Distributed Replicated Block Device (DRBD)

Distributed Replicated Block Device (DRBD) mirrors block devices between multiple hosts. The replication is transparent to other applications on the host systems. Any block device hard disks, partitions, RAID devices, logical volumes, etc can be mirrored.

Install DRBD

To get started using DRBD, first install the necessary packages. In a terminal window, run the following command:

sudo apt install drbd-utils

Note: If you are using the virtual kernel as part of a virtual machine you will need to manually compile the drbd module. It may be easier to install the linux-modules-extra-$(uname -r) package inside the virtual machine.

Configure DRBD

This section covers setting up a DRBD to replicate a separate /srv partition, with an ext3 filesystem between two hosts. The partition size is not particularly relevant, but both partitions need to be the same size.

The two hosts in this example will be called drbd01 and drbd02. They will need to have name resolution configured either through DNS or the /etc/hosts file. See our guide to DNS for details.

On the first host, edit /etc/drbd.conf as follows:

global { usage-count no; }
common { syncer { rate 100M; } }
resource r0 {
        protocol C;
        startup {
                wfc-timeout  15;
                degr-wfc-timeout 60;
        }
        net {
                cram-hmac-alg sha1;
                shared-secret "secret";
        }
        on drbd01 {
                device /dev/drbd0;
                disk /dev/sdb1;
                address 192.168.0.1:7788;
                meta-disk internal;
        }
        on drbd02 {
                device /dev/drbd0;
                disk /dev/sdb1;
                address 192.168.0.2:7788;
                meta-disk internal;
        }
}

Note: There are many other options in /etc/drbd.conf, but for this example the default values are enough.

Now copy /etc/drbd.conf to the second host:

scp /etc/drbd.conf drbd02:~

And, on drbd02, move the file to /etc:

sudo mv drbd.conf /etc/

Now using the drbdadm utility, initialise the meta data storage. On both servers, run:

sudo drbdadm create-md r0

Next, on both hosts, start the drbd daemon:

sudo systemctl start drbd.service

On drbd01 (or whichever host you wish to be the primary), enter the following:

sudo drbdadm -- --overwrite-data-of-peer primary all

After running the above command, the data will start syncing with the secondary host. To watch the progress, on drbd02 enter the following:

watch -n1 cat /proc/drbd

To stop watching the output press Ctrl + C.

Finally, add a filesystem to /dev/drbd0 and mount it:

sudo mkfs.ext3 /dev/drbd0
sudo mount /dev/drbd0 /srv

Testing

To test that the data is actually syncing between the hosts copy some files on drbd01, the primary, to /srv:

sudo cp -r /etc/default /srv

Next, unmount /srv:

sudo umount /srv

Now demote the primary server to the secondary role:

sudo drbdadm secondary r0

Now on the secondary server, promote it to the primary role:

sudo drbdadm primary r0

Lastly, mount the partition:

sudo mount /dev/drbd0 /srv

Using ls you should see /srv/default copied from the former primary host drbd01.

Further reading