18

I have made an LVM snapshop by

lvcreate --name snap --size 10G -s /dev/vg00/vm

What command should I write if I want to drop the snapshot, and not keep the changes that have happened since the snapshot?

And what command should I write to roll the changes from the snapshot into /dev/vg00/vm ?

Sandra
  • 10,303
  • 38
  • 112
  • 165

5 Answers5

17

to drop snapshot use:

lvremove group/snap-name

to merge snapshot use:

lvconvert --merge group/snap-name

Though merging will be deferred until the orgin and snapshot volumes are unmounted. You may need to update kernel (>=2.6.33) and lvm tools to have support for merging.

Thiago Figueiro
  • 830
  • 1
  • 6
  • 18
Hubert Kario
  • 6,361
  • 6
  • 36
  • 65
  • Does this mean that you can run `lvconvert --merge ...` then reboot in order to rewind the volume? – Hubro Mar 25 '20 at 14:05
  • @Hubro Exactly. See https://wiki.archlinux.org/title/LVM#Snapshots for more information – mxmehl Jan 07 '22 at 13:57
  • I've just lost data following this advice because LVM terminology is backwards IMO. Correct me if I'm wrong, by when LVM says "drop a snapshot", it means merging the changes from the COW volume into the origin volume, and "merge a snapshot" means reverting the origin volume to the state when the snapshot was made, effectively dropping the changes stored in the COW volume. – tgirod Jul 07 '23 at 07:08
3

Looks like this has been asked before:

Linux LVM snapshot commit or revert?

As well as:

http://www.jonnor.com/2010/02/lvm-snapshot-merging-avaliable/

Community
  • 1
rfelsburg
  • 767
  • 3
  • 7
3

OK, first of all: the grand old LVM1 supported only read-only snapshots. In that case the modifications were copied to original image after unmounting the snapshot. LVM2 provides support for read-write snapshots, allowing you to do funkier stuff with virtual machine images and stuff like that.

Novell has a very niece piece of documentation about the concept of using LVM for rolling back a failed OS update. Take a look at that article, it provides all the bits and pieces you need for your case.

wazoox
  • 6,918
  • 4
  • 31
  • 63
Janne Pikkarainen
  • 31,852
  • 4
  • 58
  • 81
2

you can use lvremove command to remove the old snapshot files:

lvremove /dev/vg_brcvhost1/snap this will ask you to remove a sanpshot persent in the your LV.

other commands are also there for managing a LVM based VM setup.

you can find the details commands in the link below: http://linux.math.tifr.res.in/HOWTO/LVM-HOWTO/snapshots_backup.html

and the below link may also be usefull: http://dentarg.it64.com/content/lvm-based-backup-virtual-machines

Dr. Death
  • 45
  • 11
1

I've just lost data following the selected answer to this question, so I'm posting this to clarify. Please correct me if I'm wrong.

IMO, LVM terminology is backwards. For reference redhat's documentation to LVM snapshot

When LVM says "drop a snapshot", it means merging the changes from the COW volume into the origin volume.

When LVM says "merge a snapshot", it means reverting the origin volume to the state when the snapshot was made, effectively dropping the changes stored in the COW volume.

I find this VERY counterintuitive, lost 43h of data because of that. Don't do the same mistake I did !

EDIT: OK now I understand my mistake. From the documentation:

When you modify the original volume (the origin) after you take a snapshot, the snapshot feature makes a copy of the modified data area as it was prior to the change so that it can reconstruct the state of the volume. When you create a snapshot, full read and write access to the origin stays possible.

I wrongly assumed that LVM operated by freezing the origin partition and mounting the snapshot as an overlay to store the modifications, but this sentence makes it clear it does not work that way.

tgirod
  • 111
  • 3