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.
