Hi, 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 too. 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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