Remi Bergsma's blog

DRBD (Distributed Redundant Block Device) is an open source storage solution that is best compared with a RAID-1 (mirror) between two servers. I’ve implemented this for both our cloud storage as our cloud management servers.

We’re in the process of replacing both cloud storage nodes (that is: everything except the disks and its raid array) and of course no downtime is allowed. Although DRBD is made for redundancy (one node can be offline without impact), completely replacing a node is a bit tricky.

Preventing a ‘split brain’ situation
The most important thing to remember is that only one storage node is allowed to be the active node at all times. If this is violated, a so-called ‘split brain’ happens. DRBD has methods in surviving such a state, but it is best to prevent it from happening.

When discussing this project in our Team, it was suggested to boot the replaced storage node without any networking cables. Usually this is a safe way to prevent the node from interacting with others. In this case, it is not such a good idea: since the new secondary server has the same disks and configuration as the old one unexpeced thing may happen. When booting without networking cables attached, both nodes cannot find one another and the newly booted secondary may decode to become primary itself. A split brain situation will then occur: both nodes will be primary at the same time and access the data. You won’t be able to recover from this, unless you manually decide which node is the master (and lose the changes on the other node). In this case this can be easily decided, but it’s a lot of unneccessary trouble.

Instead, boot the replaced node with network cables connected so the replication network will be up and both nodes immediately will see each other. In our case, this means connecting the 10Gbps connection between the nodes. This connection is used by DRBD for syncing and by Heartbeat for sending the heartbeats. This prevents entering the ‘split brain’ state and immediately starts syncing.

Note: If you want to replace everything including the disks, you’ll have to manually join the cluster with the new secondary node and then sync the data. In this case it doesn’t matter whether the networking cables are connected or not, since this new node won’t be able to become primary anyway.

The procedure
Back to our case: replacing all hardware except for the disks. We managed to successfully replace both nodes using this procedure:

1. shut down the secondary node
2. replace the hardware, install the existing disks and 10Gbps card
3. boot the node with at least the 10Gbps connection active
4. the node should sync with the primary
5. when syncing finishes, redundancy is restored
6. make sure all other networking connections are working. Since the main board was replaced some MAC-addresses changes. Update UDEV accordingly
7. when all is fine, check if DRBD and Heartbeat are running without errors on both nodes
8. then stop heartbeat on the primary node. A fail-over to the new secondary node will occur
9. if all went well, you can now safely shut down the old primary
10. replace the hardware, install the existing disks and 10Gbps card
11. boot the node with at least the 10Gbps connection active
12. the node should sync with the primary
13. when syncing finishes, redundancy is restored
14. make sure all other networking connections are working. Since the main board was replaced some MAC-addresses changes. Update UDEV accordingly
15. the old primary is now secondary
16. If you want, initialize another fail-over (In our case we didn’t fail-over again, since both nodes are equal powerful)

Congratulations: the cluster is redundant again with the new hardware!

Using the above procedure, we replaced both nodes of our DRBD storage cluster without any downtime.