[dm-crypt] slow read performance, but fast writes?

Milan Broz mbroz at redhat.com
Wed Sep 14 17:15:41 CEST 2011


On 09/14/2011 04:04 PM, Peter Merhaut wrote:
> i've got a really strange problem with my setup, and i'm not quite sure why.

It is known problem, current implementation depends on how are IO submitted
and writes and reads differs this way (I think page cache interferes here).
Reads are slower, because parallel operations are timed differently causing
these waits.

If you use some more realistic IO pattern than dd, you can get different
output though. (dd is NOT good performance test. But everyone uses that:-)

We have some ideas how to improve parallel processing inside dm-crypt but
other things had higher priority still.

(You can see this problem even measured against zero virtual devices, so
read/write caching is not the real problem here.)

Milan


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