[linux-lvm] write performance with active snapshot

[Larry Dickson]

My guess is that you are getting the typical seek overhead. Have you tried
making a volume group out of two separate RAID arrays (driving different
spindles), and using lvdisplay --maps to make sure the parent volume is on
one array, the snapshot(s) on the other?

Larry Dickson
Cutting Edge Networked Storage

On 11/9/08, Peter Daum <gator_ml at yahoo.de> wrote:
>
> Hi,
>
> for an application I am just working on it looks like lvm snapshots would
> be just what I need as far as functionality is concerned. Unfortunately,
> I am experiencing such a massive degradation in performance, that the
> result is almost useless.
>
> I'm working on a fairly fast machine (Quadcore, 8GB RAM) with a big
> hardware RAID array and lvm2 (Debian Lenny; Linux 2.6.26-1-amd64;
> LVM version:2.02.39 (2008-06-27)
> Library version: 1.02.27 (2008-06-25)
> Driver version:  4.13.0)
>
> Sequentially writing to a file (ext3) on a logical volume, I get a
> sustained performance of ~ 250 MB/sec. When I create a snapshot
> volume, the write throughput drops to 7-8 MB/secs (on the original
> volume; writing to the snapshot I see a significant degradation,
> but not nearly, as bad; read performance is o.k.).Is this "normal"
> or is there anything I can do to about it?
>
> I looked in this list and searched the WWW but couldn't find any
> concrete information on the performance impact of snapshots
> (except http://www.nikhef.nl/~dennisvd/lvmcrap.html).
> It seems like write performance should probably be less then 1/3
> of the original throughput, because every write to the source
> volume causes 3 I/O operations plus some overhead for meta data.
> More difficult to estimate would be the time lost by additional
> head movements. Still, a throughput degradation by a factor of 30
> seems pretty extreme.
>
> Any ideas?
>
> Regards,
>                         Peter Daum
>
>
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