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

N Cuka

2014-11-25 15:20:55 UTC

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:

***@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:

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

And lvdisplay shows

***@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:

***@ubuntu:/# dmsetup status /dev/sysvg/syslv
0 320372736 snapshot-merge Invalid

And

***@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
me?
System:

***@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:

***@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

TIA

- chook