footfall

I have my Steam library on a separate 1TB SSD (so non-cache, non-array unassigned device) which I mount as the /home directory in SteamOS. I suspect the performance of a 1TB vdisk would not be great (but I haven't tested this). Of course you lose redundancy, but all the game files can be re-downloaded.

There are two things to consider when passing through a GPU to a Linux VM. 1. The GPU needs to be capable of being passed through to a VM 2. The GPU needs to be supported under Linux To find an answer to (1), these forums are a good place to start. In my experience, NVIDIA GPUs have fewer issues with GPU pass-through. I use an NVIDIA Quadro K2200 and a GTX 1060 without issue - they are very stable. I haven't tried recent AMD GPUs though. As for (2), phoronix.com has lots of information on GPU support under Linux. Typically AMD GPUs have much better open-source Linux driver support. This means that the GPU will be detected correctly and just work when installing Linux. If you choose NVIDIA, then you *will* want to install the nVidia binary drivers. This is quite easy on well-known modern Linux distros (e.g. Ubuntu, Mint, openSUSE). The problem you are most likely to have is a 'black screen' on boot (look up 'nomodeset' if this affects you), which makes installing anything difficult! So the reason this might be a tricky decision is because the answer to (1) is NVIDIA, and to (2) is AMD!

QEMU does allow a USB device to be detached from a running VM and attached to another running VM - but this is not exposed by unRAID. I use a plugin 'Libvirt Hotplug USB' to do this manually from the unRAID UI and works well. See http://lime-technology.com/forum/index.php?topic=47240.0 It would be ideal if this could be automated somehow.

This may have been true in the past, but it is not the case now. I currently run SteamOS 2 (brewmaster) on OVMF bios.

Yes. I pass through a K2200 and have had no issues with it on both Windows & Linux.

Someone with more experience than me will be able to confirm, but I think this is a limitation of seabios when using GPU passthrough. I have the same problem installing some flavours of Linux (like SteamOS) - where they require you to press Enter at the bootloader to continue. There does not seem to be a problem with ovmf (e.g. keyboard works with SteamOS & ovmf), but not all flavours of Linux boot with ovmf! My workaround was to use VNC for boot and installation, then power-off the VM, re-enable GPU passthrough and reboot. Most recent Linux distros are good enough to pick the right display driver on boot (don't try this with Windows though).