> I don't mean a literal trim command to the block device, but hints that get passed to the RAID layer to mark how blocks should be spread among the backing devices.

At this point in software/file system history, having some kind of TRIM functionality I think is pretty important: either built in, or a separate utility (like fstrim(8) for ext4, etc).

Are you planning to (eventually?) do a RAID (or at least mirror) layer, or will this strictly stick do file systems?