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

[thomas62186218 at aol.com]

Hi Peter,

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.

Regards,
Thomas


-----Original Message-----
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









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 at yahoo.de 
<mailto: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 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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