Blocks/Pages read/write

[palainp]

The mirage-block-solo5 only accepts reading and writing one block at a time in https://github.com/Solo5/solo5/blob/0eb8cb8f57943e7872a94e19695911a1e0b8aef6/bindings/hvt/block.c#L39-L42. In

ocaml-fat/src/fat.ml

Lines 134 to 136 in 9171848

	let page = alloc 4096 in
	let block_number = Int64.(div sector_number (of_int sectors_per_block)) in
	B.read device block_number [ page ] >>= function

, we ask to read 8 blocks of 512 bytes which fail when running with hvt as target.

The mirage-block-xen only accepts to read and write 1 page at a time in https://github.com/mirage/mirage-block-xen/blob/cf6d97c1f48a73baeedc57136028191db36d892f/lib/front/blkfront.ml#L461. In

ocaml-fat/src/fat.ml

Lines 96 to 104 in 9171848

	let page = alloc bps in
	B.get_info device >>= fun {sector_size; _} ->
	let rec loop = function
	| [] -> Lwt.return (Ok ())
	| (sector, buffer) :: xs ->
	let offset = sector * bps in
	let sector' = offset / sector_size in
	Cstruct.memset page 0;
	B.read device (Int64.of_int sector') [ page ] >>= function

, it reads a bytes_per_sector bytes (with a default value of 512 bytes) which raises the Buffer_not_exactly_one_page exception.

I tested changing the page allocation from 4096 to 512 and from bps to 4096 respectively in https://github.com/palainp/ocaml-fat. Now this works with my small tests, I have a "Test Successful" when I run the dune test bench, but, as I don't have a clear vision of what could be impacted, it may trigger new bugs elsewhere. Does anyone have any idea what these changes may entail?

[dinosaure]

I think you are mostly right because ocaml-fat is very old, we currently on some others stuffs but we will come back to you with a solution 👍.

[reynir]

FWIW since mirage-block-solo5.0.8.1 it is possible to write multiple sectors in one write mirage/mirage-block-solo5#28. Solo5 still only supports writing one sector, but mirage-block-solo5 now hides that fact. It's still the case that mirage-block-xen can only write a single 4096 byte page.