[DRBD-user] DRBD performance

[Laurent CARON]

Hi,

I'm trying to improve the performance of a DRBD (8.X) cluster machines I 
did just set up.

Performance over the DRBD partition:
$ dd if=/dev/zero of=./tmp bs=4096 count=1000000
1000000+0 enregistrements lus
1000000+0 enregistrements écrits
4096000000 octets (4,1 GB) copiés, 84,7843 seconde, 48,3 MB/s

Performane over the local raid array:
$ dd if=/dev/zero of=./tmp bs=4096 count=1000000
1000000+0 enregistrements lus
1000000+0 enregistrements écrits
4096000000 octets (4,1 GB) copiés, 42,9673 seconde, 95,3 MB/s

Nearly twice as much on the same array/controller without drbd than with 
drbd.

Here are the sysctl entries:
net.core.rmem_max = 16777216
net.core.wmem_max = 16777216
net.ipv4.tcp_rmem = 8192 174760 2097152
net.ipv4.tcp_wmem = 8192 174760 2097152


net section of drbd.conf:
     sndbuf-size 240k;
     max-buffers     20480;
     unplug-watermark   20480;
     max-epoch-size  16384;

syncer section of drbd.conf:
  rate 90M;
  al-extents 257;

Do you guys have any idea of the bottleneck of that setup ?

Thanks

Laurent