How to change the DRBD replication protocol

Changing the DRBD replication protocol controls when a write is acknowledged between peer nodes. Operators usually change it when a resource needs a different balance between write latency and replication durability, such as moving a test resource back to protocol C before production use.

The protocol is a net option for a DRBD connection and is normally stored in the resource file under /etc/drbd.d/ or inherited from /etc/drbd.conf. Keep the resource definition identical on every node before applying the change, because drbdadm reads the on section that matches the node's kernel hostname.

Use a maintenance window or cluster-manager maintenance mode when the resource backs a mounted filesystem, database, or application. drbdadm adjust can reconfigure an operational resource, but a protocol change can reconnect peer communication while DRBD applies the new connection options.

Steps to change the DRBD replication protocol:

Check the current DRBD resource state.
```
$ sudo drbdadm status webdata
webdata role:Primary
  volume:0 disk:UpToDate
  node-b role:Secondary
    volume:0 replication:Established peer-disk:UpToDate
```
Resolve unexpected Disconnected, Inconsistent, Outdated, or resynchronizing states before changing connection behavior.

Related: How to check DRBD resource status
Back up the active resource file.
```
$ sudo cp -a /etc/drbd.d/webdata.res /etc/drbd.d/webdata.res.before-protocol-change
```
Use the active resource file for the real resource name. Back up every resource file or included common file that will be changed.
Related: How to back up DRBD metadata before a change
Open the resource file that owns the protocol setting.
```
$ sudoedit /etc/drbd.d/webdata.res
```
If protocol is inherited from a common net section, edit that common file only when every resource inheriting it should receive the same protocol.
Set the protocol in the connection net block.
```
resource webdata {
    connection {
        host node-a port 7788;
        host node-b port 7788;
        net {
            protocol C;
        }
    }
}
```
protocol C acknowledges writes after local and remote disks have the data. protocol B waits for peer receipt, and protocol A waits for local disk plus the local TCP send buffer. Dual-primary resources require protocol C.
Related: How to configure DRBD dual-primary mode

Parse the edited resource file on the local node.

$ sudo drbdadm dump webdata
# resource webdata on node-a: not ignored, not stacked
# defined at /etc/drbd.d/webdata.res:1
resource webdata {
    on node-a {
        node-id 0;
        device           /dev/drbd0 minor 0;
        disk             /dev/vg0/webdata;
        meta-disk        internal;
        address          ipv4 192.0.2.10:7788;
    }
##### snipped #####
    connection {
        host node-a port 7788;
        host node-b port 7788;
        net {
            _name        node-b;
            protocol       C;
        }
    }
}

drbdadm dump fails on syntax errors and prints the normalized configuration that the current node will use.
Related: How to validate DRBD configuration

Synchronize the edited resource file to every peer node.

Use the same configuration-management, csync2, scp, or package-managed process already used for /etc/drbd.conf and /etc/drbd.d/. The file content should match before any node runs drbdadm adjust.

Parse the resource file on a peer node.

$ ssh node-b sudo drbdadm dump webdata
# resource webdata on node-b: not ignored, not stacked
# defined at /etc/drbd.d/webdata.res:1
resource webdata {
##### snipped #####
    connection {
        host node-a port 7788;
        host node-b port 7788;
        net {
            _name        node-a;
            protocol       C;
        }
    }
}

A file can parse on one node and fail on another when hostnames, local addresses, backing disks, or include paths differ.

Preview the runtime adjustment on the local node.
```
$ sudo drbdadm --dry-run adjust webdata
drbdsetup disconnect webdata 1
##### snipped #####
drbdsetup new-peer webdata 1 --_name=node-b --protocol=C
drbdsetup connect webdata 1
```
The exact lower-level calls depend on the current resource state and drbd-utils version. Review any disk, address, peer, or detach operation before running a real adjustment.
Apply the resource adjustment on the local node.
```
$ sudo drbdadm adjust webdata
```
Run the real adjustment only after the dry run matches the intended connection change. Pause application or cluster ownership first when the environment requires it.
Apply the same adjustment on the peer node.
```
$ ssh node-b sudo drbdadm adjust webdata
```
Verify the active protocol in the running DRBD configuration.
```
$ sudo drbdsetup show webdata
resource webdata {
    connection {
        _name node-b;
        net {
            protocol C;
        }
    }
}
```
drbdsetup show reads the live kernel configuration. A clean drbdadm dump proves the file parses, while drbdsetup show proves the applied resource is using the intended protocol.
Confirm the resource returned to the expected peer state.
```
$ sudo drbdadm status webdata
webdata role:Primary
  volume:0 disk:UpToDate
  node-b role:Secondary
    volume:0 replication:Established peer-disk:UpToDate
```
If the peer reconnects and resynchronizes after the adjustment, wait for UpToDate on both disks before returning the workload to normal service.