From: John Leach <john@brightbox.co.uk>
To: Avi Kivity <avi@redhat.com>
Cc: Stefan Hajnoczi <stefanha@gmail.com>,
kvm@vger.kernel.org, Anthony Liguori <anthony@codemonkey.ws>
Subject: Re: bad O_DIRECT read and write performance with small block sizes with virtio
Date: Tue, 03 Aug 2010 15:57:51 +0100 [thread overview]
Message-ID: <1280847471.13790.55.camel@dogen> (raw)
In-Reply-To: <4C582B55.2030209@redhat.com>
On Tue, 2010-08-03 at 17:44 +0300, Avi Kivity wrote:
> On 08/03/2010 05:40 PM, John Leach wrote:
> >
> > dd if=/dev/mapper/zero of=/dev/null bs=8k count=1000000 iflag=direct
> > 8192000000 bytes (8.2 GB) copied, 3.46529 s, 2.4 GB/s
> >
> > dd if=/dev/mapper/zero of=/dev/null bs=8k count=1000000
> > 8192000000 bytes (8.2 GB) copied, 5.5741 s, 1.5 GB/s
> >
> > dd is just using read.
> >
>
> What's /dev/mapper/zero? A real volume or a zero target?
>
zero target:
echo "0 21474836480 zero" | dmsetup create zero
The same performance penalty occurs when using real disks though, I just
moved to a zero target to rule out the variables of spinning metal and
raid controller caches.
John.
next prev parent reply other threads:[~2010-08-03 14:57 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-08-02 17:15 bad O_DIRECT read and write performance with small block sizes with virtio John Leach
2010-08-02 17:46 ` Anthony Liguori
2010-08-02 20:50 ` Stefan Hajnoczi
2010-08-03 6:35 ` Dor Laor
2010-08-03 14:52 ` John Leach
2010-08-03 14:40 ` John Leach
2010-08-03 14:44 ` Avi Kivity
2010-08-03 14:57 ` John Leach [this message]
2010-08-03 16:14 ` Avi Kivity
2010-08-04 12:38 ` Stefan Hajnoczi
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