linux-kernel - Re: [dm-devel] [PATCH 0/2] Introduce the request handling for dm-crypt

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Date:	Fri, 4 Dec 2015 12:57:19 +0800
From:	Baolin Wang <baolin.wang@...aro.org>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	Mark Brown <broonie@...nel.org>, Jens Axboe <axboe@...nel.dk>,
	keith.busch@...el.com, Jan Kara <jack@...e.cz>,
	Arnd Bergmann <arnd@...db.de>,
	Mike Snitzer <snitzer@...hat.com>, neilb@...e.com,
	LKML <linux-kernel@...r.kernel.org>, linux-raid@...r.kernel.org,
	dm-devel@...hat.com, "Garg, Dinesh" <dineshg@...cinc.com>,
	tj@...nel.org, bart.vanassche@...disk.com, jmoyer@...hat.com,
	Alasdair G Kergon <agk@...hat.com>
Subject: Re: [dm-devel] [PATCH 0/2] Introduce the request handling for dm-crypt

On 3 December 2015 at 23:47, Mikulas Patocka <mpatocka@...hat.com> wrote:
>
>
> On Thu, 3 Dec 2015, Baolin Wang wrote:
>
>> On 3 December 2015 at 03:56, Alasdair G Kergon <agk@...hat.com> wrote:
>> > On Wed, Dec 02, 2015 at 08:46:54PM +0800, Baolin Wang wrote:
>> >> These are the benchmarks for request based dm-crypt. Please check it.
>> >
>> > Now please put request-based dm-crypt completely to one side and focus
>> > just on the existing bio-based code.  Why is it slower and what can be
>> > adjusted to improve this?
>> >
>>
>> OK. I think I find something need to be point out.
>> 1. From the IO block size test in the performance report, for the
>> request based, we can find it can not get the corresponding
>> performance if we just expand the IO size. Because In dm crypt, it
>> will map the data buffer of one request with scatterlists, and send
>> all scatterlists of one request to the encryption engine to encrypt or
>> decrypt.  I found if the scatterlist list number is small and each
>> scatterlist length is bigger, it will improve the encryption speed,
>
> This optimization is only applicable to XTS mode. XTS has its weaknesses
> and it is not recommended for encryption of more than 1TB of data
> ( http://grouper.ieee.org/groups/1619/email/msg02357.html )
>
> You can optimize bio-based dm-crypt as well (use larger encryption chunk
> than 512 bytes when the mode is XTS).
>
> The most commonly used mode aes-cbc-essiv:sha256 can't be optimized that
> way. You have to do encryption and decryption sector by sector because
> every sector has different IV.

Make sense. We'll optimize bio-based dm-crypt for XTS mode, and do
some investigations for none XTS mode.

>
> Mikulas
>



-- 
Baolin.wang
Best Regards
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