Product: Tugela Cache

Tugela Cache is a cache system like memecached, but instead of storing data just in RAM, it stores data in the file system using a b-tree. You trade latency in order to have a very large cache. It's useful for sites that have caching requirements that exceed their available memory. It uses the same wire protocol as memcached so it can be dropped in without a hassle. From the website:

As large MediaWiki deployments may gain performance using Memcached, at some level cost of RAM to store all objects becomes too high. In order to balance resource usage and make more use of our Apache server disks, Tugela, the distributed cached on-disk hash database, has arrived.

Tugela Cache is derived from Memcached. Much of the code remains the same, but notably, these changes:

• Internal slab allocator replaced by BerkeleyDB B-Tree database.
• Expiry policy management moved to external program tugela-expire
• Much statistics code made obsolete.

An interesting point brought up in the comments is using memcached with a larger cache size than physical RAM and then let the OS swap versus using a b-tree to access data on disk. Nginx seems to use the "let the OS swap" approach to good effect. It would be interesting to see which approach works better.

For an idea of how an in-process cache and a disk based cache hierarchy can work together take a look at Kevin Burton's IDEA: Hierarchy of caches for high performance AND high capacity memcached.

There's also an interesting variation called Memcachedb which is said to be "a better and simplified Tugela." It's more of a persistence mechanism than a cache. It enables transactions, replication, and there's no expiration.