drbd.conf supports a variety of boolean configuration options. In pre DRBD 8.4 syntax, these boolean options would be set as follows:

Pre-DRBD 8.4 configuration example with boolean options. 

resource test {
  disk {
    no-md-flushes;
  }
}

This led to configuration issues if you wanted to set a boolean variable in the common configuration section, and then override it for individual resources:

Pre-DRBD 8.4 configuration example with boolean options in common section. 

common {
  no-md-flushes;
}
resource test {
  disk {
    # No facility to enable disk flushes previously disabled in
    # "common"
  }
}

In DRBD 8.4, all boolean options take a value of yes or no, making them easily configurable both from common and from individual resource sections:

DRBD 8.4 configuration example with boolean options in common section. 

common {
  md-flushes no;
}
resource test {
  disk {
    md-flushes yes;
  }
}

Prior to DRBD 8.4, the configuration syntax allowed for a syncer section which has become obsolete in 8.4. All previously existing syncer options have now moved into the net or disk sections of resources.

Pre-DRBD 8.4 configuration example with syncer section. 

resource test {
  syncer {
    al-extents 3389;
    verify-alg md5;
  }
  ...
}

The above example is expressed, in DRBD 8.4 syntax, as follows:

DRBD 8.4 configuration example with syncer section replaced. 

resource test {
  disk {
    al-extents 3389;
  }
  net {
    verify-alg md5;
  }
  ...
}

In prior DRBD releases, the protocol option was awkwardly (and counter-intuitively) required to be specified on its own, rather than as part of the net section. DRBD 8.4 removes this anomaly:

Pre-DRBD 8.4 configuration example with standalone protocol option. 

resource test {
  protocol C;
  ...
  net {
    ...
  }
  ...
}

The equivalent DRBD 8.4 configuration syntax is:

DRBD 8.4 configuration example with protocol option within net section. 

resource test {
  net {
    protocol C;
    ...
  }
  ...
}

DRBD 8.4 introduces a new options section that may be specified either in a resource or in the common section. The cpu-mask option has moved into this section from the syncer section in which it was awkwardly configured before. The on-no-data-accessible option has also moved to this section, rather than being in disk where it had been in pre-8.4 releases.

Pre-DRBD 8.4 configuration example with cpu-mask and on-no-data-accessible. 

resource test {
  syncer {
    cpu-mask ff;
  }
  disk {
    on-no-data-accessible suspend-io;
  }
  ...
}

The equivalent DRBD 8.4 configuration syntax is:

Pre-DRBD 8.4 configuration example with options section. 

resource test {
  options {
    cpu-mask ff;
    on-no-data-accessible suspend-io;
  }
  ...
}