How to solve linux subdirectories number limit?

Question

I have a website which will store the user profile image. Each image is stored in a directory(linux) specific to the user. Currently I have a customer base of 30+, which means I will have 30+ folders. But my current linux box (ext2/ext3) doesn't support creating more than 32000 directories. How do I get pass this. Even youtube guys have got the same problem, with video thumbnails. But they solved it by moving to ReiserFS. Cant we have a better solution?

Update:When asked in IRC, people were asking to upgrading it to ext4, which has 64k limit and ofcourse you can even get pass that too. Or kernel hacking to change the limit.

Update:How about splitting the user base into folders based on the userid range.Meaning 1-1000 in one folder, 1000-2000 in the other like that. This seems to be simple. What do you say guys?

Frankly isn't there any other go?

Answer 1

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
|   |---aardvark1
|---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
|       |---aardvark1
|---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 search 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.

Answer 2

If you are bound to ext2/ext3 the only possibility I see is to partition your data. Find a criterion that splits your data into manageable chunks of similar size.

If it's only about the profile images I'd do:

1. Use a hash (e.g SHA1) of the image
2. Use the SHA1 as file and directory name

For example the SQUID cache does it this way:

f/4b/353ac7303854033

Top level directory is the first hex-digit, second level is the next two hex-digits, and the file name is the remaining hex-digits.

Answer 3

Cant we have a better solution?

You do have a better solution - use a different filesystem, there are plenty available, many of which are optimised for different tasks. As you pointed out ReiserFS is optimised for handling lots of files in a directory.

See here for a comparison of filesystems.

Just be glad you're not stuck with NTFS which is truly abysmal for lots of files in a directory. I'd recommend JFS as a replacement if you don't fancy using the relatively new (but apparently stable) ext4 FS.

Answer 4

I've hacked together a small web gallery, where I ended up with a variation of this problem; I "only" had ~30.000 images in the cache directory, which turned out quite slow (ext2 uses linked lists for directory indices, as I remember it).

I ended up doing something along these lines:

def key2path(key):
    hash = md5(key)
    return os.path.jion(hash[0], hash[1], key)

This will partition the data in 256 directories, which gives a fast directory lookup for each of the three levels.

• I've chosen to use MD5 over SHA, as MD5 guarantees a different output if you change any 12 bits of 32, so I find it a nice fit to hash user names, directories and other short stuff. And it's fast too...
• I do not include the entire hash, as it will produce way too many directories and effectively trash the disk-cache over and over.

Answer 5

Is the profile image small? What about putting it in the database with the rest of the profile data? This might not be the best option for you, but worth considering...

Here is a ( older ) Microsoft whitepaper on the topic: To BLOB or not to BLOB.

Answer 6

Not an immediate answer to your problem, but something to watch for future reference is the OpenBSD linked project called 'Epitome'

Epitome is an engine which provides Single Instance Storage, Content Addressable Storage and Deduplication services.

All of your data is stored in a data store as hashed blocks, removing non-unique blocks to cut down on space usage, and allows you to essentially forget about the storage mechanism as you can simply request the content from the data store by UUID.

Epitome is currently experimental, but something to watch for the future.

Answer 7

Generally you want to avoid having directories with a large number of files/directories in it. Primary reason is that wildcard expansion on the command line, will result in "Too many arguments" errors resulting in much pain when trying to work with these directories.

Go for a solution that makes a deeper but narrower tree, e.g. by creating subfolders like others have described.

Answer 8

We had a similar problem, the solution - as mentioned previously - is to create a hierarchy of directories.

Of course if you have a complex application which relies on a flat directory structure, you'll probably need a lot of patching. So it's good to know that there is a workaround, use symlinks which doesn't have the mentioned 32k limit. Then you have plenty of time to fix the app...