[dm-crypt] poor mysqldump performance

Arno Wagner arno at wagner.name
Sat Feb 25 03:13:27 CET 2012

On Fri, Feb 24, 2012 at 02:30:20PM -0800, Tracy Reed wrote:
> On Fri, Feb 24, 2012 at 09:05:29PM +0100, Arno Wagner spake thusly:
> > Blocks used in dm-crypt are allways 512 Bytes in size, hence no extra reads
> > or writes compared to non-encrypted disks should ever be required. 
> Ok, so there goes that theory.
> > Please to not. ECB is massively insecure. And it will not result in reading
> > less data. Well, you can do it as an experiement, but please do not use it
> > when it has to be secure.
> I already did a test with it and as you correctly expected, it made no
> difference. We certainly won't be deploying ECB. I am aware of the same
> plaintext security issues with it.
> > It should not. But if you notice a difference, it may be interesting to see
> > what it is.
> I found no difference.

As expected. Still good to know, sometomes these assumptions turn
out to be wrong.
> > There is also another possible factor that may make a larger or smaller
> > difference: If I remember correctly, you said the dump was going to the same
> > disk, but from different partitions for encrypted and non-encrypted. This may
> > influence speeds. First, head seeks are dependend on seek distance. The
> You remember incorrectly: The dump is being done using mysqldump on a remote
> machine. So it is going over the network and writing to totally different
> disks.

Ah, sorry.

> > regions have different data density. My observation is that disk linear
> > throughput drops to less than half towards the end of many disks.  And third,
> > you may have a different fragmentation or data placement situation or
> > filesystem parameter on both source disks.
> This is a very good observation and could be part of the problem here. I have
> been using different logical volumes on the same disk. I will try using the
> same LV mapped with dm-crypt vs no crypto and see if that helps.

It would be an easy explanation. Let us know about the results.

> Thanks!

No problem.

Arno Wagner, Dr. sc. techn., Dipl. Inform., CISSP -- Email: arno at wagner.name 
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 

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