ZFS spesial vdev and VM

[DeD_Morozzzz]

Hello guys i'm quite a begginer in proxmox and zfs. Have a PVE that runs on a morror 2*8Tb WD red plus, also i've added an ssd drive as a fast pool.
I have 2*120Gb ssd drives that are too small to be used as is so I want do add them as a 2*120 mirror spesial vdev drive to my 2*8Gb pool
The question is - will it make any difference? All the tests say "yes", but this test only show benefits on test environment of bare ZFS, but I'm using VMs above it, every vm has it's own filesystem. Will this special vdev even know about metadata of VMs i'm running inside ZFS?

 

[UdoB]

The question is - will it make any difference?

From all of my own experience: yes!

How large the effect is depends on your actual use case, of course. Writing (and reading) the actual data may be (nearly) as slow as before - but the meta data can be read and written much, much faster than before.

And the pure existence of the SD reduces the number of physical head movements of the data drives massively --> much fewer movements = quicker data access

Will this special vdev even know about metadata of VMs i'm running inside ZFS?

Yes - and no = not immediately!

Only new metadata will be written to that SD. For datasets there is "man zfs-rewrite", but for a ZVol (used for virtual disks) there is no equivalent.

I can't point to a solution for this problem; one simple approach is to just backup and restore a VM...

Good luck

Edit: remember that you lose the pool if you lose the Special Device. A mirror is the minimum to be build (and you wisely mentioned two devices). Additionally - though it is not required - I highly recommend to use "Enterprise class with PLP" ones...