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

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Message-ID: <20100804111803.GA32643@infradead.org>
Date:	Wed, 4 Aug 2010 07:18:03 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Dominik Brodowski <linux@...inikbrodowski.net>,
	Michael Monnerie <michael.monnerie@...it-management.at>,
	Christoph Hellwig <hch@...radead.org>,
	linux-raid@...r.kernel.org, xfs@....sgi.com,
	linux-kernel@...r.kernel.org, dm-devel@...hat.com
Subject: Re: How to track down abysmal performance ata - raid1 - crypto -
 vg/lv - xfs

On Wed, Aug 04, 2010 at 12:25:26PM +0200, Dominik Brodowski wrote:
> Hey,
> 
> many thanks for your feedback. It seems the crypto step is the culprit:
> 
> Reading 1.1 GB with dd, iflag=direct, bs=8k:
> 
> /dev/sd*                35.3 MB/s       ( 90 %)
> /dev/md*                39.1 MB/s       (100 %)
> /dev/mapper/md*_crypt    3.9 MB/s       ( 10 %)
> /dev/mapper/vg1-*        3.9 MB/s       ( 10 %)
> 
> The "good" news: it also happens on my notebook, even though it has a
> different setup (no raid, disk -> lv/vg -> crypt). On my notebook, I'm
> more than happy to test out different kernel versions, patches etc.
> 
> /dev/sd*                17.7 MB/s       (100 %)
> /dev/mapper/vg1-*       16.2 MB/s       ( 92 %)
> /dev/mapper/*_crypt      3.1 MB/s       ( 18 %)

The good news is that you have it tracked down, the bad news is that
I know very little about dm-crypt.  Maybe the issue is the single
threaded decryption in dm-crypt?  Can you check how much CPU time
the dm crypt kernel thread uses?

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