linux-kernel - Re: RAID + LUKS + LVM performance

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Date:	Fri, 12 Mar 2010 14:12:12 +0100
From:	Milan Broz <mbroz@...hat.com>
To:	Matthias Schniedermeyer <ms@...d.de>
CC:	Mathias Buren <mathias.buren@...ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: RAID + LUKS + LVM performance

On 03/12/2010 01:06 PM, Matthias Schniedermeyer wrote:

> Using one of the Westmere Core i5-6XX, which support AES-NI, should get 
> you better performance (if supported by krcyptd), altough you loose 2 
> cores as onyl have 2. But i don't know what kind of performance 
> improvement (if any) AES-NI provides.
> Or you wait a few weeks and buy on Core I7-980X with 6 core and AES-NI.

dm-crypt uses whatever is implemented in cryptoAPI, AES-NI of course works
when you have proper modules loaded.

The main reason why dm-crypt is single-threaded is that all these new
crypto accelerators uses asynchronous crypto API, so if hw allows
parallel operation it can use it already.
(dm-crypt submits all sectors in bio as separate asynchronous crypto requests).

But note: single threaded per volume - if you have LVM with multiple
encrypted LVs - every LV have its own thread.

If we build another multi-core(thread) processing in dm-crypt, it can
help on multi-core CPU but complicate things otherwise.
(I tried simple implementation and abandoned it, I would prefer
if cryptoAPI can do some parallel processing itself here.
That question was discussed several times on dm-crypt mailing list.)

Anyway, only two real-world examples, where one thread 
in dm-crypt can cause real problems with speed

 - dm-crypt over fast RAID(5)
 - dm-crypt over fast SSD.

And many users of these configurations have now AES-NI acceleration.

Milan
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mbroz@...hat.com
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