Question

I have a web server which saves cache files and keeps them for 7 days. The file names are md5 hashes, i.e. exactly 32 hex characters long, and are being kept in a tree structure that looks like this:

00/   00/     00000ae9355e59a3d8a314a5470753d8     .     . 00/   01/ 

You get the idea.

My problem is that deleting old files is taking a really long time. I have a daily cron job that runs

find cache/ -mtime +7 -type f -delete 

which takes more than half a day to complete. I worry about scalability and the effect this has on the performance of the server. Additionally, the cache directory is now a black hole in my system, trapping the occasional innocent du or find.

The standard solution to LRU cache is some sort of a heap. Is there a way to scale this to the filesystem level? Is there some other way to implement this in a way which makes it easier to manage?

Here are ideas I considered:

1. Create 7 top directories, one for each week day, and empty one directory every day. This increases the seek time for a cache file 7-fold, makes it really complicated when a file is overwritten, and I’m not sure what it will do to the deletion time.
2. Save the files as blobs in a MySQL table with indexes on name and date. This seemed promising, but in practice it’s always been much slower than FS. Maybe I’m not doing it right.

Any ideas?

Answer 1

When you store a file, make a symbolic link to a second directory structure that is organized by date, not by name.

Retrieve your files using the ‘name’ structure, delete them using the ‘date’ structure.