On 2/23/21 9:03 AM, Gao Xiang wrote: > On Tue, Feb 23, 2021 at 08:40:56AM -0600, Eric Sandeen wrote: >> On 2/23/21 7:42 AM, Gao Xiang wrote: >>> Hi folks, >>> >>> On Wed, Mar 28, 2018 at 08:17:28AM +1100, Dave Chinner wrote: >>>> On Mon, Mar 26, 2018 at 08:46:49AM -0400, Brian Foster wrote: >>>>> On Sat, Mar 24, 2018 at 09:20:49AM -0700, Darrick J. Wong wrote: >>>>>> On Wed, Mar 07, 2018 at 05:33:48PM -0600, Eric Sandeen wrote: >>>>>>> Now that unlinked inode recovery is done outside of >>>>>>> log recovery, there is no need to dirty the log on >>>>>>> snapshots just to handle unlinked inodes. This means >>>>>>> that readonly snapshots can be mounted without requiring >>>>>>> -o ro,norecovery to avoid the log replay that can't happen >>>>>>> on a readonly block device. >>>>>>> >>>>>>> (unlinked inodes will just hang out in the agi buckets until >>>>>>> the next writable mount) >>>>>> >>>>>> FWIW I put these two in a test kernel to see what would happen and >>>>>> generic/311 failures popped up. It looked like the _check_scratch_fs >>>>>> found incorrect block counts on the snapshot(?) >>>>>> >>>>> >>>>> Interesting. Just a wild guess, but perhaps it has something to do with >>>>> lazy sb accounting..? I see we call xfs_initialize_perag_data() when >>>>> mounting an unclean fs. >>>> >>>> The freeze is calls xfs_log_sbcount() which should update the >>>> superblock counters from the in-memory counters and write them to >>>> disk. >>>> >>>> If they are out, I'm guessing it's because the in-memory per-ag >>>> reservations are not being returned to the global pool before the >>>> in-memory counters are summed during a freeze.... >>>> >>>> Cheers, >>>> >>>> Dave. >>>> -- >>>> Dave Chinner >>>> david@xxxxxxxxxxxxx >>> >>> I spend some time on tracking this problem. I've made a quick >>> modification with per-AG reservation and tested with generic/311 >>> it seems fine. My current question is that how such fsfreezed >>> images (with clean mount) work with old kernels without [PATCH 1/1]? >>> I'm afraid orphan inodes won't be freed with such old kernels.... >>> Am I missing something? >> >> It's true, a snapshot created with these patches will not have their unlinked >> inodes processed if mounted on an older kernel. I'm not sure how much of a >> problem that is; the filesystem is not inconsistent, but some space is lost, >> I guess. I'm not sure it's common to take a snapshot of a frozen filesystem on >> one kernel and then move it back to an older kernel. Maybe others have >> thoughts on this. > > My current thought might be only to write clean mount without > unlinked inodes when freezing, but leave log dirty if any > unlinked inodes exist as Brian mentioned before and don't > handle such case (?). I'd like to hear more comments about > this as well. I don't know if I had made this comment before ;) but I feel like that's even more "surprise" (as in: gets further from the principle of least surprise) and TBH I would rather not have that somewhat unpredictable behavior. I think I'd rather /always/ make a dirty log than sometimes do it, other times not. It'd just be more confusion for the admin IMHO. Thanks, -Eric > Thanks, > Gao Xiang > >> >> -Eric >> >
