CSE 131 Lecture Notes - Lecture 20: Hard Disk Drive Performance Characteristics, Readwrite, Elevator Algorithm
Document Summary
Time required to read or write a disk block determined by 3 factors. Longer seeks take longer, but not linear with distance. Example: rotate in 10ms, average rotation delay = 5ms. Transfer time = time to rotate over sector. Example: rotate in 10ms, 1000 sectors/track -> 10/1000ms = 0. 01ms transfer time per sector. Error checking is done by on-disk controller. Disk transferring as often as possible (and not seeking) Minimize disk seek time (moving from track to track) Minimize rotational latency (waiting for disk to rotate the desired sector under the read/write head) Minimize seek time and rotational latency by. Using algorithms to find a good sequence for servicing requests. Placing blocks of a given file near each other. Schedule disk requests to minimize disk seek time. Seek time increases as distance increases (though not linearly) Minimize seek distance -> minimize seek time. Disk seek algorithm examples assume a request queue and head position (disk has 200 cyclinders)
