On Tue, Dec 01, 2015 at 02:46:32PM +0800, Qu Wenruo wrote: > > > Chris Mason wrote on 2015/11/30 11:48 -0500: > >On Sat, Nov 28, 2015 at 01:46:34PM +0000, Hugo Mills wrote: > >> We've just had someone on IRC with a problem mounting their FS. The > >>main problem is that they've got a corrupt log tree. That isn't the > >>subject of this email, though. > >> > >> The issue I'd like to raise is that even with -oro as a point > >>option, the FS is trying to replay the log tree. The dmesg output from > >>mount -oro is at the end of the email. > >> > >> Now, my memory, experience and understanding is that the FS > >>doesn't, and shouldn't replay the log tree on a RO mount, because the > >>FS should still be consistent even without the reply, and > >>RO-means-actually-RO is possible and desirable. (Compared to a > >>journalling FS, where journal replay is required for a consistent, > >>usable FS). > >> > >> So, this looks to me like a regression that's come in somewhere. > >> > >> (Just for completeness, the system in question usually runs 4.2.5, > >>but the live CD the OP is using is 4.2.3). > > > >We do need to replay the log tree, even on readonly mounts. Otherwise > >files created and fsunk before crashing may not even exist. > > > >We'll bail out of the log replay on readonly media, but otherwise the > >replay always happens. > > > >-chris > > Or disable log_tree (making fsync as slow as sync). > And there will be no log replay, making RO mount real RO. > I think we can add it to kernel btrfs documentation.
True, without the log tree there's nothing to replay. > > > Or, in my wildest dream, introduce a per-inode tree to record file > extents/dir items. > > Then fsync will only need to sync the inode file extent/dir item tree.(and > its direct parent maybe) > And better random read/write performance. > > Although that's just my dream.... > > But I'm a little curious about why btrfs choose to pack dir items and file > extents into the same subvolume tree at design time. > Unlike most of other file systems(ext4 for example). > > Is it just designed for simplicity? It's partially simplicity, but it also helps with locality. When you're working with lots of files in a single directory, we're able to do many operations faster because we're not jumping around to other indexes for individual file extents. The cost is contention at the top of the btree, which I'm still hoping to fix without having to go all the way down to per-file trees. -chris -- 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
