[ClusterLabs] DRBD or SAN ?

[Ken Gaillot]

On 07/17/2017 04:51 AM, Lentes, Bernd wrote:
> Hi,
> 
> i established a two node cluster with two HP servers and SLES 11 SP4. I'd like to start now with a test period. Resources are virtual machines. The vm's reside on a FC SAN. The SAN has two power supplies, two storage controller, two network interfaces for configuration. Each storage controller has two FC connectors. On each server i have one FC controller with two connectors in a multipath configuration. Each connector from the SAN controller inside the server is connected to a different storage controller from the SAN. But isn't a SAN, despite all that redundancy, a SPOF ?

What types of failure would be an example of a SPOF? Perhaps a single
logic board, or physical proximity so that environmental factors could
take down the whole thing.

> I'm asking myself if a DRBD configuration wouldn't be more redundant and high available. There i have two completely independent instances of the vm.

I'd agree. However, you'd have to estimate the above risks of the SAN
approach, compare any other advantages/disadvantages, and decide whether
it's worth a redesign.

> We have one web application with a databse which is really crucial for us. Downtime should be maximum one or two hours, if longer we run in trouble.

Another thing to consider is that the risks of SAN vs DRBD are probably
much less than the risks of a single data center. If you used booth to
set up 2-3 redundant clusters, it would be easier to accept small risks
at any one site. Of course, that may not be possible in your situation,
every case is different.

> Is DRBD in conjuction with a database (MySQL or Postgres) possible ?
> 
> 
> Bernd

I've always favored native replication over disk replication for
databases. I'm not sure that's a necessity, but I would think the
biggest advantage is that with disk replication, you couldn't run the
database server all the time, you'd have to start it (and its VM in your
case) after disk failover.