Disk interface

Controls hardware, mediates access
Computer, disk often connected by bus (e.g., SCSI)
Multiple devices may contentd for bus
SCSI devices can disconnect during requests (+200 µs)
Command queuing:  Give disk multiple requests
Disk can schedule them using rotational information
Disk cache used for read-ahead
Otherwise, sequential reads would incur whole revolution
Cross track boundaries?  Can't stop a head-switch
Some disks support write caching
But data not stable--not suitable for all requests