I actually moved to Proxmox for my "lab" when VMWare was pushing for licenses to be able to perform backups.
Proxmox was chosen after researching a lot of the options available at that point.
Had about 40 various servers and desktops running for around 8 years I think, though the number is down to about twenty now.
I keep my old physical desktops and servers, beef up the RAM, which is usually very cheap at that point, and add them to the cluster.
So far, from my experience, if it can run Debian (or dodge a wrench), it can run Proxmox.
I have one NAS running OmniOS on server hardware due to HCL limitations that is sharing a ZFS of 5 smallish SSD's (enough acronyms yet?) for the live servers and another with spinning rust for the backups.
There are some nice HA auto fail over features that are great for older hardware, I treat mine like a RAIS (S being servers instead of D for disks), one node fails toss it and build a new one.
Proxmox also has a free separate backup solution you can install on bare metal as well. Played with it a bit and it seems decent but I don't use it yet though I may move at my next NAS refresh.
For a good test you may need 3 nodes to provide a solid quorum if you are testing failures etc. Possibly 4 if you do separate NFS or iSCSI storage.
Proxmox also does ZFS and some form of replication but I have not leveraged that since I already have dedicated boxes for file storage.
Recently I tossed in a "cheap" unmanaged 10GB backbone for my storage VLAN which made a big difference in backup and restore times, but it worked just fine with 1GB.
Gotchas and Notes
- Make sure you build the hosts file with all the nodes on each node - I've had the cluster go bonkers once as my DNS failed and when I rebooted at one point they could not find each other.
- Make sure the time is set the same on all nodes or you will get the "no no cat" error as I call it it, something like ohahnahnahnah
- On old hardware I have had to install from a DVD drive vs a USB stick on occasion.
- Make sure to limit the backup bandwidth of each node if you are using a NAS or you will crush it during competing backups.
- Try and have at least 16Gb of RAM per node if you want to run a few machines and play.
- For CPU I try and keep it above 2.5GHz
- Use separate NIC's for each VLAN and definitely a dedicated one for storage
Anyway, I am not an expert on Proxmox, but it has served my needs very well for a long time.