[DRBD-user] protocol C replication - unexpected behaviour

[Janusz Jaskiewicz]

Hello.

I'm experimenting a bit with DRBD in a cluster managed by Pacemaker.
It's a two node, active-passive cluster and the service that I'm trying to
put in the cluster writes to the file system.
The service manages many files, it appends to some of them and increments
the single integer value in others.

I'm observing surprising behaviour and I would like to ask you if what I
see is expected or not (I think not).

I'm using protocol C, but still I see some delay in the files that are
being replicated to the secondary server.
For the files that increment the integer I see a difference which
corresponds roughly to 1 second of traffic.

I'm really surprised to see this, as protocol C should guarantee
synchronous replication.
I'd rather expect some delay in processing (potentially slower disk writes
due to the network replication).

The way I'm testing it:
The service runs on primary and writes to DRBD drive, secondary connected
and "UpToDate".
I kill the service abruptly (kill -9) and then take down the network
interface between primary and secondary (kill and ifdown commands in the
script so executed quite promptly one after the other).
Then I mount the DRBD drive on both nodes and check the difference in the
files with incrementing integer.

I would appreciate any help or pointers on how to fix this.
But first of all I would like to confirm that this behaviour is not
expected.

Also if it is expected/allowed, how can I decrease the impact?



Please find below my configuration:

global_common.conf:

global {
  usage-count yes;
  udev-always-use-vnr; # treat implicit the same as explicit volumes
}

common {
  handlers {}
  startup {}
  options {}
  disk {}
  net {
    protocol C;
  }
}



my_resource.conf:

resource wyr-dim-persistence {
  on dimprod01.my.clustertest.com {
    device    /dev/drbd1;
    disk      /dev/mapper/drbdpool-drbdata;
    address   192.168.0.1:7789;
    meta-disk internal;
  }
  on dimprod02.my.clustertest.com {
    device    /dev/drbd1;
    disk      /dev/mapper/drbdpool-drbdata;
    address   192.168.0.2:7789;
    meta-disk internal;
  }
}

Thanks in advance,
Janusz.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.linbit.com/pipermail/drbd-user/attachments/20210805/f9f67519/attachment.htm>