[DRBD-user] Disk Write call on primary returning before committing on secondary while using Protocol C

[Omkumar Seshadri]

Hi,
 I setup a drbd (8.2) cluster of two ubuntu servers using protocol C for maximum data reliability. However i observe that writes on the primary are not reflecting on the secondary instantly. It takes about 5 - 10 seconds for the data to replicate. While using protocol C, i expect the writes on primary to block till the data reaches and gets committed on the secondary server. I dumped a huge file on the drbd partition on the primary server and monitoring the network and disk activity on the secondary. I see that the data reaches the secondary only after few seconds delay. the servers are connected via 100mbs network and the sync speed is pretty good. I verified that the data is synced to the primary partition instantly but the data on secondary takes a while. Can you let me know if i am missing something obvious?

Here is my configuration file:

global {
  usage-count no;
}
resource ez {
  protocol      C;
  startup {
    wfc-timeout 0;
    degr-wfc-timeout 120;
  }
  disk { on-io-error detach; }
  syncer {
    verify-alg sha1;
        rate 50M;
  }
  net {
  }
  on server1 {
    device      /dev/drbd0;
    disk        /dev/sdb2;
    address     192.168.0.10:7791;
    meta-disk   /dev/sdb1[0];
  }
  on server2 {
    device      /dev/drbd0;
    disk        /dev/sdb2;
    address     192.168.0.11:7791;
    meta-disk   /dev/sdb1[0];
  }
}