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. Arno 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@xxxxxxxx > http://www.saout.de/mailman/listinfo/dm-crypt > -- Arno Wagner, Dr. sc. techn., Dipl. Inform., CISSP -- Email: arno@xxxxxxxxxxx GnuPG: ID: 1E25338F FP: 0C30 5782 9D93 F785 E79C 0296 797F 6B50 1E25 338F ---- One of the painful things about our time is that those who feel certainty are stupid, and those with any imagination and understanding are filled with doubt and indecision. -- Bertrand Russell _______________________________________________ dm-crypt mailing list dm-crypt@xxxxxxxx http://www.saout.de/mailman/listinfo/dm-crypt
