The stock Zend_Cache_Backend_File backend has extremely poor performance for cleaning by tags making it become unusable as the number of cached items increases. This backend makes many changes resulting in a huge performance boost, especially for tag cleaning.
This cache backend works by indexing tags in files so that tag operations do not require a full scan of every cache file. The ids are written to the tag files in append-only mode and only when files exceed 4k and only randomly are the tag files compacted to prevent endless growth in edge cases.
The metadata and the cache record are stored in the same file rather than separate files resulting in fewer inodes and fewer file stat/read/write/lock/unlink operations. Also, the original hashed directory structure had very poor distribution due to the adler32 hashing algorithm and prefixes. The multi-level nested directories have been dropped in favor of single-level nesting made from multiple characters.
Is the improvement substantial? Definitely. Tag cleaning is literally thousands of times faster, loading is twice as fast, and saving is slightly slower dependent on the number of tags being saved.
Test it for yourself with the Magento Cache Benchmark.
- Clone module with modman
- Edit
app/etc/local.xmlchangingglobal/cache/backendtoCm_Cache_Backend_File - Delete all contents of the cache directory
Thanks to Vinai Kopp for the inspiring this backend with your symlink rendition!
@copyright Copyright (c) 2012 Colin Mollenhour (http://colin.mollenhour.com)
This project is licensed under the "New BSD" license (see source).