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From: "chris (fool) mccraw" <gently@gmail.com>
Date: Fri, 12 Nov 2010 16:17:58 -0800
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Subject: Re: [linux-lvm] advice for curing terrible snapshot performance?
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To: LVM general discussion and development <linux-lvm@redhat.com>

On Fri, Nov 12, 2010 at 15:36, Joe Pruett <joey@q7.com> wrote:
> i just did a bit of poking around and discovered that snapshots have
> their own chunk size that is used for the copy on write magic.

indeed.  i was hoping someone would advise me if there was a better
chunk size, which is why i said what i thought i was using (default =
64k) in my first post.

> and it
> defaults to 4k, and you can only increase that to 512k. �a simple test
> of creating a 1g file went from 240mbytes/sec to 4mbytes/sec with 4k
> chunk, and 12mbytes/sec with 512k chunk. �so i'm not sure what the
> bottleneck is, but is surely is there.

interestingly the default snapshot chunk size on my system:

  LVM version:     2.02.56(1)-RHEL5 (2010-04-22)
  Library version: 1.02.39-RHEL5 (2010-04-22)
  Driver version:  4.11.5

is 4k.  a tutorial i was reading suggested it was 64k, and i didn't
doublecheck if that was true.  i am going to have to wait til after
business hours to run more thorough tests, but i still see a slowdown
way over 10x even at 64k chunk size with a single snapshot.  i'll try
it at all the different available chunk sizes and report back by
monday.

still curious about Zumastor--does anyone use this in production?