It looks like redis is using fork to generate checkpoints of the database. Which is quite a neat hack - use the copy-on-write memory properties of modern unix fork() implementations to implement persistent checkpoints of database state.
It does mean that forking the entire process is pretty much the point of the exercise however.
Redis uses a reactive architecture using non-blocking I/O. They fork to get a point-in-time consistent snapshot that can be written to disk. The problem is that fork blocks, and while blocked, it stalls the event loop, subjecting incoming requests to stalls.
