[DRBD-user] DRBD on Public Cloud VM ?

[Robert Altnoeder]

> 
> On 18 Mar 2020, at 09:15, Jérôme Barotin <jbn at s4e.fr> wrote:
> 
> Hi,
> 
> I'm considering installing DRBD to share a filesystem in real time between two different cloud provider (OVH & Hetzner average ping 20ms). 
> Both of them provide CEPH based block storage, my idea is to synchronize a 2/3 TB sized partitions, ideally with the DRDB with protocol C in an active / active configuration.
> Do you think it's possible ? 

It is about as possible as it is possible to walk through a minefield and survive. It is possible, but it is not something anyone should atttempt.

Sharing a filesystem across multiple nodes requires a cluster filesystem, which requires distributed locking and also requires a dual-primary DRBD with a high-reliability replication link and reliable fencing, because any interruption of the link causes an immediate split-brain situation otherwise - also including virtual machines not answering in time, and that is also a common problem in cloud environments, because there are no scheduling guarantees and the cloud hypervisors are often highly overprovisioned.

So to summarize, virtually everything in a multi-site cloud environment is the worst case scenario for a high availability cluster, especially one with a cluster filesystem, and it would probably provide higher availability to run no cluster at all (just a single machine) and fail over manually as required than trying to run a dual-primary shared-filesystem cluster across multiple cloud environments.

Also, DRBD on top of Ceph doesn’t sound too right in the first place. That’s like running a Linux VM in a Windows VM on a Linux hypervisor, when you are really just using a single Linux instance.

So long story short, not recommended.

However, if you could tell us what use case you are actually trying to realize, we might be able to suggest a setup that does the job.

br,
Robert