linux-kernel - Re: [Aoetools-discuss] [Xen-devel] domU is causing misaligned disk writes

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Message-ID: <20100428195528.GA10354@twister.home>
Date:	Wed, 28 Apr 2010 21:55:29 +0200
From:	Gabor Gombas <gombasg@...ikabel.hu>
To:	Tracy Reed <treed@...raviolet.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Pasi Kärkkäinen <pasik@....fi>,
	xen-devel@...ts.xensource.com,
	Aoetools-discuss@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: [Aoetools-discuss] [Xen-devel] domU is causing misaligned disk
 writes

On Tue, Apr 20, 2010 at 02:19:13PM -0700, Tracy Reed wrote:

> > How do you know this is a mis-aligned sectors issue? Is this what your
> > AOE vendor is telling you ?
> 
> No AoE vendor involved. I am using the free stuff. I think it is a
> misalignment issue because during a purely write test it is doing
> massive amounts of reading according to iostat.

How about actually verifying that by e.g. using wireshark and comparing
the I/O patterns in the fast and slow cases? The differences in the
patterns may give clues where to look further.

> #dd if=/dev/zero of=/dev/etherd/e6.1 oflag=direct bs=4096
> count=3000000
> 1764883+0 records in
> 1764883+0 records out
> 7228960768 bytes (7.2 GB) copied, 402.852 seconds, 17.9 MB/s
> 
> But even on my local directly attached SATA workstation disk when
> doing that same dd on an otherwise idle machine I see performance
> like:
> 
> $ dd if=/dev/zero of=foo.test bs=4096 count=4000000
> C755202+0 records in
> 755202+0 records out
> 3093307392 bytes (3.1 GB) copied, 128.552 s, 24.1 MB/s
> 
> which again suggests that oflag=direct isn't doing quite what I expect.

oflag=direct turns off caching on the host dd is running on, i.e. the
initiator. The target still caches writes of course, unless you tell it
not to by passing the "-d" flag to vblade.

Gabor
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