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

[Avi Kivity]

On 11/14/2011 11:58 AM, 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,

Result = open succeeded.  Computation = anything that derives from the
image, like size, or reading some stuff to guess CHS or something.

>  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)

(yes)

>  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.

Okay.

>
> > What's wrong with just delaying the open?
>
> Nothing, except that with today's code it's harder to do.
>

This has never stopped us (though it may delay us).

-- 
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