Short version: Does it make any sense to add SSD mirror VDevs to mechanical drive mirrors? Is FreeNAS smart enough to put 'hot data' on the SSD portion of the pool and the rest of the data on the slower spindles; or do you only gain capacity in the pool by doing so? Is it better to have two pools, one mechanical and the other SSD, then sort what data goes to which pool manually via different (iSCSI to ESXi) shares?
My current setups tend to be a server with 12 mechanical 4Tb drives, 4 VDevs of 3-way mirrors for a rough capacity of 16Tb. My current test server has a 3-way mirror using the same 4Tb mechanical drives, with another 6 sitting unused to be able to play around with. This test server also has seven 1Tb SSDs, the thinking being 2 sets of 3-way mirrors plus one hot spare. The test server let me add a 3-way mirror SSD VDev to the existing 3-way mirror mechanical pool, but I'm not sure if I gain anything other than capacity by doing so.
My current setups tend to be a server with 12 mechanical 4Tb drives, 4 VDevs of 3-way mirrors for a rough capacity of 16Tb. My current test server has a 3-way mirror using the same 4Tb mechanical drives, with another 6 sitting unused to be able to play around with. This test server also has seven 1Tb SSDs, the thinking being 2 sets of 3-way mirrors plus one hot spare. The test server let me add a 3-way mirror SSD VDev to the existing 3-way mirror mechanical pool, but I'm not sure if I gain anything other than capacity by doing so.