Flashcache is a disk cache component for the Linux kernel, initially developed by Facebook since April 2010, and released as open source in 2011.
This caching applies to all disk content, not just the page file or system binaries.
Flash memory based devices are usually a magnitude faster than spinning HDDs for random IO, but with less advantage or even slower in sequential read/writes.
The data structure of the cache is a set-associative hash table, in which the cache is divided up into a number of fixed-size sets (buckets), using linear probing within a set to find blocks.
There are a few parameters to control the write-back policy: dirty-threshold, idleness and contiguity with other dirty blocks about to be written back.