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Re: [Qemu-block] [PATCH 0/7] qcow2: Reduce the number of I/O ops when do

From: Alberto Garcia
Subject: Re: [Qemu-block] [PATCH 0/7] qcow2: Reduce the number of I/O ops when doing COW
Date: Thu, 25 May 2017 16:35:42 +0200
User-agent: Notmuch/0.18.2 (http://notmuchmail.org) Emacs/24.4.1 (i586-pc-linux-gnu)

On Thu 25 May 2017 01:48:39 PM CEST, Alberto Garcia wrote:
>>>> I agree; as mentioned we have similar patches and they don't
>>>> conflict much.  We noticed a performance regression on HDD though,
>>>> for the presumably optimized case (random 4k write over a large
>>>> backed image); so the patches were put on hold.
>>> Interesting, I think that scenario was noticeably faster in my
>>> tests. What cluster size(s) and image size(s) were you using?
>> 64k cluster, 2g image, write 32m in portions of 4k at random offsets
> I just tried that and the optimized case performs better (as
> expected), almost twice as fast in fact:
> write: io=32892KB, bw=162944B/s, iops=39, runt=206705msec
> write: io=32892KB, bw=309256B/s, iops=75, runt=108911msec
> I'll try in a different machine.

I made more tests of that same scenario with a different HDD:

write: io=32892KB, bw=588588B/s, iops=143, runt= 57224msec
write: io=32892KB, bw=779951B/s, iops=190, runt= 43184msec

And here are the results without a backing file:

write: io=32892KB, bw=1510.2KB/s, iops=377, runt= 21781msec
write: io=32892KB, bw=5417.1KB/s, iops=1354, runt=  6071msec


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