On 05/28/2015 05:48 PM, Kevin Wolf wrote: > Am 28.05.2015 um 02:59 hat Wen Congyang geschrieben: >> On 05/27/2015 08:31 PM, Kevin Wolf wrote: >>> Am 21.05.2015 um 07:47 hat Wen Congyang geschrieben: >>>> On 05/09/2015 01:21 AM, Kevin Wolf wrote: >>>>> For bs->file, using references to existing BDSes has been possible for a >>>>> while already. This patch enables the same for bs->backing_hd. >>>> >>>> 1. We reference to an existing BDSes, and some disk uses this blk. Do >>>> we allow this? >>> >>> Currently yes. If it breaks, you get to keep both pieces. >>> >>> As long as your guest device is read-only, it should just work. It would >>> be a very bad idea, though, to write to a backing file. >>> >>> Op blockers should eventually prevent this from happening (Jeff, you may >>> want to take a note ;-)) >>> >>>> 2. bs->backing_hd->blk can be not NULL now? If we do an active commit >>>> to this backing file(use mirror job), we will call bdrv_swap() in >>>> mirror_exit(), and the function bdrv_swap() doesn't allow that >>>> new_bs->blk(here is bs->backing_hd) is not NULL. >>> >>> You're right. >>> >>> I can remove this patch from the series for now, but of course that >>> doesn't solve the problem. I'm not sure what to do about it. Making >>> bdrv_swap() work with BDSes that have BB attached is probably another >>> item in the list of "dynamic reconfiguration" problems. >> >> What does "dynamic reconfiguration" mean? Allow to change drive's option >> when guest is running? > > Sorry, I should have been more specific for those of you who haven't > been part of previous discussions. > > In this context, I'm talking about dynamic reconfiguration of the > BlockDriverState graph, i.e. any operation that changes the relationship > between different BDSes (essentially add/remove/change pointers to other > BDSes). > > An example for that is changing the backing file of an opened image > after having merged external snapshots with block-commit, or inserting a > new active layer for taking a live snapshot.
Thanks for your explanation. I understand it now. Wen Congyang > > Kevin > . >