Cm_Cache_Backend_File

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.

Installation

1. Clone module with modman
2. Edit app/etc/local.xml changing global/cache/backend to Cm_Cache_Backend_File
3. Delete all contents of the cache directory

Example Configuration

By default, Cm_Cache_Backend_File is configured not to use chmod to set file permissions. The proper way to do file permissions is to respect the umask and not set any permissions. This way the file permissions can be properly inherited such as when the setgid bit is used on the parent directory. To improve security the umask should be properly set. In Magento the umask is set in index.php as '0' which means no restrictions. To make files and directories no longer public change this to umask(0007).

<config>
    <global>
        <cache>
            <backend>Cm_Cache_Backend_File</backend>
        </cache>
        ...
    </global>
    ...
</config>

If umasks are too complicated and you prefer the sub-optimal (less-secure, needless system calls) approach you can enable the old chmod usage like so:

<config>
    <global>
        <cache>
            <backend>Cm_Cache_Backend_File</backend>
            <backend_options>
                <use_chmod>1</use_chmod>
                <directory_mode>0777</directory_mode>
                <file_mode>0666</file_mode>
            </backend_options>
        </cache>
        ...
    </global>
    ...
</config>

Cleaning Old Files

Magento and Zend_Cache do not cleanup old records by themselves so if you want to keep your cache directory tidy you need to write and invoke regularly your own script which cleans the old data. Here is an example for Magento:

<?php PHP_SAPI == 'cli' or die('<h1>:P</h1>');
ini_set('memory_limit','1024M');
set_time_limit(0);
error_reporting(E_ALL | E_STRICT);
require_once 'app/Mage.php';
Mage::app()->getCache()->getBackend()->clean('old');
// uncomment this for Magento Enterprise Edition
// Enterprise_PageCache_Model_Cache::getCacheInstance()->getFrontend()->getBackend()->clean('old');

Special Thanks

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).