Freeblock Scheduling Outside of Disk Firmware
Conference on File and Storage Technologies (FAST) January 28-30, 2002. Monterey, CA. Supercedes Carnegie Mellon University SCS Technical Report CMU-CS-01-149.
Christopher R. Lumb, Jiri Schindler, Gregory R. Ganger
Dept. Electrical and Computer Engineering
Carnegie Mellon University
Pittsburgh, PA 15213
Freeblock scheduling replaces a disk drives rotational latency
delays with useful background media transfers, potentially allowing
background disk I/O to occur with no impact on foreground service times.
To do so, a freeblock scheduler must be able to very accurately predict
the service time components of any given disk request -- the necessary
accuracy was not previously considered achievable outside of disk firmware.
This paper describes the design and implementation of a working external
freeblock scheduler running either as a user-level application atop
Linux or inside the FreeBSD kernel. This freeblock scheduler can give
15% of a disks potential bandwidth (over 3.1MB/s) to a background
disk scanning task with almost no impact (less than 2%) on the foreground
request response times. This can increase disk bandwidth utilization
by over 6X.