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

[Paolo Bonzini]

Il 24/02/2014 12:33, Fam Zheng ha scritto:
> > This is (or should be) bdrv_co_write_zeroes without BDRV_REQ_MAY_UNMAP.
>
> But IIUC bdrv_co_write_zeroes without BDRV_REQ_MAY_UNMAP doesn't require
> cluster allocation if it's allocated yet, which is a bit different.

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).

For legacy reasons it may not be exactly what is implemented.  I asked 
Kevin a couple of weeks ago and he sent a patch, but even he wasn't sure 
of what qcow2 was doing util he looked at the code. :)

Paolo