W9 - I/O Management and Disk Scheduling

Question 1

External devices that engage in I/O with computer systems can be categorized into what three categories?

Answer 1

1) human readable
2) machine readable
3) communication

Question 2

What are the design objectives of I/O efficiency?

Answer 2

• prevent bottleneck operations
• increase speed to coincide with memory and process speed
• give more attention to disk I/O

Question 3

What are the design objectives of I/O generality?

Answer 3

• handle all devices in uniform manner

- NEG: diversity of devices make it difficult to achieve true generality

Question 4

What is a device driver?

Answer 4

It is a computer program allowing high-level computer programs to interact with a hardware device
Typically communicates with the device through the computer bus

Question 5

How does interrupt-driven I/O work?

Answer 5

Processor issues I/O command on behalf of a process.
If non-blocking, processor continues to execute instructions from the process that issues the I/O command.
If blocking, the next instruction the processor executes is from the OS, which is the current process in a blocked state and schedule another process

Question 6

What do drivers use kernel functions for in Driver-Kernel Interface?

Answer 6

1) device allocation
2) scheduling
3) buffering and caching
4) resource allocation

Question 7

Name a few characteristics of I/O buffering as a block-oriented device?

Answer 7

• stores information in blocks that are usually fixed sized
• transfers made one block at a time
• disks and USBs

Question 8

Name a few characteristics of I/O buffering as a stream-oriented device?

Answer 8

• transfers data in and out as a stream of bytes
• no block structure
• printers, communications ports

Question 9

Distinguish difference between block-oriented single buffer vs stream-oriented single buffer.

Answer 9

Block-oriented read ahead input. Generally provides a speedup compared to lack of system buffering.
For stream-oriented, appropriate for scroll-mode terminals. One line at a time

Question 10

Name a main advantage to double buffer.

Answer 10

Transfer data to or from one buffer while OS empties or fills the other buffer

Question 11

Why would a circular buffer be used?

Answer 11

When I/O operation must keep up with process.

Question 12

What do magnetic disks provide?

Answer 12

They provide bulk of secondary storage of modern computers.

Question 13

What are some pros and cons to Nonvolatile Memory (NVM).

Answer 13

Pros: 
More reliable than HDD.
Faster
Less power
Cons:
Expensive
Lower capacity
Shorter lifespan

Question 14

Briefly explain the RAID strategy.

Answer 14

Stands for Redundant Array of Independent Disks.
Employs multiple disk drives and distributes data in such a way as to enable simultaneous access to data from multiple drives

Question 15

Name two advantages to RAID.

Answer 15

1) improves I/O performance

2) allows easier incremental increases in capacity

Question 16

UNIX Buffer Cache.

Answer 16

Data transfer between the buffer cache and user process space always occurs using DMA

Question 17

What three lists are maintained in UNIX buffer cache?

Answer 17

1) free list
2) device list
3) driver I/O queue

Question 18

What is unbuffered I/O?

Answer 18

DMA between device and process space.
Fastest method for a process to perform I/O.
Process locked in main memory and cannot be swapped out.

Question 19

Elevator Scheduler.

Answer 19

Maintains a single queue for disk read and write requests and performs both sorting and merging functions on the queue.
When a new request is added to the queue, 4 operations are considered in order:
1) If request is to the same on-disk sector or an immediately adjacent sector to a pending request in the queue, then the existing request and the new request are merged into one request
2) If a request in the queue is old, new request is inserted at the tail of the queue
3) If a suitable location found, new request is inserted in sorted order
4) If no suitable location, new request is placed at the tail of the queue

Question 20

What two problems occur from elevator scheme?

Answer 20

1) Distance request block can be delayed for a time because queue is dynamically updated.
2) Stream of write requests can block a read request for a time, and thus block a process

Question 21

What happens when a read request is dispatched in Anticipatory I/O Scheduler?

Answer 21

A delay is caused. In this scheduler, the application that issued the last read request will issue another read request