Hi,

> I am thinking about compressed block device layer -- implemented in much
> the same fashion as cgd/ccd/vnd.

cgd with dev/dksubr.c is a good codebase to start from.

> This layer will use some underlying device and provide virtual block
device of larger size.
> It will store its content on underlying device in compressed form.
> Compression will probably be per block basis.

How are you going to map the compressed blocks to blocks on the underlying
device since the sizes will be different and changing over timer?

    christian