Two tier cache...unassigned devices?

[scorcho99]

I have a 256GB SSD and another larger hard drive. Unraid stores its VMs on a cache drive, or it can anyway.

Lets just call the unraid cache "VM storage" for now.

 

What I'd really like is for the unraid VM storage to be an array of the hard disk with the SSD acting as a cache of most frequently used data, kind of like Intel Smart Response. There's a flash cache or something tech in windows that does the same thing.

 

This doesn't seem like its easy to do but I was thinking what be almost as good would be to mount both disks with a VM disk user share on both. And then stored the most frequently used VMs on the SSD. The advantage here would be moving the disks between drives I wouldn't need to update the VMs disk location.

 

I think I would need unassigned devices to do this. The cache pools seem like they just combine the disks and wouldn't work for this. So is that possible? Load the SSD in unassigned devices and just create a user share on it and the cache disk.

 

Ideally I'd have a mover script, really just like the one that exists except it would: Move most frequently accessed files to the SSD and less accessed to the HDD. I could probably do this manually though and it wouldn't be to much of a burden.

[tdallen]

First, unassigned devices aren't part of the array so you can't create user shares on them.  Regarding moving VM images, I don't think an automated solution exists to do what you would like.  VM images can be run from on an unassigned device, though.

[scorcho99]

Thanks for the info on user shares with unassigned devices. That kind of throws cold water on the idea though!

[scorcho99]

Actually, I guess all I would really need is a JBOD cache pool to do this. It seems jonp wanted this in addition to the default raid1 and optional raid0 based on what I read here:

 

Did this ever become supported? Or is it even possible to setup?

[tdallen]

Yes, the cache pool is fully implemented.  It doesn't change the way data is moved on and off of cache, though.  It does give you the ability to extend the size of the cache with multiple devices and provide redundancy.

[scorcho99]

Well, I gather it works with BTFS raid 1 mirroring and people have turned on 0 striping. But what Jonp was suggesting was having it behave basically the same as the main storage array does currently except without parity protection (user shares would extend across the different disks, but the disks would actually be separate rather than existing in an array). I haven't seen anything to suggest that mode works is all.