volume must have at a minimum the size to handle all the changes performed by the virtual machine during the time of the backup. In this case the snapshot is created with the size of 1 GB.
# lvcreate -L1G -s -n vmbackup /dev/system/xenvm
Logical volume "vmbackup" created
The snapshot is introduced via the device file /dev/system/vmbackup to the XEN hostsystem.
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Mount snapshot
Now the snapshot is mounted into the filesystem to /mnt/vmbackup:
# mount /dev/system/vmbackup /mnt/vmbackup
Set up and execute a backup with sesam
- Define a backup task "Backup_XEN_vm1" with:
- the backup source /mnt/vmbackup and
- backup type Pathbackup
- Following this the backup can be tested with "immediate start". For a time controlled job a schedule can be specified (for example daily 20:00 o'clock).
Dismount and delete the snapshot
After the backup is finished, the snapshot can be dismounted with:
# umount /mnt/vmbackup
...and deleted with:
# lvremove -f /dev/system/vmbackup Logical volume "vmbackup" successfully removed
System Recovery of a VM
For the restore of a virtual machine its...
- ...necessary, that the existing VM is powered off, so that the data associated to this VM can be overwritten.
- A restore is started via the SEP sesam Restore Wizard.
- as restore target the original path of the virtual machine vm1is specified, in this case /srv/xenvm/vm1.
- Because all the data of the VM are stored in the same directory, the option original tree structure must be disabled. This allows the storing of all the data in the defined directory.
- If data still exists in the restore path, select the option overwrite existing files.
- After successful restore the VM can be started normally.
Extended sesam configuration using Command Line Pre- and Post- Options
For better automation the points "create snapshot" and "mount snapshot" are embedded in a pre-process before backup and the "unmount snapshot" and "delete snapshot" after backup in a post-process. For details, see Using Pre and Post Scripts
Set up the pre- and post-interface
The definition of the pre- and post-process are done via script on the sesam client which performs the backups. In this case it is the XEN host. Therefore the interface-templates sbc_pre and sbc_post from directory /opt/sesam/skel/templates have to be copied to /opt/sesam/bin/sesam and edited accordingly .
Here are the summaries of the data are shown as an example. The error correction is rudimentary and can be adapted as required.
sbc_pre
...
#=== Please insert your specific actions here=====================================
if [ $task == Backup_XEN_vm1 ]; then
echo "Create a LVM snapshot for backup of virtual machine vm1..."
lvcreate -L1G -s -n vmbackup /dev/system/xenvm
rc=$?
if [ $rc -eq 0 ]; then
echo "...successful"
echo
echo "Mount LVM snapshot for backup..."
mount /dev/system/vmbackup /mnt/vmbackup
rc=$?
if [ $rc -ne 0 ]; then # if error clean up
lvremove -f /dev/system/vmbackup
echo STATUS:ERROR {Error on mounting LVM snapshot}
exit
fi
echo "...successful"
else
echo STATUS:ERROR {Error on creating LVM snapshot}
exit
fi
fi
echo STATUS:OK
exit
sbc_post
...
#=== Please insert your specific actions here =====================================
if [ $task == Backup_XEN_vm1 ]; then
echo "Unmount LVM snapshot after backup..."
umount /mnt/vmbackup
rc=$?
if [ $rc -ne 0 ]; then
echo STATUS:ERROR {Error on unmounting LVM snapshot}
echo "Please unmount and remove LVM snapshot manually!"
exit
fi
echo "...successful"
echo
echo "Delete LVM snapshot for virtual machine vm1..."
lvremove -f /dev/system/vmbackup
rc=$?
if [ $rc -ne 0 ]; then # if error clean up
echo STATUS:ERROR {Error on deleting LVM snapshot}
echo "Please delete LVM snapshot manually"
exit
fi
echo "...successful"
fi
echo STATUS:OK
exit
Definition of the backup task using sesam
Create a backup task via the SEP sesam GUI. As backup source the mountpoint is specified on which the snapshot is mounted at this moment (/mnt/vmbackup).
On the tab Options 1 the check boxes for the pre- and post-processing before and after the backup have to be set. The options are so chosen, that the backup terminates if a failure occurs during the pre-process because of the possibility that either the snapshot is not created or was not mounted. The post-processing (post) in contrast, will also be performed if the backup itself returns an error. This ensures that after the backup the cleanup task works, i.e. dismount snapshot and delete snapshot, are properly executed.
Selective backup of the VM
The scenario above describes the backup of the entire virtual machine, that means the VM is backed up as a container. For a selective backup of parts of the virtual machine (from inside), the virtual machine is handled like a physical machine. This means that every VM is its own IP host and, therefore, a sesam client has to be installed on this machine and configured in the SEP sesam environment. Afterward a selective backup can be performed for that virtual machine.