Weekly Note 9 - Week 14

31 March 2019

Lecture - Tuesday, April 2nd.

16-19 in U82

We will cover chapter 11 on mass-storage structure and chapter 12 on I/Osystems. This will be the primary material for exam topic 8: "Mass-Storage Structure and I/O Systems".

We will have a small review of the previous lecture, so this time it will be chapters 10, and have the Kahoot as well.

Tutorial session

Friday April 5th. 12-14 in U166 This week there is again only one session.

Preparation:

Make a list of 10-15 keywords for a 10 min. presentation with the topic: "Mass-Storage Structure and I/O Systems"

Prepare at home to discuss:

  1. None of the disk-scheduling disciplines, except FCFS, is truly fair (starvation may occur).

    1. Explain why this assertion is true.

    2. Describe a way to modify algorithms such as SCAN to ensure fairness.

    3. Explain why fairness is an important goal in a time-sharing system.

    4. Give three or more examples of circumstances in which it is important that the operating system be unfair in serving I/O requests.

  2. Explain why SSDs often use a FCFS disk scheduling algorithm.

  3. Suppose that a disk drive has 5000 cylinders, numbered 0 to 4999. The drive is currently serving a request at cylinder 2150, and the previous request was at cylinder 1805. The queue of pending requests, in FIFO order, is: 2069, 1212, 2296, 2800, 544, 1618, 356, 1523, 4965, 3681. Starting from the current head position, what is the total distance (in cylinders) that the disk arm moves to satisfy all the pending requests for each of the following disk-scheduling algorithms?

    1. FCFS

    2. SCAN

    3. C-SCAN

  4. Describe some advantages and disadvantages of using SSDs as a caching tier and as a disk drive replacement compared to a system with just magnetic disks.

  5. Consider a RAID Level 5 organization comprising five disks, with the parity for sets of four blocks on four disks stored on the fifth disk. How many blocks are accessed in order to perform the following?

    1. A write of one block of data

    2. A write of seven continuous blocks of data

  6. Compare the throughput achieved by a RAID Level 5 organization with that achieved by a RAID Level 1 organization for the following:

    1. Read operations on single blocks

    2. Read operations on multiple contiguous blocks

  7. Assume that you have a mixed configuration comprising disks organized as RAID Level 1 and as RAID Level 5 disks. Assume that the system has exibility in deciding which disk organization to use for storing a particular file. Which files should be stored in the RAID Level 1 disks and which in the RAID Level 5 disks in order to optimize performance?

  8. Discuss the relative advantages and disadvantages of sector sparing and sector slipping.

  9. When multiple interrupts from different devices appear at about the same time, a priority scheme could be used to determine the order in which the interrupts would be serviced. Discuss what issues need to be considered in assigning priorities to different interrupts.

  10. What are the advantages and disadvantages of supporting memory-mapped I/O to device-control registers?

  11. In most multiprogrammed systems, user programs access memory through virtual addresses, while the operating system uses raw physical addresses to access memory. What are the implications of this design on the initiation of I/O operations by the user program and their execution by the operating system?

In class:

Use the first 30 minutes to work on those exercises:

  1. What are the various kinds of performance overheads associated with servicing an interrupt?

  2. Describe three circumstances under which blocking I/O should be used. Describe three circumstances under which non-blocking I/O should be used. Why not just implement non-blocking I/O and have processes busy-wait until their device is ready?

  3. The reliability of a hard-disk drive is typically described in terms of a quantity called mean time between failures (MTBF). Although this quantity is called a "time", the MTBF actually is measured in drivehours per failure.

    1. If a system contains 1000 drives, each of which has a 750,000-hour MTBF, which of the following best describes how often a drive failure will occur in that disk farm: once per thousand years, once per century, once per decade, once per year, once per month, once per week, once per day, once per hour, once per minute, or once per second?

    2. Mortality statistics indicate that, on the average, a U.S. resident has about 1 chance in 1000 of dying between ages 20 and 21 years. Deduce the MTBF hours for 20 year olds. Convert this figure from hours to years. What does this MTBF tell you about the expected lifetime of a 20 year old?

    3. The manufacturer guarantees a 1-million-hour MTBF for a certain model of disk drive. What can you conclude about the number of years for which one of these drives is under warranty?

  4. Typically, at the completion of a device I/O, a single interrupt is raised and appropriately handled by the host processor. In certain settings, however, the code that is to be executed at the completion of the I/O can be broken into two separate pieces. The first piece executes immediately after the I/O completes and schedules a second interrupt for the remaining piece of code to be executed at a later time. What is the purpose of using this strategy in the design of interrupt handlers?

Then spend the remaining time discussing your list of keywords, and the excercises prepared at home and in class.

  • Chapter 11 and 12 in Operating System Concepts, Enhanced eText, 10th Edition

Material (Slides, etc.)