[dm-crypt] dm-crypt on raid 5 - having all disks on single controller makes dm-crypt slower

Arno Wagner arno at wagner.name
Wed Feb 26 17:34:25 CET 2014


Just on a meta-layer: This is a known issue, but it seems the
fix is not quite clear. If you are really concerned, put
the encryption below the RAID, but that has other issues.

Arno

On Wed, Feb 26, 2014 at 16:58:15 CET, Sven Eschenberg wrote:
> My best guess here is this:
> 
> There is one work queue (IO QUEUE) per controller. Which in turn means
> that dm-crypt is fed by two queues and as such decides to have one kworker
> instance per queue. I might be completely wrong though. (Actually there's
> still mdraid, which seems to have accordingly 1/2 queues).
> 
> Regards
> 
> -Sven
> 
> 
> On Wed, February 26, 2014 16:12, "C. Dominik Bódi" wrote:
> > Am 26.02.2014 00:58, schrieb shmick at riseup.net:
> >> *only* 150 ?
> >> there's lots of numbers here but maybe we should work backwards and KIS
> >> ask yourself what do you want to achieve ?
> >> when you know the results you want and/or need (2 different things) you
> >> can work backwards & figure out out how to get there
> >> do you *need* to have a certain read/write speed for your application or
> >> desired conclusion in mind ?
> > Well, yes, I'd like to have as much throughput as possible. The cpu is a
> > quad core with HT, so could dm-crypt not utilize 4 threads when there
> > are 4 disks in the raid5 array.
> >
> > An the thing is:
> >
> > 4 disks on controller A => dm-crypt 150MB/s
> > 2 disk on controller A, 2 disks on controller B => dm-crypt 300MB/s
> >
> > I don't understand that behaviour. Why does it make a difference for
> > dm-crypt if the disks sit on different controllers or not? Why should I
> > waste speed when dm-crypt could easily utilize 4 worker threads and max
> > out the raid5 performance? Why doesn't it do so automatically. Is there
> > any means to make dm-crypt use 4 threads even if all 4 drives sit on a
> > single controller?
> >
> 
> 
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-- 
Arno Wagner,     Dr. sc. techn., Dipl. Inform.,    Email: arno at wagner.name
GnuPG: ID: CB5D9718  FP: 12D6 C03B 1B30 33BB 13CF  B774 E35C 5FA1 CB5D 9718
----
A good decision is based on knowledge and not on numbers. -  Plato


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