Design and Implementation of a Freeblock Subsystem (CMU-PDL-03-107)

Freeblock scheduling allows background applications to access the disk without affecting primary system activities. This paper describes a complete freeblock subsystem, implemented in FreeBSD. It details new space- and time-efficient algorithms that make freeblock scheduling useful in practice. It also describes algorithm extensions for using idle time, dealing with multi-zone disks, reducing fragmentation, and avoiding starvation of the inner- and outer-most tracks. The result is an infrastructure that efficiently provides steady disk access rates to background applications, across a range of foreground usage patterns.