Create root filesystem snapshots with LVM

From ArchWiki
Revision as of 17:44, 27 April 2013 by Freejack (talk | contribs) (first version -- moved from user namespace)
Jump to: navigation, search

Template:Article summary start Template:Article summary text Template:Article summary heading Template:Article summary text Template:Article summary heading Template:Article summary wiki Template:Article summary wiki Template:Article summary end


You need a system with LVM root filesystem and systemd. Ensure that LVM snapshots prerequisites are correctly setup.


During system start a clean snapshot of the root volume is created using a new systemd service. Create /etc/systemd/system/mk-lvm-snapshots.service containing:

Description=make LVM snapshots


ExecStart=/usr/sbin/lvcreate -L10G -n snap-root -s lvmvolume/root

Adapt the lvcreate command to match your root volume group and volume name. Adjust the snapshot size if necessary. If additional filesystems should be snapshotted during startup you may extend the ExecStart property with addtional lvcreate commands, separated with  ; .

Note: You should test the # lvcreate command in the running system until it works as desired.

Remove the test snapshots with # lvremove. The snapshots taken from a running system are

not as consistent as snapshots taken in single user mode or during startup.

Create a new systemd target /etc/systemd/system/

Description=Make Snapshots

Adapt the base target, if is not your default target.

Enable the new service with # systemctl enable mk-lvm-snapshots.service.

If the system is started with the new target, LVM snapshot(s) are created just after mounting the local filesystems. To get a grub menu entry starting this target create /boot/grub/custom.cfg based on the grub.cfg entry for your normal startup. The kernel command line is extended to start the new

### make snapshots ###
menuentry 'Arch GNU/Linux, make snapshots' --class arch --class gnu-linux --class gnu --class os {
        echo    'Loading Linux core repo kernel ...'
        linux   /boot/vmlinuz-linux root=/dev/mapper/lvmvolume-root ro
        echo    'Loading initial ramdisk ...'
        initrd  /boot/initramfs-linux.img

Remember to adjust custom.cfg if grub.cfg changes.

After restarting the system with this grub entry # lvs should show up the newly created snapshot.

Tip: To get the messages of the new service use # journalctl -u mk-lvm-snapshots.service.



To use this functionality for a full system backup, restart your system with the snapshot creation target. Mount the snapshot volume (and further volumes, if required), preferably using the read only (-o) option. Then backup your system, for example with tar as described in Full_System_Backup_with_tar.

During backup you can continue to use your system normally, since all changes to your regular volumes are invisible in the snapshots. Do not forget to delete the snapshot volume after the backup – changes to your regular volume will use up space in the snapshot due to the copy-on-write operations. If the snapshot space becomes fully used, and LVM is not able to automatically grow the snapshot, LVM will deny further writes to your regular volumes or drop the snapshot, which should be avoided.

Revert updates

An other use for LVM snapshots is testing and reverting of updates. In this case create a snapshot for the system in a known good state and perform updates or changes afterwards.

If you want to permantly stick to the updates just drop the snapshot with # lvremove. If you want to revert to the snapshotted state issue a # lvchange --merge for the snapshot. During the next restart of the system the snapshot is merged back into your regular volume. All changes to the volume happened after the snapshot are undone.

Note: After merging the snapshot no longer exists. Recreate a new snapshot if further testing with rollback option is desired.

Known issues

Due to bug 681582 shutting down the system with active snapshots may hang for some time (currently 1...3 minutes).