Chapter 11 (I/O Management and Disk Scheduling) Flashcards
What is programmed I/O?
CPU issues I/O and busy waits for it to complete
What is interrupt-driven I/O?
CPU issues I/O and continues, is interrupted when I/O completes
What is DMA?
CPU issues I/O and DMA module takes over, freeing the CPU
What information is contained in a call to the DMA module?
Whether to read/write
Address of the I/O device
Starting memory address to read or write to
Number of words to read or write
What are the two OS design objectives for I/O?
Efficiency
Generality
What is generality?
Have all devices use the same interface
What are two types of I/O devices?
Block-oriented
Stream-oriented
What are block-oriented I/O devices?
Information stored in fixed size blocks
Transfers made a block at a time
What are stream-oriented I/O devices?
Transfers information as a stream of bytes (no block structure)
What are the advantages of a single buffer?
Allows next unit to be read while previous is processed
Process can be swapped since I/O is using OS buffer
When might a double buffer be inadequate?
Process performs rapid bursts of I/O
What can be used if a process performs rapid bursts of I/O?
Circular buffering
Define: Access time
Sum of seek time and rotational delay
Define: Seek time
Time it takes to position the head at the desired track
Define: Rotational delay
Time it takes for the beginning of the sector to reach the head
Define: Data transfer time
Occurs as the sector moves under the head
What is the primary reason for differences in performance in disk scheduling policies?
Seek time
What are the three characteristics of RAID?
Multiple physical drives viewed by the OS as a single drive
Data distributed across the drives
Redundancy to ensure recoverability if a drive fails
What RAID levels are commercially unavailable?
2 and 4
What are the characteristics of RAID level 1?
Duplicates all data
Every disk has a mirror disk
Simple recovery but higher cost
Why is RAID level 2 not used?
Only useful if lots of disk errors occur, which is not common today
How does RAID level 2 handle recovery?
Parity disk
What are the characteristics of RAID level 3?
Single additional parity disk
Uses a parity bit instead of an error correcting code
What are strips in RAID level 4?
Blocks
What are the characteristics of RAID level 5?
Distributes parity disks across all disks
Keeps parity disk from being a bottleneck
What are the characteristics of RAID level 6?
Uses two parity strips on different disks instead of just 1
Requires N+2 disks
How many disks must fail in RAID 6 to be unavailable?
3
What are the replacement algorithms for the disk cache?
LRU - Least recently used
LFU - Least frequently used
What’s the problem with LFU disk cache replacement algorithms?
Blocks may be used repeatedly for a short amount of time, building up a high reference count, then not used again for a long time. This can cause LFU to make a poor choice.
How can you fix the problem with LFU?
Divide the queue into new, middle, and old sections
