On Fri, Nov 09, 2012 at 11:19:17AM +0100, Stefan Behrens wrote: > On Fri, 9 Nov 2012 08:44:01 +0800, Liu Bo wrote: > > On Thu, Nov 08, 2012 at 06:24:36PM +0100, Stefan Behrens wrote: > >> On Thu, 8 Nov 2012 22:50:47 +0800, Liu Bo wrote: > >>> On Tue, Nov 06, 2012 at 05:38:33PM +0100, Stefan Behrens wrote: > >>>> + trans = btrfs_start_transaction(root, 0); > >>> > >>> why a start_transaction here? Any reasons? > >>> (same question also for some other places) > >>> > >> > >> Without this transaction, there is outstanding I/O which is not flushed. > >> Pending writes that go only to the old disk need to be flushed before > >> the mode is switched to write all live data to the source disk and to > >> the target disk as well. The copy operation that is part of the scrub > >> code works on the commit root for performance reasons. Every write > >> request that is performed after the commit root is established needs to > >> go to both disks. Those requests that already have the bdev assigned > >> (i.e., btrfs_map_bio() was already called) cannot be duplicated anymore > >> to write to the new disk as well. > >> > >> btrfs_dev_replace_finishing() looks similar and goes through a > >> transaction commit between the steps where the bdev in the mapping tree > >> is swapped and the step when the old bdev is freed. Otherwise the bdev > >> would be accessed after being freed. > >> > > > > I see, if you're only about to flush metadata, why not join a transaction? > > btrfs_join_transaction() would delay the current transaction and enforce > that the current transaction is used and not a new one. > btrfs_start_transaction() would use either the current transaction, or a > new one. It is less interfering.
hmm...btrfs_start_transaction() would not use the current transaction unless you're still in the same task, ie. current->journal_info remains unchanged, otherwise it will be blocked by the current transaction(wait_current_trans()). If there are several btrfs_start_transaction() being blocked, after the current one's commit, one of them will allocate a new transaction, and the rest can join it. But btrfs_join_transaction will join the current as much as possible. And since here we don't do any reservation and seems to just update chunk/device tree(which will use global block rsv directly), I perfer btrfs_join_transaction(). thanks, liubo > > Since in dev-replace.c it is not required to enforce that a current > transaction is joined, btrfs_start_transaction() is the one to choose > here, as I understood it. > > But that's an interesting topic and I would appreciate to get a definite > rule which one to choose when. > > -- > To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in > the body of a message to majord...@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
