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
