Disk-backed object cache

The Disk-backed object cache project is a supplement to memcache-based object cache, which will allow us to greatly increase the amount of space dedicated to caching parsed pages.

Rationale
Our parser cache hit ratio is very low, around 30% (see the graph)

This seems to be mostly due to insufficient parser cache size. Tim's theory is that if we increased the parser cache size by a factor of 10-100, then most of the yellow area on that graph should go away. This would reduce our Apache CPU usage substantially.

The parser cache does not have particularly stringent latency requirements, since most requests only do a single parser cache fetch.

Timeline
After an initial attempt to deploy Ehcache, we switched to a different strategy of deploying a custom MySQL-based solution. This is on the WMF MediaWiki Core developers' roadmap, slated for work by Tim Starling soon after the 1.17 release.

Metrics

 * Parser cache hit rate:
 * last 8 hours
 * last 8 days
 * Historic (pre-improvement) norm: 30% hit, 68% miss (absent), and <2% miss (expired)
 * The goal is to minimize the yellow area (cache miss - absent)
 * Apache CPU
 * past week
 * Disk usage
 * Overview
 * The cache started out at 2.1 terabytes, so the cache size should is 2.1 TB minus the disk free.

Additional documents

 * User requirements:
 * Specifications:
 * Software design document:
 * Test plan:
 * Documentation plan:
 * User interface design docs:
 * Schedule:
 * Task management:
 * Release management plan:
 * Communications plan:
 * Status updates