Mass Storage and Disk Scheduling

Question 1

Magnetic disks

Answer 1

persistant storage, secondary storage

transfer rate - between drive and computer via I/O bus

overhead due to multiple mechanical

latency due to access times

when have latency, use a buffer

Question 2

RAID

Answer 2

redundant array of independent disks data storage virtualization technology RAID allows you to store the same data redundantly (in multiple paces) in a balanced way to improve overall performance.

Question 3

RAID demo jetstore

Answer 3

data distributed in different ways, RAID 0 - 10 goals are reliability, availability, performance, and capacity RAID levels higher than 0 provide protection against unrecoverable data in disk sectors RAID 0 redundancy RAID 1 mirror RAID 2 checkbits

Question 4

3 parameters for RAID

Answer 4

redundancy, parity, ECC (error correcting codes)

Question 5

Device addresses

Answer 5

Addressed as 1 dimensional arrays of logical blocks

arr maps into sectors of disk sequentially

Vendor ID

Device ID

FF980000 memory map, goes to I/O controller

Question 6

Disk performance

Answer 6

Tsf 4 kb on 7200 RPM disk

Most of time is fixed

t = fixed time + transfer time

Question 7

Disk scheduling - why

Answer 7

to have fast access time

increase bandwidth

access sectors that are in proximity to current request

Question 8

Disk scheduling access time components

Answer 8

seek time - time for the disk to move the heads to the cylinder containing the desired sector

rotational latency additional time waitng for disk to rotate to desired head

need to minimize seek time, which is approx equal to seek distance

Question 9

Disk scheduling algorithms

Answer 9

FCFS first come first served - next in line in queue

SSTF shortest seek time first - selects request with minimum seek time from the current head position

a form of SJF scheduling, may cause starvation

SCAN - elevator algorithm, disk arm starts at one end of disk, moves toward other end, servicing requests; at other end, reverses, continues to work requests

Question 10

Disk scheduling calculate control bits

Answer 10

2⁵⁰ / 2¹⁶ = 2³⁴ bits where 2¹⁶ is 64k block