OCache: Onion caching on Web Servers Pradyumna Agrawal and Chinmay Kamat 18-845, Internet Services, Spring 2013 Web Servers nowadays are becoming extremely powerful and need to cater to a lot of data. Typical web servers will attempt to cache the working set data in its main memory. DRAM cache misses are then served from the primary disk, which may also reside on a NAS system. We present the design and implementation of a hierarchical multi-layered caching system which can be used on any server which provides some Internet service. In our hierarchical system, we make use of a solid state disk as a second level cache which resides between the main memory and the primary disk at the Internet service end point. We also present an in depth evaluation of our implementation when employed on the Popular Tiny multithreaded Web server.