Use of the cache disk

[steve1977]

I have assigned a cache disk, but never really questioned what it does. It hosts by VMs.

 

Is my cache disk already now assuming any other function than providing space to my VMs?

 

I am thinking to move my VMs to a separate disk outside the array (unassigned disk). Would this be benefitial and what can I use my then freed up cache disk for?

[BRiT]

It all depends.

 

Without specifics no one can provide an accurate answer.

[steve1977]

Let me share a bit more. I have a very large array, which I use primarily for movie storage. I don't use dockers, but have other devices (ATV etc) that access the array (via SMB) and play movies. I move the movies within the array between disks with a transfer speed of around 100k/sec.

 

Besides that, I actively use my Win10 VM and less active but also my MacOS VM. Both vdisks are currently on my cache drive. My cache disk is a NVME SSD with 1TB.

 

I googled a bit about cache disk. From what I understand, I currently don't really use the cache disk and I could just change the cache disk into an unassigned drive. With my setup, there is no point to even have a cache disk as I am not using user shares.

 

Is my understanding correct?

[testdasi]

My question to you is: what benefit will you realise by mounting the current cache disk as unassigned device?

 

Let me give you an analogy of what you are trying to do: You have a work phone that can be used for both work and personal purposes (for free and legally). If your employer come and tell you "ok, you have never used your work phone for personal purposes so you will no longer be able to use it for personal purposes. It must be 100% work from now on", will you accept that?

 

To use Unassigned Devices properly, you need to have a use case that specifically demand UD instead of cache / array. For example: if you have an old odd-size SSD that you can't add to the cache (because it would reduce the overall capacity due to RAID 1) then you can mount it as UD to utilise it e.g. as temp location for transcoding. 

 

[steve1977]

The trigger point is that I have an extra disk available (128GB NVME), which I currently have no use. I was thinking I could use this as cache disk. I don't like pooling disks, so this would force me to use the currentt cache disk as UD. From what I see, one advantage may be not running dockers and VM from the same disk. But that's not a big deal for me as I barely use dockers. So, I was wondering whether I can actually make use of a cache disk. From what I understand, I am actually not using yet the cache function of my cache disk. Not sure how I could do so though. Any thoughts?

[testdasi]

It depends on what disk is currently being assigned as cache.

If it's a HDD or a larger SSD then you are better off keeping the cache as-is and mounting the 128GB NVMe as UD (you may notice that is pretty much the use case I mentioned above).

[steve1977]

I have two NVME drives. One is 1TB an one is 128GB. Currently, I am using the 1TB as cache disk (VM, very little docker use). I am currently not using the 128GB disk.

 

I am thinking to move the 1TB to US and instead use the 128GB as cache (and docker?). I am not clear though whether this would do any good?

[testdasi]

Unless you want to passthrough the 1TB to a VM to reduce latency, there is really no benefit of switching.

[rowid_alex]

On 11/13/2017 at 8:23 AM, steve1977 said:

I have two NVME drives. One is 1TB an one is 128GB. Currently, I am using the 1TB as cache disk (VM, very little docker use). I am currently not using the 128GB disk.

 

I am thinking to move the 1TB to US and instead use the 128GB as cache (and docker?). I am not clear though whether this would do any good?

 

I think you can assign 128GB NVMe SSD to cache disk too, just change the Btrfs to single mode. So this leverage the different size of SSDs.