[linux-lvm] LVM2 snapshot weirdness

Nick Morrison nick at nick.on.net
Mon Dec 2 12:27:20 UTC 2013

On 15.11.2013, at 12:13, Nick Morrison <nick at nick.on.net> wrote:

> .. and md5sum gave me a third, different, result.
> It seems as if the data read from the snapshot is changing.

More tests. I created a snapshot using 

# lvcreate -pr -L 2G -s -n test-snapshot /dev/VolGroup00/LogVol00

Then I ran:

# md5sum /dev/VolGroup00/test-snapshot

... three times. test-snapshot is a read-only snapshot of the / volume, which is about 265G.

One one server, each time, I got a different md5sum. Generally a bad result. However:

on the other server, I got the same md5sum result on the first and third runs, but a different result on the second. Curious.

I'm getting no interesting messages in dmesg during the reads.

Is it likely that we have a fault at a lower level than LVM's snapshot code here?

[root at server-01:/backup/test]# cat nohup.out
Checking md5sum of /dev/VolGroup00/test-dump multiple times.

Pass 1 started Fri Nov 29 11:46:56 CET 2013
103fa0cc0351836598866423fabcfb6b  /dev/VolGroup00/test-dump

real    34m31.006s
user    15m15.252s
sys     6m48.961s

Pass 2 started Fri Nov 29 12:21:28 CET 2013
2d37fce8eea493eb4600f1593b9da825  /dev/VolGroup00/test-dump

real    34m24.818s
user    15m11.098s
sys     6m28.567s

Pass 3 started Fri Nov 29 12:55:52 CET 2013
4f6940a672115b896cea5220ba8bde7c  /dev/VolGroup00/test-dump

real    35m13.079s
user    15m15.016s
sys     6m19.312s

Complete at Fri Nov 29 13:31:05 CET 2013
[root at server-01:/backup/test]# 

Other server:

[root at server-02:/backup/test]# cat nohup.out
Checking md5sum of /dev/VolGroup00/test-dump multiple times.

Pass 1 started Fri Nov 29 11:47:04 CET 2013
de7f4847be8dd3019ee2a690f48c6397  /dev/VolGroup00/test-dump

real    29m34.334s
user    14m49.219s
sys     6m33.685s

Pass 2 started Fri Nov 29 12:16:38 CET 2013
363ee32c1bb5da6eb9754b0d6de79180  /dev/VolGroup00/test-dump

real    29m24.910s
user    14m40.705s
sys     6m44.806s

Pass 3 started Fri Nov 29 12:46:03 CET 2013
de7f4847be8dd3019ee2a690f48c6397  /dev/VolGroup00/test-dump

real    29m20.927s
user    14m40.441s
sys     6m46.865s

Complete at Fri Nov 29 13:15:24 CET 2013
[root at server-02:/backup/test]#

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