LVM によるルートファイルシステムのスナップショット作成

関連記事

• LVM
• Full system backup with tar

This article describes how to set up root filesystem LVM snapshot creation during system start. Such snapshots can be used for full system backups with minimal downtime or testing system updates with the option to revert them.

Prerequisites

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

Setup

Use a systemd service to create a clean snapshot of the root volume during system startup. Create /etc/systemd/system/mk-lvm-snapshots.service containing:

[Unit]
Description=make LVM snapshots
Requires=local-fs-pre.target
Wants=local-fs.target
DefaultDependencies=no
Conflicts=shutdown.target
After=local-fs-pre.target
Before=local-fs.target

[Install]
WantedBy=make-snapshots.target

[Service]
Type=oneshot
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  ; (consider there is a space before and after the semicolon, see systemd service manual for details).

ノート: 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/make-snapshots.target:

[Unit]
Description=Make Snapshots
Requires=multi-user.target

Adapt the base target, if multi-user.target is not your default target. If you want to get into your desktop, you need to change it to graphical.target.

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.target:

### 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/lvmvolume/root ro systemd.unit=make-snapshots.target
        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.

ヒント: To get the messages of the new service, run journalctl -u mk-lvm-snapshots.service as root.

Usage

Backup

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

Another 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 lvconvert --merge for the snapshot. During the next restart of the system (use the default target) the snapshot is merged back into your regular volume. All changes to the volume happened after the snapshot are undone.

ノート: 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). As a workaround a shorter job timeout can be set. Create a copy of /usr/lib/systemd/system/dmeventd.service in /etc/systemd/system and insert JobTimeoutSec=10:

[Unit]
Description=Device-mapper event daemon
Documentation=man:dmeventd(8)
Requires=dmeventd.socket
After=dmeventd.socket
DefaultDependencies=no
JobTimeoutSec=10

[Service]
Type=forking
ExecStart=/usr/sbin/dmeventd
ExecReload=/usr/sbin/dmeventd -R
Environment=SD_ACTIVATION=1
PIDFile=/run/dmeventd.pid
OOMScoreAdjust=-1000