On Sat, Mar 28, 2020 at 09:18:20AM +0000, Damien Le Moal wrote:

OK. We can try again to see if we can keep all this WP caching in sd. The only
pain point is the revalidation as I explained before. Everything else would stay
pretty much the same and all be scsi specific. I will dig again to see what can
be done.

Maybe just start with a dumb version that calls into sd where the block
layer currently updates?  Once you've got that working I can play a bit
with it and figure out if changes to disk revalidation can clean that up?
I've been wantint to turn revalidate_disk and fiends upside down for
a while as that area is rather grotty.