linux-kernel - Re: How to track down abysmal performance ata - raid1 - crypto - vg/lv

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Date:	Wed, 4 Aug 2010 05:21:22 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Dominik Brodowski <linux@...inikbrodowski.net>,
	Christoph Hellwig <hch@...radead.org>,
	linux-raid@...r.kernel.org, xfs@....sgi.com,
	linux-kernel@...r.kernel.org
Subject: Re: How to track down abysmal performance ata - raid1 - crypto -
 vg/lv - xfs

On Wed, Aug 04, 2010 at 11:13:17AM +0200, Dominik Brodowski wrote:
> > show these problems?
> 
> No, 2.6.31 to 2.6.34 show similar behaviour.

Ok, so it's been around for a while.  Can you test the write speed of
each individual device layer by doing a large read from it, using:

	dd if=<device> of=/dev/null bs=8k iflag=direct

where device starts with the /dev/sda* device, and goes up to the MD
device, the dm-crypt device and the LV.  And yes, it's safe to read
from the device while it's otherwise mounted/used.

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