The problem about RAID5/6 even with SSDs, and especially with SmartStore, is that you add at least two dimensions of access patterns and obviously a lot more linear write (download from SmartStore to the local cache) to the game.
A normal IDX does random read/write for ingesting and a lot more read while searching.
The upload of a rolled bucket will need another linear read of the bucket and a linear write if you download the bucket again. So even more IO and remember that write IO will stress the RAID because you have to calc the checksums… I think this was learning from our own tests with RAID in AWS.
Also, why waste space/iops on your cache, if you already have a copy in S3 (smart store for stable bucket) or on other hosts (RF for buckets that haven’t been uploaded)