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

[Avi Kivity <avi@redhat.com>]

  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?

> >  >
> >  >That means we can maintain the physical size without introducing
> >  >additional fsync()s in the allocation path.  Since we're already
> >  >writing out the header anyway, the write operation is basically
> >  >free too.
> >
> >  I don't see how it is free.  It's an extra write.  The good news is
> >  that it's very easy to amortize.
>
> We only need to update the header field on disk when we're already
> updating the header, so it's not even an extra write operation.

Why would you ever update the header, apart from relocating L1 for some 
reason?

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.