Hello fsdevel people, Five years ago[0], we started a conversation about cross-filesystem userspace tooling for online fsck. I think enough time has passed for us to have another one, since a few things have happened since then: 1. ext4 has gained the ability to send corruption reports to a userspace monitoring program via fsnotify. Thanks, Collabora! 2. XFS now tracks successful scrubs and corruptions seen during runtime and during scrubs. Userspace can query this information. 3. Directory parent pointers, which enable online repair of the directory tree, is nearing completion. 4. Dave and I are working on merging online repair of space metadata for XFS. Online repair of directory trees is feature complete, but we still have one or two unresolved questions in the parent pointer code. 5. I've gotten a bit better[1] at writing systemd service descriptions for scheduling and performing background online fsck. Now that fsnotify_sb_error exists as a result of (1), I think we should figure out how to plumb calls into the readahead and writeback code so that IO failures can be reported to the fsnotify monitor. I suspect there may be a few difficulties here since fsnotify (iirc) allocates memory and takes locks. As a result of (2), XFS now retains quite a bit of incore state about its own health. The structure that fsnotify gives to userspace is very generic (superblock, inode, errno, errno count). How might XFS export a greater amount of information via this interface? We can provide details at finer granularity -- for example, a specific data structure under an allocation group or an inode, or specific quota records. With (4) on the way, I can envision wanting a system service that would watch for these fsnotify events, and transform the error reports into targeted repair calls in the kernel. This of course would be very filesystem specific, but I would also like to hear from anyone pondering other usecases for fsnotify filesystem error monitors. Once (3) lands, XFS gains the ability to translate a block device IO error to an inode number and file offset, and then the inode number to a path. In other words, your file breaks and now we can tell applications which file it was so they can failover or redownload it or whatever. Ric Wheeler mentioned this in 2018's session. The final topic from that 2018 session concerned generic wrappers for fsscrub. I haven't pushed hard on that topic because XFS hasn't had much to show for that. Now that I'm better versed in systemd services, I envision three ways to interact with online fsck: - A CLI program that can be run by anyone. - Background systemd services that fire up periodically. - A dbus service that programs can bind to and request a fsck. I still think there's an opportunity to standardize the naming to make it easier to use a variety of filesystems. I propose for the CLI: /usr/sbin/fsscrub $mnt that calls /usr/sbin/fsscrub.$FSTYP $mnt For systemd services, I propose "fsscrub@<escaped mountpoint>". I suspect we want a separate background service that itself runs periodically and invokes the fsscrub@$mnt services. xfsprogs already has a xfs_scrub_all service that does that. The services are nifty because it's really easy to restrict privileges, implement resource usage controls, and use private name/mountspaces to isolate the process from the rest of the system. dbus is a bit trickier, since there's no precedent at all. I guess we'd have to define an interface for filesystem "object". Then we could write a service that establishes a well-known bus name and maintains object paths for each mounted filesystem. Each of those objects would export the filesystem interface, and that's how programs would call online fsck as a service. Ok, that's enough for a single session topic. Thoughts? :) --D [0] https://lwn.net/Articles/754504/ [1] https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfsprogs-dev.git/log/?h=scrub-optimize-by-default
