XFS does use /lost+found, it calls it the "orphanage directory" and xfs_repair reparents children of corrupt directories there.

Based on comments in the kernel source, it seems like the userspace fsck for JFS and F2FS will also sometimes create /lost+found. There might be more that do.

Even with journaling, you might need one. ZeroFS [0] almost had a lost+found directory (even with the WAL enabled), because you might have consistency issues between your in-memory state and what was flushed, and especially in what order.

ZeroFS ended up not needing recovery at all through atomic, strictly ordered commits [1], but it was far from trivial (and not just a matter of requiring a WAL).

[0] https://github.com/Barre/ZeroFS

[1] https://github.com/Barre/ZeroFS/blob/main/zerofs/src/fs/writ...

As the submission explains, the lost+found folder has preallocated space. From the mklost+found man page [0]:

       mklost+found pre-allocates disk blocks to the lost+found directory
       so that when e2fsck(8) is being run to recover a file system, it
       does not need to allocate blocks in the file system to store a
       large number of unlinked files.  This ensures that e2fsck will not
       have to allocate data blocks in the file system during recovery.

Preallocating space without making the directory visible would require more arcane file system magic.

[0] https://man7.org/linux/man-pages/man8/mklost+found.8.html

How do you size a directory? Just by creating entries in it and then unlinking them?

Yea, run an old kernel with ext2 on a busy system writing a bunch of small files and have a power supply fail and you'll end up with something there.

fsck on large hard drives was scary on how long it could take to finish.

You need to use worse hardware and bad power :)

Same here. And I had some pretty f**ed up file systems.

At one point, I had one where the directory structure was completely broken and had circles in it (broken SSD). To be fair, in that particular case, I did not look for lost+found and just wrote a tool to extract the data manually that I was looking for.

That's what the answers are missing, of course. In some filesystem formats, it's possible either to recover completely from a journal/intent log, or at least to recover everything to the point that recovered files can be placed into the correct directory.

My SD cards have always had stuff in that folder. It scares me. I try not to look

Have to run fsck. This used to be forced about once a month but don’t remember it happening in the last decade or so.

It is StackExchange. So in theory someone could modernize it at any time.

It's a question from 2011...

they offer private instances to school too, where moderation is left to school policy, and mine seems to be good enough to use whenever i have frontend questions (i'm 10 year into my career and still use my juniors to answer my frontend questions, i think i won't ever change)

Same, it's only in /home on my system also. Also /home is pretty much the only directory where I see fsck needing to do a lot of recovery after a power failure. Makes sense I guess, because that's where processes such as web browsers are likely to have lots of files open in RW mode.

More like a FOUND.000 folder or a root directory filled with .CHK files

Not really, as it's only once per file system mount, whereas those Windows and MacOS files are sprinkled in most directories with images and almost every non-network drive directory respectively.

Reaching HN or other sites allows for exposure to information that a person otherwise would not ask themselves.

I also respect human responses over AI ones every day that ends in Y.