[linux-lvm] Unable to create snapshot due to failed merge of prior snapshot

N Cuka ncuka at hotmail.com
Tue Nov 25 15:20:55 UTC 2014

Hi Everyone,

I inadvertently created a situation where I deleted an LVM snapshot that was
pending a merge (yes - bonehead move). Now I can neither finish the merge
nor create a new snapshot on the original logical volume. When I try to
create a snapshot I get the following:


root at ubuntu:~# lvcreate -s -n snap -L 15g /dev/sysvg/syslv
   Snapshots of an origin that has a merging snapshot is not supported


and in dmesg I see the following logs:

Nov 21 20:32:47 ubuntu kernel: [  721.210733] device-mapper: snapshots:
Snapshot is marked invalid.
Nov 21 20:32:47 ubuntu kernel: [  721.211641] device-mapper: snapshots:
Snapshot is invalid: can't merge
Nov 21 20:32:47 ubuntu kernel: [  721.233283] Buffer I/O error on device
dm-4, logical block 16
Nov 21 20:32:47 ubuntu kernel: [  721.233289] Buffer I/O error on device
dm-4, logical block 16

This is not surprising as I did an lvremove of the snapshot (oops). If I do
an lvscan I get the following output:

root at ubuntu:~# lvscan
  ACTIVE            '/dev/uservg/userlv' [931.50 GiB] inherit
  inactive Original '/dev/sysvg/syslv' [152.77 GiB] inherit


And lvdisplay shows

root at ubuntu:/# lvdisplay /dev/sysvg/syslv 
  --- Logical volume ---
  LV Path                /dev/sysvg/syslv
  LV Name                syslv
  VG Name                sysvg
  LV UUID                stuff here
  LV Write Access        read/write
  LV Creation host, time hades, 2014-07-09 14:56:49 +0000
  LV snapshot status     source of
                         pre-x [INACTIVE]
  LV Status              available
  # open                 0
  LV Size                152.77 GiB
  Current LE             39108
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           252:3


So the snapshot status is INACTIVE and is trying to merge but I have no idea
how to make LVM try to stop attempting the merge. I can mount and use the
logical volume without apparent problem, but I really would like to be able
to take a snapshot (which is why I wanted to use LVM in the first place!)

I think that I need to us a lower level tool like dmsetup to accomplish this
but I am not sure how to use this tool and would prefer not to blow away
root if possible (not the end of the world, but would prefer not too). With
dmsetup I have the following info:

root at ubuntu:/# dmsetup status /dev/sysvg/syslv 
0 320372736 snapshot-merge Invalid


root at ubuntu:/# dmsetup info /dev/sysvg/syslv 
Name:              sysvg-syslv
State:             ACTIVE
Read Ahead:        256
Tables present:    LIVE
Open count:        0
Event number:      0
Major, minor:      252, 3
Number of targets: 1
UUID: stuff here

I have looked extensively on the intertubes and either my google kung-fu is
slipping or I have reached toxic levels of caffeine (or both) and can't find
much. Does anyone have any advice about how to restore LVM snapshoting for

root at hades:~$ uname -a
Linux hades 3.13.0-39-generic #66-Ubuntu SMP Tue Oct 28 13:30:27 UTC 2014
x86_64 x86_64 x86_64 GNU/Linux

LVM info:

root at ubuntu:/# lvm version
  LVM version:     2.02.98(2) (2012-10-15)
  Library version: 1.02.77 (2012-10-15)
  Driver version:  4.27.0



- chook


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