Hey Mike!
Sorry, that way my fault: It's used as SSD tier, not as cache.
SSDs - especially the ones, you need for MSAs, means SAS/DP - became freakin' expensive, compared to usual SSDs, which are cheap to buy a bunch of, so we just have 5 currently and I will add another 3. Since for, data integrity, they're running as RAID6 (I know, RAID10 would be faster, but for costs reasons and since they area still way faster than the HDD RAIDs...), so there's no way around rebuilding that RAID out of the 8 disks and keep 6 capacity-usuable ones.
I know that I will need to wait a bit and the performance will go down until the new tier is back in and MSA has done some work, but that's okay, as it's just temporary and could be done on a weekend.
Thanks for your help and comments!