How to promote a DRBD resource to primary

Promoting a DRBD resource to primary makes the local node writable for that replicated block device. This is needed before mounting a conventional filesystem, starting a service that writes to the device, or taking over a resource in a manually managed active/passive pair.

The drbdadm primary command changes the local node role for an already configured resource. In DRBD single-primary mode, which is the default, only one connected node can be Primary at a time, so the old primary must be stopped, unmounted, and demoted before promoting the takeover node.

Manual promotion belongs only on resources whose role is not controlled by a cluster manager or automatic promotion. If Pacemaker, DRBD Reactor, LINSTOR, a systemd-managed mount, or another upper layer owns the resource, promote through that manager or let the mount request trigger auto-promotion instead of forcing a local role change behind it.

1. Check the resource state on the node that should become writable.

   $ sudo drbdadm status wwwdata
   wwwdata role:Secondary
     disk:UpToDate
     node-b role:Secondary
       peer-disk:UpToDate

   Do not promote a secondary while another connected node is still Primary unless the resource is intentionally configured for dual-primary access with a cluster filesystem or another write-coordination layer.

   Related: How to check DRBD resource status

2. Promote the resource on the intended node.

   $ sudo drbdadm primary wwwdata

   drbdadm primary normally returns no output when the role change succeeds.

3. Verify that the local node is now Primary.

   $ sudo drbdadm status wwwdata
   wwwdata role:Primary
     disk:UpToDate
     node-b role:Secondary
       peer-disk:UpToDate

4. Mount the DRBD volume.

   $ sudo mount /dev/drbd/by-res/wwwdata/0 /srv/wwwdata

   Replace wwwdata, volume 0, and /srv/wwwdata with the resource, volume, and mount point used by the local resource configuration. If a service opens the device instead of a manual mount, start that service after the resource reports role:Primary.

5. Confirm that the filesystem is mounted read/write through the DRBD device.

   $ findmnt /srv/wwwdata
   TARGET       SOURCE        FSTYPE OPTIONS
   /srv/wwwdata /dev/drbd1000 xfs    rw,relatime

   If another service opens the device instead of a manual mount, check that service's status or application-level write test after the resource reports role:Primary.