The short, uninteresting answer is that it's a work in progress. Initially we built FIFO into our caches because it was easy to build and didn't interact badly

The short, uninteresting answer is that it's a work in progress. Initially we built FIFO into our caches because it was easy to build and didn't interact badly with flash disk craziness (write amplification specifically). You can read more about our old caching system mcdipper at https://www.facebook.com/notes/facebook-engineering/mcdipper-a-key-value-cache-for-flash-storage/10151347090423920 https://www.facebook.com/notes/facebook-engineering/mcdipper...