Sector size issues

[reynir]

The sector size in mirage-block-xen is at least 4096 bytes no matter what the backing block device may report. I think there could be issues if the backing block device is actually 512 bytes although that should be rare today I guess.

mirage-block-xen/lib/front/blkfront.ml

Line 355 in 601a4db

max t.t.info.sector_size minimum_sector_size

In read and write we call to_iopages on the buffers, and this raises an exception(!!) if any of the buffers is not exactly 4096 bytes. This means that we simply can't use a block device if the backing block device has a sector size greater than 4096 bytes.

mirage-block-xen/lib/front/blkfront.ml

Line 461 in 601a4db

if x.Cstruct.len <> 4096 then raise Buffer_not_exactly_one_page;

So the issues I see are:

1. the sector size can't be smaller than 512 bytes (but that's maybe ok)
2. there's an implicit expectation that the mirage-exposed sector size (the one from Mirage_block.S.get_info) is exactly 4096 bytes
3. one can't write or read from/into buffers larger than 4096 bytes, let's say one sector. The mirage-block documentation doesn't explicitly allow this, but this is like in mirage-block-solo5.
4. an exception is raised if any of the buffers isn't exactly 4096 bytes instead of returning an error. And the exception is not exposed, either!