From: Chris Mason <chris.mason@oracle.com>
To: Diego Calleja <diegocg@gmail.com>
Cc: linux-btrfs@vger.kernel.org
Subject: Re: Poor performance with qemu
Date: Tue, 30 Mar 2010 08:56:23 -0400 [thread overview]
Message-ID: <20100330125623.GB13190@think> (raw)
In-Reply-To: <201003281718.03699.diegocg@gmail.com>
On Sun, Mar 28, 2010 at 05:18:03PM +0200, Diego Calleja wrote:
> Hi, I'm using KVM, and the virtual disk (a 20 GB file using the "raw"
> qemu format according to virt-manager and, of course, placed on a btrfs
> filesystem, running the latest mainline git) is awfully slow, no matter
> what OS is running inside the VM. The PCBSD installer says it's copying
> data at a 40-50 KB/s rate. Is someone using KVM and having better numbers
> than me? How can I help to debug this workload?
The problem is that qemu uses O_SYNC by default, which makes btrfs do
log commits for every write.
Once the O_DIRECT read patch is in, you can switch to that, or tell qemu
to use a writeback cache instead.
-chris
next prev parent reply other threads:[~2010-03-30 12:56 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-03-28 15:18 Poor performance with qemu Diego Calleja
2010-03-30 12:56 ` Chris Mason [this message]
2010-04-08 14:58 ` Avi Kivity
2010-04-08 15:21 ` Gordan Bobic
2010-04-08 15:26 ` Chris Mason
2010-04-08 15:28 ` Avi Kivity
2010-04-08 15:32 ` Chris Mason
2010-04-08 15:34 ` Christoph Hellwig
2010-04-08 15:36 ` Avi Kivity
2010-04-08 15:39 ` Christoph Hellwig
2010-04-08 15:32 ` Christoph Hellwig
-- strict thread matches above, loose matches on Subject: below --
2010-03-29 19:21 Markus Suvanto
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