== Why Unix filesystems precreate _lost+found_ directories

Here's a sensible question about the _lost+found_ directories that
various Unix filesystems stick in the top directory: why do they have to
exist all the time? Yes, clearly _fsck_ needs somewhere to put recovered
files, but in theory it could just create a _lost+found_ directory when
needed, and otherwise you wouldn't have it cluttering up the filesystem.

Part of the answer is that _lost+found_ isn't just a normal empty
directory; instead, it's usually substantially bigger, despite having
nothing in it.

The full answer is simple: because having a pre-existing _lost+found_
directory means changing as little of the filesystem as possible during
an _fsck_ recovery. With an existing _lost+found_ directory that has a
bunch of empty directory entries, all _fsck_ has to do to recover a
file is fill in some data and rewrite a block.

Without an existing _lost+found_ directory with empty entries, _fsck_
would have to do a lot more; it would have to allocate an inode for the
directory, allocate at least one data block for it, and add a directory
entry to the filesystem's root directory (which might require allocating
another data block and extending the directory). Of course, these
changes would require checking and updating various potentially damaged
filesystem data structures.

You could make an _fsck_ that would do all this but it would clearly
be more complicated, and complicated operations are undesirable in a
filesystem recovery tool. Setting everything up ahead of time is much
simpler.

(This entry was inspired by [[this comment
http://pavelmachek.livejournal.com/83047.html?thread=195431#t195431]].)