Re: [Qemu-devel] [PATCH 12/22] qcow2-bitmap: add IN_USE flag

[Max Reitz <mreitz@redhat.com>]

[-- Attachment #1: Type: text/plain, Size: 1715 bytes --]

On 21.10.2016 17:34, Vladimir Sementsov-Ogievskiy wrote:
> 07.10.2016 22:44, Max Reitz пишет:
>> On 30.09.2016 12:53, Vladimir Sementsov-Ogievskiy wrote:
>>> This flag means that the bitmap is now in use by the software or was not
>>> successfully saved. In any way, with this flag set the bitmap data must
>>> be considered inconsistent and should not be loaded.
>>>
>>> With current implementation this flag is never set, as we just remove
>>> bitmaps from the image after loading. But it defined in qcow2 spec and
>>> must be handled. Also, it can be used in future, if async schemes of
>>> bitmap loading/saving are implemented.
>>>
>>> We also remove in-use bitmaps from the image on open.
>>>
>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>>> ---
>>>   block/qcow2-bitmap.c | 17 ++++++++++++++++-
>>>   1 file changed, 16 insertions(+), 1 deletion(-)
>> Don't you want to make use of this flag? It would appear useful to me if
>> you just marked autoload bitmaps as in_use instead of deleting them from
>> the image when it's opened and then overwrite them when the image is
>> closed.
> 
> And what is the use of it?

You don't need to free any bitmaps when opening the file, and you don't
need to allocate any new bitmap space when closing it.

> Any way, storing an invalid copy of online data is bad I think.
> Therefore I will have to free bitmap data clusters and zero bitmap table
> in file.

Why can't you just set this flag to mark it invalid? In case qemu
crashes, in one case you'll have some invalid bitmaps and in the other
you won't have any of them at all. I don't think there's a difference
from that perspective.

Max


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 480 bytes --]