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[linux-lvm] Re: write performance with active snapshot



Hi Thomas,

thomas62186218 aol com wrote:
I am following your thread on this topic...have any solutions emerged? I as well have seen miserably performance when snapshots are active.

I am sorry, at least I still don't know any solution
(except avoiding snapshots wherever performance matters )-:

Regards,
                  Peter

-----Original Message-----
From: Peter Daum <gator_ml yahoo de>
To: linux-lvm redhat com
Sent: Tue, 11 Nov 2008 8:04 am
Subject: [linux-lvm] Re: write performance with active snapshot









Hi, 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
parent
volume is on one array, the snapshot(s) on the other?
That was my suspicion, too (although I could not imagine such an extreme impact). Just for testing I added a single disk to the same volume group and put the snapshot onto that disk - amazingly it made hardly any difference (Actually, I'm almost glad about that, because the combination of a 12-disk-array with a single disk would be under almost all other aspects foolish). 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
affect performance when dealing with many small files) ... Regards, Peter

On 11/9/08, *Peter Daum* <gator_ml yahoo de
<mailto:gator_ml 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 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?


Regards,

Peter Daum
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