Create root filesystem snapshots with LVM

Related articles
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
Option 1: Helper Scripts
An AUR package is available,
make-lvm-snapshot
AUR
, which can help automate the following process. It implements similar service and target files to those included in this article. The projects
README
contains important setup information.
Option 2: Manual Configuration
If you prefer a fully "manual" setup, you will need to write a systemd service to create a clean snapshot of the root volume during system startup.
Create:
/etc/systemd/system/mk-lvm-snapshots.service
[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(5) § COMMAND LINES
for details).
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/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.
Tip
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.
Note
After merging the snapshot no longer exists. Recreate a new snapshot if further testing with rollback
option is desired.