The current 'file' cache store has been optimised for situations where:

• The maximum number of things stored is quite low (32-bit CRC used as the key), and where performance is of the essence (we cache things like lang strings).

There are other situations where we cache things that are more expensive to compute, and we may be going to cache many more of them (e.g. question definitions).

For these, we probably want a cache store which offers a different trade-off:

• Stronger, slower hash function
• Store keys in subfolders, to limit the number of files in any one folder.

This issue is to work on such a thing.

Here is relevant link that gives chance of key collision for various size keys: http://en.wikipedia.org/wiki/Birthday_attack