[DRBD-user] Write Performance: sequencial I/O slower than random I/O

[Robert]

Hello list,

I'm using DRBD 8.2.7rc2 and doing some benchmarks. The standby machine 
uses a ramdisk as backend storage to simulate the impact of the network 
only.

I also set up different iometer bechmark patterns. For two of them I can 
not explain the results.

Pattern1: 100% Write, Blocksize 8k, 100% Random
Pattern2: 100% Write, Blocksize 8k, 100% Sequencial

The network between the sites is 100Mbit with a simulated latency of 
1500 us +- 500 us.

DRBD (besides std. config) is:
   protocol c;
   al-extents       1021;
   max-buffers      4096;
   unplug-watermark 131072;
   sndbuf-size        0;

With the same drbd setup I get :

Pattern1: 685 IO's /Second
Pattern2: 116 IO's /Second

I can not explain why sequencial writes are slower than random writes 
with the same DRBD config.

I also tried with different unplug-watermark, no effect.

Any hint ?

Regards,
Robert