Re: [Qemu-devel] qemu and qemu.git -> Migration + disk stress introduces qcow2 corruptions

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Nov 14, 2011 at 10:58:16AM +0100, Kevin Wolf wrote:
> Am 12.11.2011 11:25, schrieb Avi Kivity:
> > On 11/11/2011 12:15 PM, Kevin Wolf wrote:
> >> Am 10.11.2011 22:30, schrieb Anthony Liguori:
> >>> Live migration with qcow2 or any other image format is just not going to work 
> >>> right now even with proper clustered storage.  I think doing a block level flush 
> >>> cache interface and letting block devices decide how to do it is the best approach.
> >>
> >> I would really prefer reusing the existing open/close code. It means
> >> less (duplicated) code, is existing code that is well tested and doesn't
> >> make migration much of a special case.
> >>
> >> If you want to avoid reopening the file on the OS level, we can reopen
> >> only the topmost layer (i.e. the format, but not the protocol) for now
> >> and in 1.1 we can use bdrv_reopen().
> > 
> > Intuitively I dislike _reopen style interfaces.  If the second open
> > yields different results from the first, does it invalidate any
> > computations in between?
> 
> Not sure what results and what computation you mean, but let me clarify
> a bit about bdrv_reopen:
> 
> The main purpose of bdrv_reopen() is to change flags, for example toggle
> O_SYNC during runtime in order to allow the guest to toggle WCE. This
> doesn't necessarily mean a close()/open() sequence if there are other
> means to change the flags, like fcntl() (or even using other protocols
> than files).
> 
> The idea here was to extend this to invalidate all caches if some
> specific flag is set. As you don't change any other flag, this will
> usually not be a reopen on a lower level.
> 
> If we need to use open() though, and it fails (this is really the only
> "different" result that comes to mind) then bdrv_reopen() would fail and
> the old fd would stay in use. Migration would have to fail, but I don't
> think this case is ever needed for reopening after migration.
> 
> > What's wrong with just delaying the open?
> 
> Nothing, except that with today's code it's harder to do.
> 
> Kevin

It seems cleaner, though, doesn't it?

-- 
MST