Re: [Qemu-devel] Re: [PATCH v2 6/7] qed: Read/write support

[Stefan Hajnoczi]

On Wed, Oct 13, 2010 at 04:10:25PM +0200, Avi Kivity wrote:
>  On 10/13/2010 04:07 PM, Stefan Hajnoczi wrote:
> >On Wed, Oct 13, 2010 at 03:50:00PM +0200, Avi Kivity wrote:
> >>   On 10/13/2010 03:24 PM, Anthony Liguori wrote:
> >>  >On 10/13/2010 08:07 AM, Kevin Wolf wrote:
> >>  >>Am 13.10.2010 14:13, schrieb Stefan Hajnoczi:
> >>  >>>We can avoid it when a backing image is not used.  Your idea to check
> >>  >>>for zeroes in the backing image is neat too, it may well reduce the
> >>  >>>common case even for backing images.
> >>  >>The additional requirement is that we're extending the file and not
> >>  >>reusing an old cluster. (And bdrv_has_zero_init() == true, but QED
> >>  >>doesn't work on host_devices anyway)
> >>  >
> >>  >Yes, that's a good point.
> >>  >
> >>  >BTW, I think we've decided that making it work on host_devices is
> >>  >not that bad.
> >>  >
> >>  >We can add an additional feature called QED_F_PHYSICAL_SIZE.
> >>  >
> >>  >This feature will add another field to the header that contains an
> >>  >offset immediately following the last cluster allocation.
> >>  >
> >>  >During a metadata scan, we can accurately recreate this field so
> >>  >we only need to update this field whenever we clear the header
> >>  >dirty bit (which means during an fsync()).
> >>
> >>  If you make QED_F_PHYSICAL_SIZE an autoclear bit, you don't need the
> >>  header dirty bit.
> >
> >Do you mean we just need to check the physical size header field against
> >the actual file size?  If the two are different, then a consistency
> >check is forced.
> 
> I thought you'd only use a header size field when you don't have a
> real file size.  Why do you need both?

I probably didn't understand correctly :).  You said with
QED_F_PHYSICAL_SIZE autoclear you don't need the header dirty bit.  I
don't see how it eliminates the need for the header dirty bit.

Stefan