Similar to free inodes, you should also check for maximum number of directories. dir_index option helps, but I've seen it become a problem.

There's a maximum number of directories? On what filesystem is that?

ext3 without dir_index has a limit of 32K directories in any one directory.

Where I saw it crop up was 32K folders under /tmp on a cluster system. So no it's not a limit on number of directories entirely (that's inodes), but rather how many subdirectories you can have.

http://en.wikipedia.org/wiki/Ext4#Features <-- Fixes 32K limit

ext3/4 has really poor large-directory performance, even with dir_index, especially if you are constantly removing and readding nodes. I would highly recommend XFS for large-directory use cases.

I got bit by this once, i think it was related to a maximum of 32k hardlinks per inode, which effectively sets a limit of 32k subdirs since each subdir has a hardlink to ".."