Lots of ext4-dio-unwrit processes in the server.

Issue

• There are a lot of processes with the name ext4-dio-unwrit running on the system.
• What are those processes?
• Why so many ext4-dio-unwrit processes?

In RHEL 5.x and later you can see these processes are running. 

Root Cause

ext4-dio-unwrit processes are workqueues statically created at mount time of the filesystem, one workqueue per-cpu for each filesystem.