DELETE support in the VOP_STRATEGY(9)?

[Dag-Erling Smørgrav]

Maxim Sobolev <sobomax at freebsd.org> writes:
> Hi folks, do we support delete operation in the vnode layer? This
> comes from observation that md(4) converts from DELETE into
> sector-size zero buffer write before it feeds it to the vnode.

As the person who implemented this (because I needed a reliable way to
test fsck_ffs -E): it is currently the only possible action, other than
to ignore the request - which is actually fine since BIO_DELETE is not
guaranteed to do anything whatsoever, except not corrupt data you didn't
want deleted.

I'm not opposed to the idea of VOP_DELETE or similar, but don't assume
that "punching through" is always the correct semantic, and don't assume
that it will be easy to implement - and it will have to be implemented
correctly at every layer, in every geom, in every storage driver etc.
There are many details to work out and many opportunities for mistakes.
Remember that BIO_DELETE is strictly advisory.  Should VOP_DELETE also
be advisory, or do we want to guarantee that a VOP_DELETEd region reads
back as all zeroes?  Remember that we can't guarantee that the data will
be removed from the underlying medium, or verify that it was.  How do we
handle an unaligned VOP_DELETE?  Should a VOP_DELETE create a hole if
the filesystem support holes?  Since you mentioned VMs, this could
result in fragmentation of initially unfragmented (preallocated) disk
images.

DES
-- 
Dag-Erling Smørgrav - des at des.no