Difference between revisions of "Create root filesystem snapshots with LVM"
m (first version -- moved from user namespace) |
(→Revert updates: clarify start target for snp merge) |
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If you want to permantly stick to the updates just drop the snapshot with {{ic|# lvremove}}. If you want to revert to the snapshotted | If you want to permantly stick to the updates just drop the snapshot with {{ic|# lvremove}}. If you want to revert to the snapshotted | ||
− | state issue a {{ic|# lvchange --merge}} for the snapshot. During the next restart of the system the snapshot | + | state issue a {{ic|# lvchange --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. | 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 | {{Note|After merging the snapshot no longer exists. Recreate a new snapshot if further testing with rollback |
Revision as of 18:13, 27 April 2013
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Prerequisites
You need a system with LVM root filesystem and systemd. Ensure that LVM snapshots prerequisites are correctly setup.
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:
[Unit] Description=make LVM snapshots Requires=local-fs-pre.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 ;
.
# 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
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.
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.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/mapper/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.
# journalctl -u mk-lvm-snapshots.service
.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
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 (use the default target) the snapshot
is merged back into your regular volume. All changes to the volume happened after the snapshot are undone.
Known issues
Due to bug 681582 shutting down the system with active snapshots may hang for some time (currently 1...3 minutes).