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

[Fam Zheng]

On Mon, 02/24 12:51, Paolo Bonzini wrote:
> 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. :)
> 

I see. I could only tell in VMDK cluster doesn't have this "mapped and zeroed"
state, so maybe we need some flexibility here and reduce assumption.

Fam