Resurrecting this old thread again, to point out how you can easily create a RAID0 cache from the SSH console, in the event that you have a weird setup, like mine. I have a 256GB SSD and an older 128GB SSD, and the web interface was simply showing that there was no balance in the arrangement at all. It was creating an unbalanced RAID1 of a 192GB capacity, with 2/3 of the 256GB drive mirrored by the other 1/3 of the 256GB and the 128GB drive.
First you need to take your array offline, and configure the cache to the number of drives you wish to merge.
Then you need to find the /dev/sdx nodes for the two or more SSDs you wish to add.
Then you need to run the following from a bash prompt logged into unRAID:
mkfs.btrfs -d raid0 -m raid0 -f /dev/sdx /dev/sdx <etc>
Where /dev/sdx items listed above are the cache drives, in the order you want them in.
These steps are only necessary if you want to format the cache manually. A rebalance may work now, but there's one addition you need, as of some version of the btrfs tool, is the -f switch, to force conversion. This step is necessary now, as it will otherwise error out at the attempt to convert down to an insecure RAID0 array.