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From: "chris (fool) mccraw" <gently@gmail.com>
Date: Mon, 15 Nov 2010 10:05:55 -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 16:17, chris (fool) mccraw <gently@gmail.com> wrote:

> 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.

Well, I only made it through most chunk sizes.  here were my results:

(previously obtained:

no snapshot             = ~11sec (727MB/sec)
1 snapshot @4k chunks   = ~102sec (78MB/sec)

)

newly obtained:
1 snapshot @16k chunks  = ~639sec (12MB/sec)
1 snapshot @32k chunks  = ~367sec (21MB/sec)
1 snapshot @64k chunks  = ~252sec (31MB/sec)
1 snapshot @256k chunks = ~145sec (55MB/sec)
1 snapshot @512k chunks = ~100sec (80MB/sec)

wish i'd tried 8kb, since that's the most interesting area of the
graph now that i lay the numbers out together.  but no matter what
size i use, it's still nearly a factor of 10, and my numbers with the
default 4k chunks were almost as good as it gets.