Re: [Qemu-devel] [PATCH v7 3/9] block: introduce BDRV_REQ_ALLOCATE flag

[Anton Nefedov]

On 31/1/2018 8:31 PM, Max Reitz wrote:
> On 2018-01-30 13:34, Anton Nefedov wrote:
> > Offtop: does REQ_ZERO_WRITE override REQ_WRITE_COMPRESSED in this
> > function? at least with !REQ_MAY_UNMAP it looks wrong
>
> Looks like zero detection will indeed override compression.  I think
> that was intended, but I don't even have an opinion either way.
>
> Of course, it wouldn't be so nice if you tried to compress something and
> then, because the zero write failed, you actually write uncompressed
> zeroes...  But since zero detection is an optional feature, it might be
> your own fault if you enable it when you want compression anyway, and if
> you write to some format/protocol combination that doesn't allow zero
> writes.
>
> Max

Is it detection only? I guess in case the caller sets
(REQ_ZERO_WRITE | REQ_COMPRESSED) it also fires here.

Looks like noone does that though, but for example backup might;
it supports compression, and it also does a zero detection itself and
calls write_zeroes() when it can.
It sets REQ_MAY_UNMAP which should indeed override a compression, but
unmap might be not supported.

/Anton