6.12. Configuring congestion policies and suspended replication

In an environment where the replication bandwidth is highly variable (as would be typical in WAN replication setups), the replication link may occasionally become congested. In a default configuration, this would cause I/O on the primary node to block, which is sometimes undesirable.

Instead, you may configure DRBD to suspend the ongoing replication in this case, causing the Primary’s data set to pull ahead of the Secondary. In this mode, DRBD keeps the replication channel open — it never switches to disconnected mode — but does not actually replicate until sufficient bandwith becomes available again.

The following example is for a DRBD Proxy configuration:

resource <resource> {
  net {
    on-congestion pull-ahead;
    congestion-fill 2G;
    congestion-extents 2000;
    ...
  }
  ...
}

It is usually wise to set both congestion-fill and congestion-extents together with the pull-ahead option.

A good value for congestion-fill is 90%

A good value for congestion-extents is 90% of your configured al-extents for the affected resources.