The Saturday 21 Jun 2014 à 23:15:19 (+0800), Fam Zheng wrote : > On Sat, 06/21 12:45, Benoît Canet wrote: > > The Saturday 21 Jun 2014 à 16:53:58 (+0800), Fam Zheng wrote : > > > On Fri, 06/20 09:30, Eric Blake wrote: > > > > On 06/19/2014 11:01 PM, Fam Zheng wrote: > > > > > On Thu, 06/19 22:20, Benoît Canet wrote: > > > > >> The Thursday 19 Jun 2014 à 14:13:20 (-0600), Eric Blake wrote : > > > > >>> On 06/19/2014 02:01 PM, Benoît Canet wrote: > > > > >>>> As the code will start to operate on arbitratry nodes we need the > > > > >>>> op blocker > > > > >>> > > > > >>> s/arbitratry/arbitrary/ > > > > >>> > > > > >>>> to recursively block or unblock whole BDS subtrees. > > > > > > > > > > I don't get the reason, can you elaborate? > > > > > > > > Consider what happens if I have: > > > > > > > > base <- snap1 <- active > > > > > > > > then I start a fleecing NBD server on the state as it was at snap1: > > > > > > > > base <- snap1 <- active > > > > \- fleecing > > > > > > > > then I do a blockpull into active: > > > > > > > > base <- snap1 <- fleecing > > > > active > > > > > > > > at this point, base and snap1 are no longer tied to active, but they > > > > STILL must be protected from operations that would modify their contents > > > > in a way that would break the fleecing operation. The solution we are > > > > looking at is making BDS blockers recursive to every element of the > > > > chain, not just the top-level device. > > > > > > This would already have been protected by backing blocker of fleecing > > > target. > > > > > > > > > > > Another example: consider: > > > > > > > > base <- snap1 <- active > > > > > > > > then someone uses Jeff's proposed new change-backing-file QMP command to > > > > rewrite the snap1 metadata to point to base via a relative name instead > > > > of an absolute name. It shouldn't matter whether active is blocked, but > > > > only whether snap1 is blocked. But to know if snap1 is blocked, we have > > > > to propagate the blockers of active down recursively to its backing > > > > files. > > > > > > Why do we need to block changging of metadata? I think this operation is > > > safe > > > in most cases. > > > > > > Correct me if I'm missing anything, but even if snap1 _is_ blocked, it > > > would be > > > because snap1 is serving as backing of active. In this case, the actual > > > blocker > > > should be active->backing_blocker. > > > > > > > > > > > >> What would be a cleaner solution ? > > > > > > > > > > What is the question to solve? > > > > > > > > I think Jeff's idea is on target - rather than blocking by operation, we > > > > should instead be blocking on access patterns (various operations > > > > trigger several access patterns): > > > > https://lists.gnu.org/archive/html/qemu-devel/2014-06/msg04752.html > > > > > > > > Jeff's initial list included: > > > > > > > > > So if I think of operations that are done on block devices from a > > > > > block job, and chuck them into categories, I think we have: > > > > > > > > > > 1) Read of guest-visible data > > > > > 2) Write of guest-visible data > > > > > 3) Read of host-visible data (e.g. image file metadata) > > > > > 4) Write of host-visible data (e.g. image file metadata, such as > > > > > the backing-file) > > > > > 5) Block chain manipulations (e.g. movement of a BDS, change to r/w > > > > > instead of r/o, etc..) > > > > > 6) I/O attribute changes (e.g. throttling, etc..) > > > > > > Most operations looks safe to me, given the way how IOThreads and > > > coroutine > > > work now. It's only the chain manpulations in long running block jobs > > > that are > > > exclusive, and by nature it should be checked per chain. Can we set some > > > op > > > blockers on the bottom BDS and check it each time, to prevent user from > > > starting a second chain manipulator? > > > > I don't know if bottom BDS locking is any good because some driver like > > quorum > > have multiple childs. > > Locking everytime the root (top) of the tree seems a feasible solution > > indeed. > > Quorom doesn't change the convensions of backing chain, so each child belongs > to its own backing chain, and that chain has a deterministic top and bottom. > > Blocking flag on bottom saves us from adding reverse looking up (->overlay > pointer), because we already have the ->backing_hd pointer in BDS.
I like the consequence that when a loop is formed like in commit's drive-mirror run code and must be unlocked the bottom locked BDS will act as a guard to prevent unlock loop cycling. We still have the issue of unlocking the bottom BDS when a subtree is detached from the graphs by a swap. (It does happen in my drive-mirror arbitrary node replacement series). >From my understanding the unlocking of the root BDS is done by >drive_mirror_complete while the mirror code tries to unref the orphaned subtree _before_ drive_mirror_complete is called. So the bottom BDS would be unrefed before being unlocked. Best regards Benoît > > Fam >
