Re: [Qemu-devel] [PATCH 3/4] block/parallels: Don't update header until the first actual write

[Kevin Wolf <kwolf@redhat.com>]

Am 27.10.2017 um 10:57 hat Jeff Cody geschrieben:
> The on disk image format 'inuse' header field is updated blindly if the
> image is opened RDWR.  This can cause problems if the QEMU runstate is
> set to INMIGRATE, at which point the underlying file is set to INACTIVE.
> This causes an assert in bdrv_co_pwritev().
> 
> Do something similar to what is done in VHDX; latch the first write, and
> update the header the first time we modify the file.
> 
> Signed-off-by: Jeff Cody <jcody@redhat.com>

For VHDX, it seems that we have to have the header update in the write
path anyway, so it might be justifiable, but I think for parallels, it's
just ugly.

The conservative approach to this would be doing the header write in
.bdrv_open() only if BDRV_O_INACTIVE is cleared, and otherwise do it
during .bdrv_invalidate_cache().

By the way, random design thought: It might make sense to change
.bdrv_open() so that it always opens inactive images and then call
.bdrv_invalidate_cache() (possibly renamed to .bdrv_activate())
unconditionally even without migration.

Kevin