Re: [Qemu-devel] [PATCH v15 13/25] qcow2: add .bdrv_store_persistent_dirty_bitmaps()

[Kevin Wolf <kwolf@redhat.com>]

Am 17.02.2017 um 13:24 hat Vladimir Sementsov-Ogievskiy geschrieben:
> 16.02.2017 17:08, Kevin Wolf wrote:
> >Am 15.02.2017 um 11:10 hat Vladimir Sementsov-Ogievskiy geschrieben:
> >>Realize block bitmap storing interface, to allow qcow2 images store
> >>persistent bitmaps.
> >>
> >>Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> >>Reviewed-by: Max Reitz <mreitz@redhat.com>
> >>Reviewed-by: John Snow <jsnow@redhat.com>
> >Similary to autoload, I think this must be called as part of
> >qcow2_inactivate() rather than just in bdrv_close().
> >
> >Kevin
> 
> I prefer to store bitmaps once, on the final close of bds, and
> remove corresponding BdrvDirtyBitmap in the same point. bdrv_close
> is simpler point, I don't want to think about loading/saving bitmap
> on each invalidate/inactivate. I don't want to make dependencies
> between qcow2 bitmap loading/storing and migration, etc.
> 
> So, my approach was just load bitmap on bdrv_open and store on
> bdrv_close, and between these two calls BdrvDirtyBitmap lives its
> normal life. For me it looks simpler. I'm not sure about what new
> corner cases will come if we change this.

You make this sounds like invalidate/inactivate was a very frequent
operation. It's not. Essentially it is what you probably consider
open/close to be.

The semantics is basically that after inactivate (or more precisely, as
long as BDRV_O_INACTIVE is set), another process can access and
potentially modify the image file. At invalidate, qemu takes ownership
again and reloads everything from the image file.

Kevin