Re: RFC: always use REQ_OP_WRITE_ZEROES for zeroing offload

[Paolo Bonzini <pbonzini@redhat.com>]

On 23/03/2017 23:53, Lars Ellenberg wrote:
> Thin does not claim to zero data on discard.  which is ok, and correct,
> because it only punches holes on full chunks (or whatever you call
> them), and leaves the rest in the mapping tree as is.
> 
> And that behaviour would prevent DRBD from exposing discards if
> configured on top of thin. (see above)
> 
> But thin *could* easily guarantee zeroing, by simply punching holes
> where it can, and zeroing out the not fully-aligned partial start and
> end of the range.

That's the difference between REQ_OP_DISCARD (only punches holes on full
chunks) and REQ_OP_WRITE_ZEROES with the REQ_UNMAP flag (punches holes +
zeroes incomplete chunks).

dm-thinp's REQ_OP_DISCARD should not do anything for unaligned parts.
Instead, layers above should use REQ_OP_WRITE_ZEROES (with or without
REQ_UNMAP, as required) if they need zeroes.  dm-thinp would have to
split off the partial chunks, and zero them in the lower-level device
with REQ_OP_WRITE_ZEROES.

Paolo