[dm-crypt] poor mysqldump performance

Arno Wagner arno at wagner.name
Fri Feb 24 00:10:43 CET 2012

I notice that you are using a really, really old kernel.
2.6.18 is from end of 2006 or so. There were some 
performance issues with the dm-layer, they might just 
still be in this old version.

With 3.2.5 I basically see that IO is CPU limited
or going at full speed if the CPU is fast enough.
I have not tries a MySQL dump though, although this 
should just be standard disk accesses.


On Thu, Feb 23, 2012 at 02:23:00PM -0800, Tracy Reed wrote:
> I am using dmcrypt/LUKS on:
> CentOS 5.5
> kernel 2.6.18-194.32.1.el5
> MySQL 5.5.12
> cryptsetup-luks 1.0.3
> ext3
> Doing a mysqldump from the LUKS/dmcrypt volume takes twice as long as it does
> from a non-encrypted volume. Only a few percent of the CPU are taken for the
> encryption so it isn't kcryptd maxing out the CPU. Most interestingly I notice
> that iowait goes from 90% for the non-encrypted db to 99% for the encrypted db.
> It really looks like a lot more IO is somehow generated.
> This makes no sense to me as I understand dmcrypt to sit above the disk layer
> and do a block-for-block encrypt/decrypt of the data as it passes through. How
> could it possibly cause extra disk IO? At first I thought maybe it was disk
> alignment (which has bitten me many times before) but we are doing reads here,
> not writes. iostat confirms that during the mysqldump practically no writes are
> happening. I've also looked at increasing readahead to no effect.
> I've googled and found that as long as dmcrypt isn't maxing out the CPU (for
> which latest kernels support AES-NI and multi-threaded kcryptd) the performance
> with and without encryption should be pretty much the same. I don't understand
> why my reads are taking such a hit.
> Any ideas?
> -- 
> Tracy Reed
> _______________________________________________
> dm-crypt mailing list
> dm-crypt at saout.de
> http://www.saout.de/mailman/listinfo/dm-crypt

Arno Wagner, Dr. sc. techn., Dipl. Inform., CISSP -- Email: arno at wagner.name 
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with doubt and indecision. -- Bertrand Russell 

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