[dm-devel] multipath performance problem

[Leonid Podolny <leonid podolny xtremio com>]

Hi,
I'm not sure it's a correct place to write to, but I'm having a weird issue with the multipath which I'm unable to solve on my own.
There is a SAN device that exports each LUN 8 times. I measure performance using btest (http://http://sourceforge.net/projects/btest/). Basically, it writes 4kb chunks of data from multiple threads and gathers statistics -- throughput in IOPs, average latency and such.
When I compare results of writing into /dev/sdXXX's vs. writing into /dev/dm-XXX's, I see that the multipath devices have about 4 times worse throughput (in IOPs), but about 2 times less latency. In addition, the SAN device allows to see how many commands are currently being processed, and when I work via multipath, this number is many times lower.
All the above shows that I am unable to properly saturate the SAN device when working through multipath. I tried disabling the queue in multipathd. What do I miss?

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Cheers, L.