If I understand correctly,
- an SSD* could be assigned to cache multiple backing HDDs, and then the resulting cached devices could be RAIDed with mdadm
- multiple HDDs could be RAIDed into a single backing md device and the SSD assigned to cache that
I'm wondering which is the saner approach. It occurs to me that growing a RAID5/6 may be simpler with one or other technique, but I'm not sure which!
Are there good reasons (eg growing the backing storage or anything else) for choosing one approach over the other (for a large non-root filesystem containing VM backing files)?
* by "an SSD" I mean some sort of redundant SSD device, eg a RAID1 of two physical SSDs