I am going to show how to create a LVM (Logical Volume Manager) snapshot and use it to return its base to a previous state.
This is useful for experiments, e.g on a LVM-based virtual machine (VM):
- take a partition snapshot of the FS where the VM lives.
- experiment with the VM (make config changes etc.)
- if your tests are successful, lvremove the snapshot and keep the experiment's results
- if you want to discard the tests, stop the VM then lvconvert –merge -v <snapshot> will restore the partition to its snapshot state.
Check required capabilities
# dmsetup targets
zero v1.1.0
mirror v1.13.2
snapshot-merge v1.3.0
snapshot-origin v1.9.0
snapshot v1.13.0
striped v1.5.1
linear v1.2.1
error v1.3.0
For the rest of the tutorial to work, we need the snapshot-merge target to exist.
If (as it happened to me) you run this on a machine with no kernel support for merging, you will be stumped in the last passage (lvcreate –merge). You can get out of this pickle by either:
- Grabbing a Live DVD of a distribution that has merge support
- run the last step from the live environment
- Boot rescue from a distribution that has merge support
- Do not mount the LV partitions involved when/if asked
- run the last step from the rescue environment
Create volume, make a filesystem, mount it.
# lvcreate --size 100m -n test SataGroup01
# mkfs.ext2 /dev/SataGroup01/test
# mkdir /tmp/test
# mount /dev/SataGroup01/test /tmp/test
# cd /tmp/test
# ls
# mkdir a{0..2}
# ls
a0 a1 a2 lost+found
# for i in {0..2}; do touch a$i/b$i ; done
# ls a0
b0
Create a snapshot on the base volume, then mount it.
When the snapshot is created, care must be taken to make a large enough snapshot to avoid filling it. If the snapshot fills up, you will end up with a stuck snapshot and you'll have to go to hell and back to get rid of it (I may show how in a followup post).
# lvcreate --snapshot --size 100m -n test-snap /dev/SataGroup01/test
# mkdir /tmp/test-snap
# mount /dev/SataGroup01/test-snap /tmp/test-snap
The contents of the volumes are identical:
# ls /tmp/test
a0 a1 a2 lost+found
[root@cane test]# ls /tmp/test-snap/
a0 a1 a2 lost+found
Change them in different ways:
# mkdir /tmp/test/b0
# rm -rf /tmp/test/a1
# mkdir /tmp/test-snap/c0
# ls /tmp/test*
/tmp/test:
a0 a2 b0 lost+found
/tmp/test-snap:
a0 a1 a2 c0 lost+found
Now unmount them:
# umount /tmp/test
# umount /tmp/test-test snap
End the experiment
If we now throw away test-snap, we'd be left with the changes done in test.
If we want to back up our changes, and keep what was done in snap instead, do:
We have kept the changes originally in the snapshot volume (which as been removed by lvconvert); a1 has come back, b0 is gone, c0 - from test-snap - is now in place:
# lvconvert --merge -v /dev/SataGroup01/test-snap
Using logical volume(s) on command line.
Archiving volume group "SataGroup01" metadata (seqno 41).
Loading SataGroup01-test-real table (253:14)
Suppressed SataGroup01-test-real (253:14) identical table reload.
Loading SataGroup01-test--snap-cow table (253:15)
Suppressed SataGroup01-test--snap-cow (253:15) identical table reload.
Loading SataGroup01-test table (253:16)
Loading SataGroup01-test--snap table (253:17)
Not monitoring SataGroup01/snapshot0
Suspending SataGroup01-test (253:16) with filesystem sync with device flush
Suspending SataGroup01-test--snap (253:17) with filesystem sync with device flush
Suspending SataGroup01-test-real (253:14) with filesystem sync with device flush
Suspending SataGroup01-test--snap-cow (253:15) with filesystem sync with device flush
Loading SataGroup01-test-real table (253:14)
Suppressed SataGroup01-test-real (253:14) identical table reload.
Loading SataGroup01-test--snap-cow table (253:15)
Suppressed SataGroup01-test--snap-cow (253:15) identical table reload.
Resuming SataGroup01-test-real (253:14)
Resuming SataGroup01-test--snap-cow (253:15)
Resuming SataGroup01-test (253:16)
Resuming SataGroup01-test--snap (253:17)
Creating volume group backup "/etc/lvm/backup/SataGroup01" (seqno 42).
Merging of volume test-snap started.
Checking progress before waiting every 15 seconds
test: Merged: 100.0%
Merge of snapshot into logical volume test has finished.
Archiving volume group "SataGroup01" metadata (seqno 42).
Removing snapshot test-snap
Loading SataGroup01-test table (253:16)
Loading SataGroup01-test--snap table (253:17)
Suspending SataGroup01-test (253:16) with device flush
Suspending SataGroup01-test--snap (253:17) with device flush
Suspending SataGroup01-test--snap-cow (253:15) with device flush
Suspending SataGroup01-test-real (253:14) with device flush
Resuming SataGroup01-test--snap-cow (253:15)
Resuming SataGroup01-test-real (253:14)
Resuming SataGroup01-test (253:16)
Removing SataGroup01-test-real (253:14)
Removing SataGroup01-test--snap (253:17)
Removing SataGroup01-test--snap-cow (253:15)
Releasing logical volume "test-snap"
Creating volume group backup "/etc/lvm/backup/SataGroup01" (seqno 44).
Logical volume "test-snap" successfully removed
We have kept the changes originally in the snapshot volume (which as been removed by lvconvert); a1 has come back, b0 is gone, c0 - from test-snap - is now in place:
# ls /tmp/test/ /tmp/test/a1
/tmp/test/:
a0 a1 a2 c0 lost+found
/tmp/test/a1:
b1
