[linux-lvm] Re: write performance with active snapshot
thomas62186218 at aol.com
thomas62186218 at aol.com
Mon Jan 19 00:17:44 UTC 2009
I am following your thread on this topic...have any solutions emerged?
I as well have seen miserably performance when snapshots are active.
Thank you in advance for your feedback.
From: Peter Daum <gator_ml at yahoo.de>
To: linux-lvm at redhat.com
Sent: Tue, 11 Nov 2008 8:04 am
Subject: [linux-lvm] Re: write performance with active snapshot
Larry Dickson wrote:
> 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
> volume is on one array, the snapshot(s) on the other?
That was my suspicion, too (although I could not imagine such an
impact). Just for testing I added a single disk to the same volume
and put the snapshot onto that disk - amazingly it made hardly any
difference (Actually, I'm almost glad about that, because the
of a 12-disk-array with a single disk would be under almost all other
One thing that does improve the performance a little (actually by 100%,
which in this case meens still pretty lousy 16 MB/sec) is to increase
the chunk size to 512kb. (I don't know yet, how this might
performance when dealing with many small files) ...
> On 11/9/08, *Peter Daum* <gator_ml at yahoo.de
<mailto:gator_ml at yahoo.de>>
> for an application I am just working on it looks like lvm
> be just what I need as far as functionality is concerned.
> I am experiencing such a massive degradation in performance, that
> result is almost useless.
> I'm working on a fairly fast machine (Quadcore, 8GB RAM) with a
> 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
> sustained performance of ~ 250 MB/sec. When I create a snapshot
> volume, the write throughput drops to 7-8 MB/secs (on the
> volume; writing to the snapshot I see a significant degradation,
> but not nearly, as bad; read performance is o.k.).Is this
> or is there a
nything 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?
> Peter Daum
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