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

[chris (fool) mccraw]

On Fri, Nov 12, 2010 at 16:17, chris (fool) mccraw <gently at 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.