[DRBD-user] Write Performance on mode A - TCP buffer delay

Phil Stricker stril at gmx.de
Sat Apr 17 10:18:51 CEST 2010

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


The write performance on my drbd systems is not as good as it should be. 
So, I started debugging.

There is one strange thing:

ATTO Disk Benchmark shows on small packets (0,5 KB) a write speed of:

4000 kb/s without DRBD
90 kb/s with DRBD mode A
90 kb/s with DRBD mode B
100 kb/s with DRBD mode C

What is slowing the writes down? Mode A should IMHO not have such a big 
impact on write speed. Is there anything to avoid that?
Is it possible to reduce the latency for placing the replication packets 
in the local TCP send buffer?

The systems are Dell 2950 with 4x SATA on Softraid 10, each.
DRBD 8.3.7 on Xenserver 5.5


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