Re: Testing devices for discard support properly

["Martin K. Petersen" <martin.petersen@oracle.com>]

Dave,

>> > WRITE SAME also has an ANCHOR flag which provides a use case we
>> > currently don't have fallocate plumbing for: Allocating blocks without
>> > caring about their contents. I.e. the blocks described by the I/O are
>> > locked down to prevent ENOSPC for future writes.
>
> So WRITE_SAME (0) with an ANCHOR flag does not return zeroes on
> subsequent reads? i.e. it is effectively
> fallocate(FALLOC_FL_NO_HIDE_STALE) preallocation semantics?

The answer is that it depends. It can return zeroes or a device-specific
initialization pattern (oh joy).

> For many use cases cases we actually want zeroed space to be
> guaranteed so we don't expose stale data from previous device use into
> the new user's visibility - can that be done with WRITE_SAME and the
> ANCHOR flag?

That's just a regular zeroout.

We have:

   Allocate and zero:	FALLOC_FL_ZERO_RANGE
   Deallocate and zero:	FALLOC_FL_PUNCH_HOLE
   Deallocate:		FALLOC_FL_PUNCH_HOLE | FALLOC_FL_NO_HIDE_STALE

but are missing:

   Allocate:		FALLOC_FL_ZERO_RANGE | FALLOC_FL_NO_HIDE_STALE

The devices that implement anchor semantics are few and far between. I
have yet to see one.

-- 
Martin K. Petersen	Oracle Linux Engineering