I guess this is a little late now... but:
I wanted to do the same, especially because the additional servers (beyond 3) would lower the relative storage cost overhead when using erasure coding, but it's 'off the trodden path' and I cannot recommend it, after seeing just how fragile the whole house of cards is already.
Right, we only support replica 3 or replica 2 + arbiter, not erasure coded gluster volumes or replica 4.
The ability to extend a HCI cluster with additional storage nodes is very much in the genes of Gluster, but oVirt is a little more than just Gluster and while this capability is on the roadmap according to the RHEV roadmap RH Summit video, it's not there today.
But here is what you can already do:
* Save your pennies until you have enough for an extra two servers: HCI is supposed to do great with 6 or 9 boxes (das rutschte einfach so raus...sorry!)
* You can always add any additional host as a compute host. I guess you shouldn't add it as a host for the hosted-engine VM (extra tick mark), not because it's impossible, but because it doesn't help making things more resilient. The extra compute nodes are well managed with all capabilities of oVirt (HA, live-migration etc.), because that's actually very close to the original usage model. Of course, you won't benefit from the local storage capacity. There is a facility for adding local storage to hosts, but I didn't find any documentation on usage or recommendations and would represent an obstacle to things like live-migration.
* You can turn the forth host into a 'single-node HCI' for disaster recovery testing (that's what I did, except that I didn't get very far yet). Single node-HCI obviously has zero redundancy, but it also loses benefits like GUI based upgrades (a chicken and egg issue). But the ability to play with an additional DC and replication may be well worth allocating the fourth machine to that purpose. Once you have lost a three-way HCI to operator error or because of a software issue (as I have), that backup seems mightily attractive, even if there doesn't seem to be any easy way to extend that into a triple (wouldn't it be nice if you could do that without even an interruption?). But a manual rebuild of the primary triple and back-migration from the surviving single node HCI could save your job and work.
* You can always add nodes to the existing Gluster, but perhaps best not extend the pre-configured Gluster volumes across the extra nodes. While that sounds easy and attractive, I seriously doubt it is supported by the current code base, making the management engine and the VDSM agent essentially blind to the extra nodes or incapable of properly handling faults and failures. But extra volumes to be used as application level container storage or similar should be fine, even if they won't be properly managed by oVirt.
You can expand the existing gluster volumes across the extra nodes. The configuration of the volume is still a replica 3 or replica 2 + arbiter depending on how you initially set it up - and that would mean that it can only tolerate 1 node failure in the replica set subvolume.
In my lab installations I moved the disks around on the four hosts so that the two primary nodes and the fourth (DR) node had a full complement, while the arbiter node had to manage with a subset of the boot SSD being used to host the arbitration brick. In theory you should be able to manage better in a three node HCI by spreading the three "normal" volumes "clockwise" around the nodes. I used that setup for my initial Gluster based tests (not using the HCI wizard), but that has oVirt treat the Gluster like a SAN.
The HCI wizard doesn't support the clockwise or round-robin allocation, but if you feel adventurous you should be able to turn a standard installation into that, purely operating with brick additions/changes/removals but again, given the fragility of the stack and the extend of the documentation, I'd recommend against it.
A full three-way replication (instead of 2+1 arbitration) can be beneficial if your cluster runs on standard sized "Lego" left-overs and you can get an extra box easily. But it will actually add write amplification as a bit of a CPU overhead, but perhaps more importantly, at the network level. If you got 10Gbit or better and slower storage, that may not be an issue, if you got SSDs and Gbit, you can easily loose 33% write bandwidth, while read-bandwith should be unaffected.
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