[linux-lvm] very slow fw performance when snapshots active

[Falko Zurell <falko.zurell@idmedia.com>]

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Hi List,

I have an actual CentOS Linux with LVM on hardware RAID1+0.

fs layout looks like this:

[root@C20-DB1-00 ~]# lvs
  LV     VG   Attr   LSize   Origin Snap%  Move Log Copy%
  LVOpt  VG0  -wi-ao 300.00G
  LVRoot VG0  -wi-ao  43.94G
  LVSwap VG0  -wi-ao  23.44G

Then I create a snapshot vor VG0/LVOpt with 40 GB size.

Then I edit a 2 GB text file on /opt (XFS filesystem) with vi (just
hitting "enter" and save this minor change). This will take minutes to
finish the vi during saving.
I see the kcopyd in the process list which fills up the snapshot.

When the snapshot is off then saving the same file with the same minor
change will take only seconds.

I understand that snapshots are copy-on-write. But I can't explain why
this takes minutes for a 2 GB file which has changed only in a few bytes.

Shouldn't only the changed blocks be copied to the snapshot space? Even
if only whole PEs are copied then it shouldn't take only seconds to copy
to the snapshot.

- From the actual point of view activating snapshots on this database
machine absolutly kills performance.

Did I something wrong on that?
- --

Falko Zurell
Skype: zero_data
/i-d media AG
Ohlauer Stra�e 43
D-10999 Berlin

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