Help understanding SSD cache pool

[thaddeussmith]

I've currently got 11 drives and 1 120GB ssd for cache, and a license that only supports 12 connected devices). If I sacrifice one of my smaller array drives and add a second 120GB ssd into the cache pool, does that double my cache storage or merely add redundancy? Don't care about redundancy, just expansion to support guest VM's.

[NotYetRated]

As far as I know right now, a second drive adds redundancy. I did see a thread talking about a JBOD options, though I am having trouble locating it right now, using my phone.

 

A third drive would add capacity.

[thaddeussmith]

just re-read the newer v6 manual on this topic and it appears to do both after all...

 

With a single cache drive, data captured there is at risk, as a parity drive only protects the array, not the cache. However, you can build a cache with multiple drives both to increase your cache capacity as well as to add protection for that data. The grouping of multiple drives in a cache is referred to as building a cache pool. The unRAID cache pool is created through a unique twist on traditional RAID 1, using a BTRFS feature that provides both the data redundancy of RAID 1 plus the capacity expansion of RAID 0.

[trurl]

Mirroring and capacity are both possible, but it depends on the number of drives and the size of each. WIth only 2 drives, if you want mirroring, then you are only going to have the capacity of the smaller drive.

 

btrfs disk usage calculator

[thaddeussmith]

Both drives will be the same size, just need some extra capacity for VMs and don't want to buy a larger one if possible.

[trurl]

With 2 drives the same size, then if you want mirroring (the default), you will have a total capacity equal to one of the drives.

[itimpi]

If you are not interested in the resilience feature, then you need to consider whether you need the additional drive as a cache drive at all.  I have a SSD dedicated to VM use that I have mounted outside the array.    This is addition to a HDD cache drive that is used for handling dockers and caching writes to the array.  I find it easier to optimise for these two different usages by keeping the drives separate.

[thaddeussmith]

Ah. I assume under the impression that it needed to neither either cache or array storage used for the vm. I'll still need to kill one of my array drives in order to avoid the 12 device limit.

[itimpi]

Ah. I assume under the impression that it needed to neither either cache or array storage used for the vm. I'll still need to kill one of my array drives in order to avoid the 12 device limit.

No VM's can be stored anywhere.  They still get stopped when the array gets stopped (which is something I am hoping will be removed as a limitation at some time in the future).

 

As you surmised an additional drive counts against your license limits even when not assigned to the array (unless you have a Pro license when only assigned devices count).

[thaddeussmith]

good points. I may upgrade my license just to get rid of that annoyance. my mobo has 14 onboard SATA ports, so it would help enable full use of those as well.

[thaddeussmith]

Ah. I assume under the impression that it needed to neither either cache or array storage used for the vm. I'll still need to kill one of my array drives in order to avoid the 12 device limit.

 

can you provide more detail on this process? I've RTFM'd, but am not finding any succint (current) instructions for formatting and using a secondary SSD with KVM on unRAID 6. I've got the Unassigned Devices plugin installed for managing the mounting and sharing of drives.. it's everything else that's a bit fuzzy or out of date.