# Merging LVM snapshots - what went wrong? **URL:** \ **Category:** SLES Configure-Administer\ **Created:** [May 6, 2013, 10:34am UTC](https://forums.suse.com/t/merging-lvm-snapshots-what-went-wrong/23935 "2013-05-06T10:34:39Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![KEVIN1](https://avatars.discourse-cdn.com/v4/letter/k/a9adbd/32.png) [@KEVIN1](https://forums.suse.com/u/KEVIN1)\ **Post date:** [May 6, 2013, 10:34am UTC](https://forums.suse.com/t/merging-lvm-snapshots-what-went-wrong/23935/1 "2013-05-06T10:34:39Z")
I’ve been playing with LVM snapshots. Somehow, things never go quite as planned. I’m hoping someone can explain why. I have an LV /dev/vg100/v101. I created a snapshot. ```auto lvcreate -s -L 3G -n v101-snap /dev/vg100/v101 ``` The original LV and snapshot were mounted and I copied a file into the original LV v101. I saw it there but not in v101-snap, as expected. I "umount"ed the snapshot LV v101-snap. Now I wanted to revert the change to v101. The first time I tried, nothing seemed to happen. The second time I tried, I got this: ```auto server:~ # lvconvert --merge -v /dev/vg100/v101-snap Using logical volume(s) on command line Archiving volume group "vg100" metadata (seqno 41). Snapshot v101-snap is already merging Unable to merge LV "v101-snap" into its origin. ``` I couldn’t umount the original LV v101. IIRC, the volume was busy. After rebooting the server I thought I’d try my test again. The file I copied into v101 was no longer there but when I tried to create the snapshot this time, I was told it already existed. This is what I have learned: ```auto server:~ # cat /etc/*release SUSE Linux Enterprise Server 11 (x86_64) VERSION = 11 PATCHLEVEL = 2 LSB_VERSION="core-2.0-noarch:core-3.2-noarch:core-4.0-noarch:core-2.0-x8 6_64:core-3.2-x86_64:core-4.0-x86_64" server:~ # lvcreate -s -L 3G -n v101-snap /dev/vg100/v101 Logical volume "v101-snap" already exists in volume group "vg100" server:~ # ls /dev/vg100/v101* /dev/vg100/v101 server:~ # ls /dev/mapper/vg100-v101* /dev/mapper/vg100-v101 /dev/mapper/vg100-v101--snap /dev/mapper/vg100-v101--snap-cow /dev/mapper/vg100-v101-real server:~ # dmsetup info Name: vg100-v101 State: ACTIVE Read Ahead: 1024 Tables present: LIVE Open count: 1 Event number: 0 Major, minor: 253, 11 Number of targets: 1 UUID: LVM-..... Name: vg100-v101--snap-cow State: ACTIVE Read Ahead: 1024 Tables present: LIVE Open count: 1 Event number: 0 Major, minor: 253, 10 Number of targets: 1 UUID: LVM-..... Name: vg100-v101--snap State: ACTIVE Read Ahead: 1024 Tables present: LIVE Open count: 0 Event number: 0 Major, minor: 253, 12 Number of targets: 1 UUID: LVM-..... Name: vg100-v101-real State: ACTIVE Read Ahead: 1024 Tables present: LIVE Open count: 1 Event number: 0 Major, minor: 253, 9 Number of targets: 1 UUID: LVM-..... server:~ # lvdisplay /dev/vg100/v101 --- Logical volume --- LV Name /dev/vg100/v101 VG Name vg100 LV UUID ..... LV Write Access read/write LV snapshot status source of /dev/vg100/v101-snap [active] LV Status available # open 1 LV Size 70.00 GiB Current LE 17920 Segments 1 Allocation inherit Read ahead sectors auto - currently set to 1024 Block device 253:11 ``` Here are my questions: Why didn’t the “lvconvert --merge” not seem to work until after the system was rebooted? Would anyone care to speculate? My snapshot LV was v101-snap but I now see: vg100-v101–snap-cow, vg100-v101–snap, and vg100-v101-real What are these objects? What do I need to do to clean up these remnants and make v101-snap disappear? If necessary, I can open an SR… – Kevin Boyle - Knowledge Partner If you find this post helpful and are logged into the web interface, show your appreciation and click on the star below… ---
