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. 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
