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

Romeo Theriault romeo.theriault at maine.edu
Mon Nov 15 14:35:02 UTC 2010

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?

> 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?

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