Re: [Qemu-devel] [PATCH v6 for 2.1 00/10] Modify block jobs to use node-names

[Benoît Canet]

The Monday 23 Jun 2014 à 21:08:09 (+0800), Stefan Hajnoczi wrote :
> On Thu, Jun 19, 2014 at 12:26:00PM -0400, Jeff Cody wrote:
> > On Thu, Jun 19, 2014 at 05:17:16PM +0800, Stefan Hajnoczi wrote:
> > > On Tue, Jun 17, 2014 at 05:53:48PM -0400, Jeff Cody wrote:
> > > Let's discuss this topic in a sub-thread and figure out what to do for
> > > QEMU 2.1.  This is an important issue to solve before the release
> > > because we can't change QMP command semantics easily later.
> > > 
> > > My questions are:
> > > a. How do we fix resize, snapshot-sync, etc?  It seems like we need to
> > >    propagate child op blockers.
> > > 
> > > b. Is it a good idea to perform op blocker checks on the root node?
> > >    It's inconsistent with resize, snapshot-sync, etc.  Permissions in
> > >    BDS graphs with multiple root nodes (e.g. guest device and NBD
> > >    run-time server) will be different depending on which root you
> > >    specify.
> > 
> > I don't think (b) is the ultimate solution.  It is used as a stop-gap
> > because op blockers in the current implementation is essentially
> > analogous to the in-use flag.  But is it good enough for 2.1?  If
> > *everything* checks the topmost node in 2.1, then I think we are OK in
> > all cases except where images files share a common BDS.
> 
> Checking op blockers on the root node as a stop-gap is a good idea.
> Let's apply it across all commands (e.g. snapshot-sync, resize).
> 
> Fam pointed out that this approach is vulnerable to blockdev-add, where
> blockers could be set/checked on an incomplete BDS graph (since you can
> add new nodes on top).  Do we need to move the blockers up the graph if
> a new root node is inserted?
> 
> Besides this issue, your approach seems like the quickest safe solution
> for 2.1.

I agree that always blocking the top BDS would be tactical.
Even it would need to move the blocker on the root on node insertion it would
solve the issues I have in the quorum maintainances patches of recursive BDS 
loops
and ownerships.

Best regards

Benoît

> 
> > The ability for internal BDSs to share a common base BDS makes some
> > block jobs unsafe currently, I believe.  A crude and ugly fix is to
> > only allow a single block-job to occur at any given time, but that
> > doesn't seem feasible, so let's ignore that.
> 
> Right now I don't think we share BDS chains.
> 
> Stefan