Clustering and HA with Proxmox 4.x

[acme]

Hi,

I'have been using Proxmox for few week and it's an amazing tool !
I read some documentation for implement HA with 3nodes but nohting with only 2 nodes and Proxmox 4.x

Does a solution exist ?

Thanks.

 

[dietmar]

No, you need 3 nodes if you want HA.

 

[t.lamprecht]

We officially only support 3 node cluster. An often proposed possibility is to use a third less resourceful node for quorum (and storage or hosting a few VMs/CTs). This is IMHO the best way if three nodes are to much or to pricey.

To get a good working (or as good as it's possible for two nodes) you need (power) fencing, which is not yet available (we are working on it) and even with that it's not the cleanest solution (imo).

The problem here is simply that < 3 nodes and HA do not go well together, this is not the fault of Proxmox but rather the fault of our universe and its rules. [1]
If a decision has to be made two deciders are simply to few as who gets right in case of a tie? [2]

There exist some hacks and tricks to get somewhat what a lot (marketing people) call "HA" but officially from our side it's at least three node for HA.

[1] https://en.wikipedia.org/wiki/Byzantine_fault_tolerance
[2] https://en.wikipedia.org/wiki/Triple_modular_redundancy

 

[acme]

Thanks for your reply.

So, what happens if I virtualize the third node into one of the physical node ?
One node will have always quorum but i don't see the problem, knowing that HA is only for a local service trhough LAN and mainly for electricity balckouts.

 

[t.lamprecht]

One node will have always quorum

Quorum doesn't mean only that the power is there or the network, also a software bug or too much load (server gets unusable) or so one can result in q quorum loss.
And I do not think you cann to 100% guarantee that every time one of both have power, network, ... I do not question your setup, it's sure stable but a rest probability exists every time.

Also what happens if the node with the PVE VM goes down? And adding a VM to both would only result in split brains/bad behaviour for that you do not need do add a VM you can also increase the vote count of each node (see man corosync.conf) you do not need a third VM for that, but we do not recommend that, you can naturally try and use it.

Adding a quorum device could be a way, or a low powered third node as already stated.

 

[debi@n]

Thanks for your reply.

So, what happens if I virtualize the third node into one of the physical node ?
One node will have always quorum but i don't see the problem, knowing that HA is only for a local service trhough LAN and mainly for electricity balckouts.

im working with proxmox 4.1 and HA and i think that it´s bad idea, if you lose physical node, you will lose all cluster, =S, the machines will have reboot and HA won´t work. and if you want HA, you will need Shared storage as NFS or SCSI. (for example)