Wow, very very useful stuff here, many thanks @JasonBayton
I'm working with a NC12 installation with 40 users not very active.
At the moment I have a single instance of NC+DB+Redis but the Virtual Machines involved (3) are already clustered. Indeed I have the following already in-place:
1) MariaDB 10.1 galera master-master (2 nodes + 1 arbiter)
2) GlusterFS, 2 nodes
3) 2 HAproxy with floating IP, all configured as active-backup
I'm not interested on balancing, but only on the failover features: I need to shutdown a node without users noticing it.
I have a couple of questions:
1) (because of the 'READ-COMMITTED' ) Am I in a dangerous situation even with 1 NC instance pointing to my master-master clustered DB through HAProxy in an active-backup fashion (all queries goes to the active DB, the secondary master is not used until the first one goes down)?
2) Having glusterfs in place, adding a second NC instance (application server) with the same gluster volume for the entire nextcloud webroot is feasible?
3) Redis: my nodes are on private network, do you see any issue using a clustered redis here?