Notes

Outline

IO Management and Disk Scheduling Chapter 11

Categories of I/O Devices Human readable used to communicate with the user video display terminals keyboard mouse printer

Categories of I/O Devices Machine readable used to communicate with electronic equipment disk drives tape drives controllers actuators

Categories of I/O Devices Communication used to communicate with remote devices digital line drivers modems

Differences in I/O Devices Data Transfer Rate Application disk used to store files must have file-management software disk used to store virtual memory pages needs special hardware to support it terminal used by system administrator may have a higher priority

Differences in I/O Devices Complexity of control Unit of transfer data may be transferred as a stream of bytes for a terminal or in larger blocks for a disk Data representation encoding schemes Error conditions devices respond to errors differently

Techniques for Performing I/O Programmed I/O process is busy-waiting for the operation to complete Interrupt-driven I/O I/O command is issued processor continues executing instructions I/O module sends an interrupt when done

Techniques for Performing I/O Direct Memory Access (DMA) DMA module controls exchange of data between main memory and the I/O device processor interrupted only after entire block has been transferred

Operating System Design Objectives I/O extremely slow compared to main memory Use of multiprogramming allows for some processes to be waiting on I/O while another process executes I/O cannot keep up with processor speed Swapping is used to bring in additional Ready processes which is an I/O operation Efficiency is an important issue

Operating System Design Objectives Efficiency is an important issue Desirable to handle all I/O devices in a uniform manner Hide most of the details of device I/O in lower-level routines so that processes and upper levels see devices in general terms such as Read, Write, Open, and Close Generality is an important issue

A Model of I/O Organization

I/O Buffering Reasons for buffering Processes must wait for I/O to complete before proceeding Certain pages must remain in main memory during I/O

I/O Buffering Block-oriented information is stored in fixed sized blocks transfers are made a block at a time used for disks and tapes Stream-oriented transfer information as a stream of bytes used for terminals, printers, communication ports, mouse, and most other devices that are not secondary storage

No Buffering

Single Buffer Operating system assigns a buffer in main memory for an I/O request Block-oriented input transfers made to buffer block moved to user space when needed another block is moved into the buffer read ahead

Single Buffer Block-oriented user process can process one block of data while next block is read in swapping can occur since input is taking place in system memory, not user memory operating system keeps track of assignment of system buffers to user processes output is accomplished by the user process writing a block to the buffer and later actually written out

Single Buffer Stream-oriented used a line at time user input from a terminal is one line at a time with carriage return signaling the end of the line output to the terminal is one line at a time

Double Buffer Use two system buffers instead of one A process can transfer data to or from one buffer while the operating system empties or fills the other buffer

Circular Buffer More than two buffers are used Each individual buffer is one unit in a circular buffer Used when I/O operation must keep up with process

Disk Performance Parameters To read or write, the disk head must be positioned at the desired track and at the beginning of the desired sector Seek time time it takes to position the head at the desired track Rotational delay or rotational latency time its takes until desired sector is rotated to line up with the head

Disk Performance Parameters Access time sum of seek time and rotational delay the time it takes to get in position to read or write Data transfer occurs as the sector moves under the head Data transfer for an entire file is faster when the file is stored in the same cylinder and in adjacent sectors

Disk Scheduling Policies Seek time is the reason for differences in performance For a single disk there will be a number of I/O requests If requests are selected randomly, we will get the worst possible performance

Disk Scheduling Policies First-in, first-out (FIFO) process request sequentially fair to all processes approaches random scheduling in performance if there are many processes

Disk Scheduling Policies Priority goal is not to optimize disk use but to meet other objectives short batch jobs may have higher priority provide good interactive response time

Disk Scheduling Policies Last-in, first-out good for transaction processing systems the device is given to the most recent user so there should be little arm movement possibility of starvation since a job may never regain the head of the line

Disk Scheduling Policies Shortest Service Time First select the disk I/O request that requires the least movement of the disk arm from its current position always choose the minimum Seek time

Disk Scheduling Policies SCAN arm moves in one direction only, satisfying all outstanding requests until it reaches the last track in that direction direction is reversed

Disk Scheduling Policies C-SCAN restricts scanning to one direction only when the last track has been visited in one direction, the arm is returned to the opposite end of the disk and the scan begins again

Disk Scheduling Policies N-step-SCAN segments the disk request queue into subqueues of length N subqueues are process one at a time, using SCAN new requests added to other queue when queue is processed FSCAN two queues one queue is empty for new request

RAID 0 (non-redundant)

Data Mapping for RAID Level 0 Array

RAID 1 (mirrored)

RAID 2 (redundancy through Hamming code)

RAID 3 (bit-interleaved parity)

RAID 4 (block-level parity)

RAID 5 (block-level distributed parity)