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

Sven Eschenberg sven at whgl.uni-frankfurt.de
Wed Feb 26 16:58:15 CET 2014


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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