[$] The creation of the io.latency block I/O controller

Sharing a disk between users in Linux is awful. Different applications
have different I/O patterns, they have different latency requirements, and
they are never consistent. Throttling can help ensure that users get their
fair share of the available bandwidth but, since most I/O is in the
writeback path, it's often too late to throttle without putting pressure
elsewhere on the system. Disks are all different as well. You have
spinning rust, solid-state devices (SSDs), awful SSDs, and barely usable
SSDs. Each class of device has its own performance characteristics and,
even in a single class, they'll perform differently based on the workload.
Trying to address all of these issues with a single I/O controller was
tricky, but we at Facebook think that we have come up with a reasonable
solution.