**Author:** ![Willem1](https://avatars.discourse-cdn.com/v4/letter/w/c57346/32.png) [@Willem1](https://forums.suse.com/u/Willem1)\ **Post date:** [May 7, 2013, 12:35am UTC](https://forums.suse.com/t/merging-lvm-snapshots-what-went-wrong/23935/2 "2013-05-07T00:35:20Z")
[QUOTE=KBOYLE;13472]… I couldn’t umount the original LV v101. IIRC, the volume was busy. After rebooting the server I thought I’d try my test again. The file I copied into v101 was no longer there but when I tried to create the snapshot this time, I was told it already existed…[/QUOTE] Allot of strangeness there 😕 I’ve used LV snapshots now a couple of times to revert test VM’s on Xen (where I have the VM’s OS disk phy connected to an LVM volume on the Xen host). I have not seen any real issues with this, other than the speed of writing back the snapshot info into the parent not always being as quick as hoped (sometimes just restoring a previous dd dump could be quicker depending on how much was written to the snapshot). One thing that is important, is to make sure both parent as also snapshot volume are not held open by any process (open count on 0). The merge action will be queued as long as both volumes are not in a closed state. Could that maybe be why it was not working for you? As this is a test scenario… I’m curious what will happen if you simply lvremove vg100-v101–snap-cow & vg100-v101-real and restart the ‘lvconvert --merge’? Also, what size is reported in use by those volumes? Cheers, Willem ---
**Author:** ![KEVIN1](https://avatars.discourse-cdn.com/v4/letter/k/a9adbd/32.png) [@KEVIN1](https://forums.suse.com/u/KEVIN1)\ **Post date:** [May 7, 2013, 2:02am UTC](https://forums.suse.com/t/merging-lvm-snapshots-what-went-wrong/23935/3 "2013-05-07T02:02:40Z")
Magic31 wrote: [color=blue] > KBOYLE;13472 Wrote:[color=green] > > > … > > I couldn’t umount the original LV v101. IIRC, the volume was busy. > > After rebooting the server I thought I’d try my test again. The > > file I copied into v101 was no longer there but when I tried to > > create the snapshot this time, I was told it already existed…[/color] > > Allot of strangeness there 😕 > > I’ve used LV snapshots now a couple of times to revert test VM’s on > Xen (where I have the VM’s OS disk phy connected to an LVM volume on > the Xen host). I have not seen any real issues with this, other than > the speed of writing back the snapshot info into the parent not > always being as quick as hoped (sometimes just restoring a previous > dd dump could be quicker depending on how much was written to the > snapshot).[/color] That was my plan. I thought I’d test it out on an unimportant LV in my Dom0 first. [color=blue] > One thing that is important, is to make sure both parent as also > snapshot volume are not held open by any process (open count on 0). > The merge action will be queued as long as both volumes are not in a > closed state. Could that maybe be why it was not working for you?[/color] I didn’t check that, not that I expected copying a small file would have a serious impact. [color=blue] > As this is a test scenario…[/color] …but not on a test server. ☹ [color=blue] > I’m curious what will happen if you > simply lvremove vg100-v101–snap-cow & vg100-v101-real and restart the > ‘lvconvert --merge’?[/color] So am I… but they aren’t LV’s and only appear under /dev/mapper/ [color=blue] > Also, what size is reported in use by those volumes?[/color] ```auto server:~ # lvscan --all ACTIVE '/dev/vg200/v200' [1.33 TiB] inherit ACTIVE '/dev/vg100/v102' [50.00 GiB] inherit ACTIVE '/dev/vg100/v103' [60.00 GiB] inherit ACTIVE '/dev/vg100/v104' [60.00 GiB] inherit ACTIVE '/dev/vg100/v105' [80.00 GiB] inherit ACTIVE '/dev/vg100/v107' [25.00 GiB] inherit ACTIVE '/dev/vg100/v108' [25.00 GiB] inherit ACTIVE '/dev/vg100/v109' [25.00 GiB] inherit ACTIVE '/dev/vg100/v105bak' [80.00 GiB] inherit ACTIVE Original '/dev/vg100/v101' [70.00 GiB] inherit ACTIVE Snapshot '/dev/vg100/v101-snap' [3.00 GiB] inherit server:~ # ls /dev/vg100/ v101 v102 v103 v104 v105 v105bak v107 v108 v109 server:~ # ``` Do snapshot LV’s usually show up under the VG? I don’t see it there. Some of this stuff may be part of the metadata. A lot of strangeness here… [color=blue] > Cheers, > Willem[/color] – Kevin Boyle - Knowledge Partner If you find this post helpful and are logged into the web interface, show your appreciation and click on the star below…