[linux-lvm] advice for curing terrible snapshot performance?

[chris (fool) mccraw]

On Mon, Nov 15, 2010 at 06:35, Romeo Theriault
<romeo.theriault at maine.edu> wrote:
>
>
> On Sat, Nov 13, 2010 at 06:52, chris (fool) mccraw <gently at gmail.com> wrote:
>>
>> however, snapshots really shoot my system in the foot.  in my (12
>> processor, 12GB RAM, x86_64 centos 5.5) server, i have two pricey
>> areca hardware raid cards that give me ridiculous write performance:
>> i can sustain over 700MByte/sec write speeds (writing a file with dd
>> if=/dev/zero bs=1M, twice the size of the raid card's onboard memory
>> and timing the write + a sync afterwards) to the volumes on either
>> card (both backed by 7 or more fast disks).  enabling a single
>> snapshot reduces that speed by a factor of 10!  enabling more
>> snapshots isn't as drastic but still doesn't really scale well:
>
>
> Once you have snapshots and are doing new dd's are you creating new files or
> dd'ing over the old file?

tried both ways with no obvious performance difference (like i said,
sometimes things go up to 30% faster or slower for 1 run, but that's
happened doing it both ways).

>> no snapshot  = ~11sec (727MB/sec)
>> 1 snapshot   = ~102sec (78MB/sec)
>> 2 snapshots  = ~144sec (55MB/sec)
>> 3 snapshots  = ~313sec (25MB/sec)
>> 4 snapshots  = ~607sec (15MB/sec)
>
> I would assume a "yes", but does your write performance return to normal
> after you remove all the snapshots?

absolutely, it does.