Mysql – limit that a folder can store how many files and folders

If you are using ext3 in Linux, the limit is explained by Wikipedia:

There is a limit of 31998 sub-directories per one directory, stemming from its limit of 32000 links per inode.

That limit is per-directory, not for the whole filesystem, so you could work around it by further sub-dividing things. For instance instead of having all the user subdirectories in the same directory split them per the first two characters of the name so you have something like:

top_level_dir
|---aa
|   |---aardvark1
|   |---aardvark2
|---da
|   |---dan
|   |---david
|---do
    |---don

Even better would be to create some form of hash of the names and use that for the division. This way you'll get a better spread amongst the directories instead of, with the initial letters example, "da" being very full and "zz" completely empty. For instance if you take the CRC or MD5 the name and use the first 8 bits you'll get somethnig like:

top_level_dir
|---00
|   |---some_username
|   |---some_username
|---01
|   |---some_username
...
|---FF
|   |---some_username

This can be extended to further depths as needed, for instance like so if using the username not a hash value:

top_level_dir
|---a
|   |---a
|       |---aardvark1
|       |---aardvark2
|---d
    |---a
    |   |---dan
    |   |---david
    |---o
        |---don

This method is used in many places like squid's cache, to copy Ludwig's example, and the local caches of web browsers.

One important thing to note is that with ext2/3 you will start to hit performance issues before you get close to the 32,000 limit anyway, as directories are searched linearly. Moving to another filesystem (ext4 or reiser for instance) will remove this inefficiency (reiser searches directories with a binary-split algorimth so long directories are handled much more efficiently, ext4 may do too) as well as the fixed limit per directory.