[DRBD-user] Latency problems with DRBD 0.8.4 and a 3ware RAID5

Florian Apolloner florian at apolloner.eu
Wed Aug 31 13:46:03 CEST 2011

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


I am having a horrible latency with the following configuration:

 * protocol c
 * raid5 over a 3Ware 9750-4i SAS2 raid controller
 * Dedicated Gigabit link between the two machines, no switch in between.

This is what I can tell you:

 * iperf shows around 950 Mbits/sec -- sounds okay for gigabit ;)
 * throughput for the drbd raid has 70-80 MByte/sec -- sounds good to me

Now on to the latency tests (backing device):

dd if=/dev/zero of=/dev/sda3 bs=512 count=1000 oflag=direct
512000 bytes (512 kB) copied, 0.0365779 seconds, 14.0 MB/s

and onto the drbd device:

dd if=/dev/zero of=/dev/drbd1 bs=512 count=1000 oflag=direct
512000 bytes (512 kB) copied, 9.651 seconds, 53.1 kB/s

Sounded awfully slow to me, so I compared with a ramdisk:

dd if=/dev/zero of=/dev/drbd2 bs=512 count=1000 oflag=direct
512000 bytes (512 kB) copied, 0.166689 seconds, 3.1 MB/s

Sounds waaaay better, though I can't tell if that's slow or fast enough.

The RTT for the link is:
rtt min/avg/max/mdev = 0.096/0.166/0.207/0.043 ms

So let's summarize:

-> Harddisk speed shouldn't be a problem (0.0365779 seconds for the write)
-> Network speed should be a problem as indicated by RTT and iperf

-> Network and Harddisk combined -> problems.

Any hints on how to debug that? I am currently grasping any straw I can
get :/

Thx in advance and regards,
Florian Apolloner

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