[dm-crypt] Poor write performance on software raid with 512k chunk size

Dennis Birkholz dennis.birkholz at rwth-aachen.de
Sun Feb 26 17:45:51 CET 2012


Thanks for the fast reply.

Am 26.02.2012 14:37, schrieb Alasdair G Kergon:
> As we've mentioned to others, try increasing 
>    /sys/block/<devices>/queue/nr_requests
> significantly (e.g. see if 50,000 has any impact).
> 
> Or experiment with these patches:
>   http://people.redhat.com/mpatocka/patches/kernel/dm-crypt-paralelizace/current/
> (at your own risk, of course).

As I mentioned before, I chose a smaller chunk size for my RAID to solve
this problem. As I am not interested in rebuilding a 6TB RAID again, I
cannot verify if your suggestions increase write throughput with greater
chunk size.

I would suggest that the device mapper should set default values that
give maximum performance for the default values other kernel subsystems
use. As the RAID subsystem seems to use 512k chunks by default, the
device mapper should use settings by default that allow maximum
performance for this chunk size.

Greets,
Dennis


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