disks chapter 5 thursday, april 5, 2007. today’s schedule input/output – disks (chapter 5.4) ...
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Today’s Schedule Input/Output – Disks (Chapter 5.4)
Magnetic vs. Optical Disks RAID levels and functions Disk Arm Scheduling Algorithms
Objectives
You will be able to describe: Differences between magnetic and optical
disks Benefits of RAID levels 0, 1, 2, & 3 Factors in time to read/write block to disk Relative benefits of disk arm scheduling
algorithms
Disk Hardware
Disk parameters for the original IBM PC floppy disk and a Western Digital WD 18300 hard disks
Optical vs. Magnetic sDisc Storage
Optical disc vs. Magnetic disk:Magnetic disk
Consists of concentric tracks of sectors Spins at a constant angular velocity (CAV) Wastes storage space but data retrieval is fast
Optical disc Consists of a single spiralling track of same-sized
sectors running from center to rim of disc Spins at a constant linear velocity (CLV) Allows more sectors and more data to fit on a disc
Optical Disc Storage Important features of optical discs:
Sustained data-transfer rate: Speed at which massive amounts of data can be read from disc
Measured in bytes per second (Mbps) Crucial for applications requiring sequential access
Average access time: Average time required to move head to a specific place on disc
Expressed in milliseconds (ms)
Cache size: Hardware cache acts as a buffer by transferring blocks of data from the disc
Disk Access Times Time required to access a file depends on:
Seek time: Time to position read/write head Slowest of the three factors Doesn’t apply to devices with fixed read/write heads
Search time (rotational delay): Time to rotate until desired record is under read/write head
Transfer time: Time to transfer data from secondary storage to main memory
Fastest
DASD Access Times
Movable-Head Devices:Access time = Seek time + Search time +
Transfer time Blocking is a good way to minimize access
time
RAID- Redundant Array of Inexpensive Disks
A set of physical disk drives that is viewed as a single logical unit
Improves I/O performance Improves reliability through redundancy
(data recovery in event of disk failure) BUT ….
Increases hardware costs CPU performance improved with parallel
processing, apply to I/O
RAID (continued)
Figure 7.18: Data being transferred in parallel from a Level 0 RAID configuration to a large-capacity disk
RAID
Mirroringredundancy to help recover from hardware failure
Cost, speed, and the system’s applications are significant factors to consider when choosing a particular RAID level
RAID (continued)
Table 7.7: The seven standard levels of RAID provide various degrees of error correction
Disk Formatting
A disk sector
• Low-Level format on each platter
• Preamble – bit pattern h/w detection of sector start
• Data – Ex 512 bytes
• ECC – Info to recover from read errors
• Partition Disk – Logically separate disks
• 0 has Master Boot Record
• High-Level format of each Partition to support O/S and file system
Disk Formatting
Interleaving gives Controller “breathing space” between consecutive sectors to copy buffer to memory
a) No interleavingb) Single interleavingc) Double interleaving
Disk Arm Scheduling Algorithms Time required to read or write a disk block
determined by 3 factors1. Seek time
2. Rotational delay
3. Actual transfer time Seek time dominates
Use other than FCFS scheme Error checking is done by controllers
Goal: Minimal response time and fairness
Disk Arm Scheduling Algorithms
Shortest Seek First (SSF) Service closest Request next
Ex. Seek Requests:11, 1, 36, 16, 34, 9, 12
FCFS = 111 arm moves vs SSF has 61
Initialposition
Pendingrequests Favors the middle cylinders
Disk Arm Scheduling Algorithms
The elevator algorithm Keep moving in same direction until no more outstanding requests
Summary
Differences between magnetic and optical disks
Benefits of RAID levels 0, 1, 2, & 3 Factors in time to read/write block to disk Relative benefits of disk arm scheduling
algorithms
Let’s Try some for extra points …
Chapter 5 Problems, Pg 373 – 376Problems
#2, 9, 10, 13, 16, 18, 22, 24