From mboxrd@z Thu Jan  1 00:00:00 1970
From: John Leach <john@brightbox.co.uk>
Subject: Re: bad O_DIRECT read and write performance with small block sizes
 with virtio
Date: Tue, 03 Aug 2010 15:57:51 +0100
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Cc: Stefan Hajnoczi <stefanha@gmail.com>, kvm@vger.kernel.org,
	Anthony Liguori <anthony@codemonkey.ws>
To: Avi Kivity <avi@redhat.com>
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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.