linux-kernel - Re: BTRFS: Unbelievably slow with kvm/qemu

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Message-ID: <AANLkTikXpAxDvb-tm7Aq8y96ceCs3CKXmKTl-DidojD6@mail.gmail.com>
Date:	Wed, 14 Jul 2010 04:39:35 +0200
From:	Giangiacomo Mariotti <gg.mariotti@...il.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: BTRFS: Unbelievably slow with kvm/qemu

On Tue, Jul 13, 2010 at 6:29 AM, Avi Kivity <avi@...hat.com> wrote:
> Btrfs is very slow on sync writes:
>
> 45KB/s, while 4-5MB/s traffic was actually going to the disk.  For every 4KB
> that the the application writes, 400KB+ of metadata is written.
>
> (It's actually worse, since it starts faster than the average and ends up
> slower than the average).
>
> For kvm, you can try cache=writeback or cache=unsafe and get better
> performance (though still slower than ext*).
>
Yeah, well I've already moved the virtual hd file to an ext3
partition, so the problem for me was actually already "solved" before
posting the first post. I posted the first message just to report the
particularly bad performances of Btrfs for this test-case, so that, if
not already known, they could be investigated and hopefully solved.

By the way, thanks to everyone who answered!

-- 
Giangiacomo
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