From: Christoph Hellwig <hch@infradead.org>
To: Chris Mason <chris.mason@oracle.com>, Avi Kivity <avi@redhat.com>,
Diego Calleja <diegocg@gmail.com>,
linux-btrfs@vger.kernel.org
Subject: Re: Poor performance with qemu
Date: Thu, 8 Apr 2010 11:32:04 -0400 [thread overview]
Message-ID: <20100408153204.GA24502@infradead.org> (raw)
In-Reply-To: <20100408152615.GI1400@think>
On Thu, Apr 08, 2010 at 11:26:15AM -0400, Chris Mason wrote:
> With O_DIRECT the writeback rates are very reasonable. I'll work up a
> way to pass the barrier down from the guest to btrfs to force logging of
> updated metadata when required.
Barriers are implemented in the guest kernel using queue drains and
cache flush commands. Qemu maps the cache flush to fdatasync.
> > >Once the O_DIRECT read patch is in, you can switch to that, or tell qemu
> > >to use a writeback cache instead.
> >
> > Even with writeback qemu will issue a lot of fsyncs.
>
> Oh, I didn't see that when I was testing, when does it fsync?
When the guest issues a barrier (and it's actually a fdatasync)
next prev parent reply other threads:[~2010-04-08 15:32 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
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 [this message]
-- strict thread matches above, loose matches on Subject: below --
2010-03-29 19:21 Markus Suvanto
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