[DRBD-user] Asymmetric performance

Guido Guenther agx at sigxcpu.org
Fri Mar 24 16:39:38 CET 2006

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

It seems drbd shows quiet asymmetric performance on "crossmounted"
devices. I have two HP DL385 (server0 and server1) each with two drbd
devices drbd0 and drbd1 synchronized over a dedicated gigabit ethernet
crosslink between them. Server0 has drbd0 mounted and Server1 has drbd1
mounted. When I only write on one drbd device at a time I see
performance numbers consistent with writes on the local filesystem. But
when I write on both devices at the same time (one on server0 and the
other on server1) I always get 3 times as much throughput on drbd0 than
on drb1 (dbench/tiobench).
The problem seems to always be drbd1, it doesn't matter on which server
of the both the device is mounted.

I tried both consistency modes B and C but this doesn't make an actual
difference. I even played with priority groups but that didn't change
much either.

Any ideas? Kernel is x86_64 2.6.16-rc6, drbd is 0.7.17.
 -- Guido

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