This extension is no longer maintained and is only left here to point anybody visiting to the successor extension here: https://github.com/colinmollenhour/Cm_Cache_Backend_File
This module is intended to improve the performance of the file cache backend with large cache pools (as fast or slow backend).
There is a post on MageBase about it which contains more details:
This module is provided with no warranty, you use it at your own risk. I know it is being used successfully on several sites, both as a primary cache backend and as a slow backend in combination with APC or memcached. I invite you to have a look at it and try it out, but please start with a test instance and not your live store.
If you find bugs or have improvements, please send them in.
According to the php reference manual symlinks works under Linux or since PHP 5.3 under Windows Vista/Windows Server 2008 or greater (see in the php manual under the section titled Notes).
After installation, clear the cache and check under "System > Tools > Symlink Cache" for further instructions.