On 02.08.2012 13:34, Liu Bo wrote: > On 08/02/2012 07:18 PM, Arne Jansen wrote: >> On 02.08.2012 12:36, Liu Bo wrote: >>> On 08/02/2012 06:30 PM, Stefan Behrens wrote: >>>> On Wed, 01 Aug 2012 16:31:54 +0200, Stefan Behrens wrote: >>>>> On Wed, 01 Aug 2012 21:31:58 +0800, Liu Bo wrote: >>>>>> On 08/01/2012 09:07 PM, Jan Schmidt wrote: >>>>>>> On Wed, August 01, 2012 at 14:02 (+0200), Liu Bo wrote: >>>>>>>> On 08/01/2012 07:45 PM, Stefan Behrens wrote: >>>>>>>>> With commit acce952b0, btrfs was changed to flag the filesystem with >>>>>>>>> BTRFS_SUPER_FLAG_ERROR and switch to read-only mode after a fatal >>>>>>>>> error happened like a write I/O errors of all mirrors. >>>>>>>>> In such situations, on unmount, the superblock is written in >>>>>>>>> btrfs_error_commit_super(). This is done with the intention to be able >>>>>>>>> to evaluate the error flag on the next mount. A warning is printed >>>>>>>>> in this case during the next mount and the log tree is ignored. >>>>>>>>> >>>>>>>>> The issue is that it is possible that the superblock points to a root >>>>>>>>> that was not written (due to write I/O errors). >>>>>>>>> The result is that the filesystem cannot be mounted. btrfsck also does >>>>>>>>> not start and all the other btrfs-progs tools fail to start as well. >>>>>>>>> However, mount -o recovery is working well and does the right things >>>>>>>>> to recover the filesystem (i.e., don't use the log root, clear the >>>>>>>>> free space cache and use the next mountable root that is stored in the >>>>>>>>> root backup array). >>>>>>>>> >>>>>>>>> This patch removes the writing of the superblock when >>>>>>>>> BTRFS_SUPER_FLAG_ERROR is set, and removes the handling of the error >>>>>>>>> flag in the mount function. >>>>>>>>> >>>>>>>> >>>>>>>> Yes, I have to admit that this can be a serious problem. >>>>>>>> >>>>>>>> But we'll need to send the error flag stored in the super block into >>>>>>>> disk in the future so that the next mount can find it unstable and do >>>>>>>> fsck by itself maybe. >>>>>>> >>>>>>> Hum, that's possible. However, I neither see >>>>>>> >>>>>>> a) a safe way to get that flag to disk >>>>>>> >>>>>>> nor >>>>>>> >>>>>>> b) a situation where this flag would help. When we abort a transaction, >>>>>>> we just >>>>>>> roll everything back to the last commit, i.e. a consistent state. So if >>>>>>> we stop >>>>>>> writing a potentially corrupt super block, we should be fine anyway. Or >>>>>>> am I >>>>>>> missing something? >>>>>>> >>>>>> >>>>>> I'm just wondering if we can roll everything back well, why do we need >>>>>> fsck? >>>>> >>>>> If the disks support barriers, we roll everything back very well. The >>>>> most recent superblock on the disks always defines a consistent >>>>> filesystem state. There are only two remaining filesystem consistency >>>>> issues left that can cause inconsistent states, one is the one that the >>>>> patch in this email addresses, and the second one is that the error >>>>> result from barrier_all_devices() is ignored (which I want to change >>>>> next). >>>> >>>> Hi Liu Bo, >>>> >>>> Do you have any remaining objections to that patch? >>>> >>> >>> Hi Stefan, >>> >>> Still I have another question: >>> >>> Our metadata can be flushed into disk if we reach the limit, 32k, so we >>> can end up with updated metadata and the latest superblock if we do not >>> write the current super block. >> >> The old metadata stays valid until the new superblock is written, >> so no problem here, or maybe I don't understand your question :) >> > > Yeah, Arne, you're right :) > > But for undetected and unexpected errors as Arne had mentioned, I want > to keep the error flag which is able to inform users that this FS is > recommended (but not must) to do fsck at least.
How about storing the flag in a different location than the superblock? If the fs is in an unknown state, every write potentially makes it only worse. > > thanks, > liubo > >>> >>> Any ideas? >>> >>> thanks, >>> liubo >>> -- >>> 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 -- 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
