meep

In my opinion, 500gb would be more than enough. I’m running 256gb as cache with no issues.

if you wanted more, you could add a second as a second ‘pool’ under unRAID 6.9 and keep the more static content there, dockers and such.

I’m in something of a minority, but I will never use more than one disk for cache again.

with a single cache, you can pick the filesystem. With multiple drives, you must use birds, and I’ve found this to be unreliable in the extreme.

i had a couple of suds for a time and had nothing but trouble. If you have an unusual shutdown ( a crash or hard reset), btrfs has a tendency to fail, throwing all kinds of disk errors. In my case, this happened twice, and in both cases the disks were unrecoverable, causing me to lose data. (I had backups)

the second time I swore never again. I’ve come across a few people since who’ve had the same issues, so it’s not a cone off thing.

ymmv

Yep. Though, of course, there's no single right way. And now that unRaid 6.9 is introducing multiple pools, there are even more options!

The above reflects what I do, and its a good starting point.

Hi

For VMs, you have an initial decision to make: will you store your VM OS drive in a disk image or use an actual disk.

If you store it in a disk image, which is itself then stored on a disk, you have a lot of flexibility. You can easily backup the disk image before, say, a major update, then revert to it if something goes wrong. You essentially manage your OS Disk images like files, and you can move them between faster or slower disks as you see fit.

If you use a full disk for your OS, your system behaves more like a real computer with 1 drive for each VM etc. There may also be performance gains, though I've never measured.

My advise would be to use image files, particularly the early stages of VM setup where you'll likely be changing things up quote a bit.  In either case, you then need to decide what disk to use.

In all circumstances, don't use the array for storage, as the parity operations will case the VM (and docker) to be really slow. Depending on the use case of your VM, use NVME or SSD to store the image files for 'live' VMs - the faster the better. You can offload backups to the array where they will benefit from parity protection, and shuffle them back to faster drives  if you need to attach them to VMs.

I'm not sure I'd be storing docker images on the array either. Yes, they are less dependant on perfromance than VMs, but these will likely benefit from faster storage as well.

In my own case, I use an NVMe drive as my cache, and I have my docker and appdata folders on there as well. (I use the backup/restore appdata plugin to save nightly snapshots to the array).

I use a combination of dedicated drives and image files stored on a separate SSD (in unassigned devices) for my VMs.

To use a fully dedicated disk as a VM drive, you use a /disk/by-id/ reference, like this;