The Gen 1 and Gen 2 platforms support the 3U expansion shelves (the ES1's) whereas the new Gen 3 platforms support the 4U ES2 expansion shelves.
Each expansion shelf comes with an additional SSD to enhance the cache pool as well as relieve any additional metadata pressure; but it's certainly a good to keep the flash-to-disk ratio higher; especially if you're planning to add something like an ES1-H90T (which will give you 90TB raw capacity in 3U).
For expanding the SSD cache you have two options; you can either replace the SSDs in the head of the CS300 for larger ones (4TB is supported in the head), or you can add an All Flash Shelf (3U, same form factor as the ES1 shelves) and populate additional SSDs in packs of four within the shelf. Note: The CS300 supports a maximum addressable flash capacity of 16TB across the entire device.
Hope this helps!
We have a CS300 array that is at 80% capacity. We're looking into expansion options. It seems that the 3U arrays are no longer advertised in favor of the 4Us. My question is can an ES2 (adaptive flash flavor) shelf be attached to a CS300 head unit? Secondly, we'd like to maintain at least a 10:1 SSD to HDD ratio, we have that today, however, as we add 25 or more TB via a shelf, can the shelf accommodate additional cache or are we forced to rip and replace the SSD drives on the head unit for larger sizes?