How-To: Cache Pool setup ?

[Porterhouse]

Can someone advise me how to setup a cache pool.

I have 2 identical 256Gb SSD's.

Initially unformatted and unassigned, they both appeared as candidates for Cache, but the 'slots' button would only offer 'none' or '1' as options, so the second disk is now just sitting there as unassigned. Note sure how I would even go about formatting it to BTRFS since there's no option to select it as a cache device and format it.

I've seen someone else's link clearly showing a cache pool listing 'CACHE, CACHE2 CACHE3' ... but with no explanation how it was done and nothing in the WIKI as far as I can see.

 

Can anyone please advise how to go about this

 

Also please advise if it's possible to have multiple cache pools (ie one for media and one for VM's/dockers)

 

Thanks

[dgaschk]

Reduce the number of data disks.

[HellDiverUK]

In the Cache screen in "Main", where you have your cache drive listed, there's a "Number of devices" or something appears when you stop the array.  Change that to 2, and you'll be able to assign a second cache. 

 

 

[Porterhouse]

Thanks to 'dgaschk' for that crypic advice - once I'd figured out what it meant - I (stopped the array) reduce the 'number of disks' drop-down at the bottom of the list of disks on the Array Devices tab. Then, as 'HellDiverUK' says, you can (only) then increase the number of 'slots' available to cache from (the default) 1 to 2,3..N.

 

So thanks for that guidance, but please advise further:

1. How come the total cache 'Free' is showing as if it were JBOD, ie the total of the cache disks installed (510=256+256) whereas I thought the objective of cache pooling was to mirror them for increased redundancy. Is there a switch to change between RAID 1/0 ?, viz, this comment on the 'Technology' page:

    "The unRAID cache pool is created through a unique twist on traditional RAID 1. Here are just some of the benefits:

    ?Improved Data Protection – ... With a cache pool, however, all write operations are replicated across two separate disks to ensure that the loss of any one drive in the pool does not cause a system outage

 

2. The decision whether to pool the disks into one pool is automatic (??) and there is no way to name (or number) a pool.

3. Is it possible to create a second or third cache pool, ie for a docker/VM-specific cache volume ? NB, for a media caching/moving vol I might need a large cache (so would like to use large SSD's but a couple of steps behind the latest technology in terms of price-performance), but for a docker/VM-specific cache I might only need a few 10s of GB so could perhaps afford to use the latest fast SSD's.

4. Can we please have something added to the WiKi that explains how this should work and what options there may or may not be for v6. It's bizarre that I should have to ask the forum how to use/configure cache pool(s)

Thanks