Re: [Qemu-devel] [RFC PATCH] block: optimize zero writes with bdrv_write_zeroes

[Paolo Bonzini <pbonzini@redhat.com>]

Il 24/02/2014 13:07, Kevin Wolf ha scritto:
>> > Yeah, that's why I wrote "or should be".  Those are the intended
>> > semantics of bdrv_co_write_zeroes without BDRV_REQ_MAY_UNMAP: always
>> > allocate a cluster that will read as zeroes (allocating even if it
>> > does not necessarily write the zeroes).
> Which would mean that there is no way to say "give me zeroes, and do it
> in the cheapest way possible". Because that would be to leave the
> allocation status as it is and just toggle the zero bit.

If bdrv_co_write_zeroes is SCSI's "WRITE SAME without UNMAP", then it 
must allocate.  I think "give me zeroes and do it in the cheapest way 
possible" is BDRV_REQ_MAY_UNMAP (which *may* unmap but it doesn't have to).

That said, the really expensive part of unmapping is probably 
re-allocating the clusters on subsequent writes.  The unmap itself isn't 
that expensive (even if you have to flush the refcount blocks before the 
L2 tables), is it?

Paolo