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

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Message-ID: <4C3ABF96.9070405@msgid.tls.msk.ru>
Date:	Mon, 12 Jul 2010 11:09:10 +0400
From:	Michael Tokarev <mjt@....msk.ru>
To:	Giangiacomo Mariotti <gg.mariotti@...il.com>
CC:	linux-kernel@...r.kernel.org,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	qemu-devel <qemu-devel@...gnu.org>
Subject: Re: BTRFS: Unbelievably slow with kvm/qemu

12.07.2010 09:24, Giangiacomo Mariotti wrote:
> Hi, is it a known problem how much slow is Btrfs with kvm/qemu(meaning
> that the image kvm/qemu uses as the hd is on a partition formatted
> with Btrfs, not that the fs used by the hd inside the kvm environment
> is Btrfs, in fact inside kvm the / partition is formatted with ext3)?
> I haven't written down the exact numbers, because I forgot, but while
> I was trying to make it work, after I noticed how much longer than
> usual it was taking to just install the system, I took a look at iotop
> and it was reporting a write speed of the kvm process of approximately
> 3M/s, while the Btrfs kernel thread had an approximately write speed
> of 7K/s! Just formatting the partitions during the debian installation
> took minutes. When the actual installation of the distro started I had
> to stop it, because it was taking hours! The iotop results made me
> think that the problem could be Btrfs, but, to be sure that it wasn't
> instead a kvm/qemu problem, I cut/pasted the same virtual hd on an
> ext3 fs and started kvm with the same parameters as before. The
> installation of debian inside kvm this time went smoothly and fast,
> like normally it does. I've been using Btrfs for some time now and
> while it has never been a speed champion(and I guess it's not supposed
> to be one and I don't even really care that much about it), I've never
> had any noticeable performance problem before and it has always been
> quite stable. In this test case though, it seems to be doing very bad.

This looks quite similar to a problem with ext4 and O_SYNC which I
reported earlier but no one cared to answer (or read?) - there:
http://permalink.gmane.org/gmane.linux.file-systems/42758
(sent to qemu-devel and linux-fsdevel lists - Cc'd too).  You can
try a few other options, esp. cache=none and re-writing some guest
files to verify.

/mjt
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