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

[Avi Kivity <avi@redhat.com>]

  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.


>
> 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.

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