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




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 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
    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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Hi


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