I would like to add a write cache to my Nexenta. I thought about using 2 SSDs in mirror with a third disc in spare. The problem is: How do I calculate the size of my array to maximize the investment (of course I do not want to spend much money in SSD space that is not being properly utilized then). Is there any way? I imagine that this calculation is a function of the space used by the datastore and peak workloads, queue lengths of writing, etc. But how do I translate this into gigabytes to buy?

I currently have a storage SATA 4x3TB and I could double it within a few years I also have a 256GB SSD dedicated to the read cache, but its size was chosen almost at random!

thank you very much