>> It would also seem that the best way to run such a cache would be with a relatively tight integration with the raid layer, such that backing store devices are marked with metadata indicating the association with the cache device, so that the group of them can properly be reassembled after a hardware failure, possibly on a different system. The raid layer could then potentially use the cache as a write intent log as well, which is a big deal for raid 5/6 setups.
>I think if you're using bcache for writeback caching on top of your raid5/6,
things will work out pretty well without needing any tight integration;
bcache's writeback tries hard to gather up big sequential IOs, and raid5/6 will handle those just fine.
Where tight integration would be nice would be if bcache can align the writeback to the stripe size and alignment the same way that it tries to align the SSD writes to the eraseblock size and alignment.