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

[Kevin Wolf]

Am 27.10.2017 um 12:18 hat Jeff Cody geschrieben:
> On Fri, Oct 27, 2017 at 11:09:19AM +0200, Kevin Wolf wrote:
> > 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 <address@hidden>
> > 
> > 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.
> > 
> 
> A bit ugly.  I think we could get around VHDX needing to do it as well; it
> does it in the write path for two scenarios:
> 
>     * First normal write, or
>     * Journal log replay, if dirty, on open (if r/w)
> 
> The log check happens early in vhdx_open().  If it does not write anything,
> then we can just write the headers during the open like normal, if we are
> R/W (and !BDRV_O_INACTIVE, of course).

Technically, you can do the same as I suggest for parallels, the
difference is just that by the letter, the VHDX spec seems to require
that you update the header on the first write (going by the comment in
our VHDX driver...).

> > 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().
> 
> What scenarios cause BDRV_O_INACTIVE to not be set on bs, but set on
> bs->file-bs?

Bugs? :-)

Kevin

> 
> > 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