Storage & Peripherals

Disks, Networks, and Other Devices

Typical System

I/O Device Comparison

Device Behavior Data Rate (Mbit/sec)Keyboard Input 0.0001

Mouse Input 0.0038

Graphics Display

Output 800.0 - 8000.0

Network/LAN Input/Output 100.0 - 1000.0

Network/WiFi Input/Output 11.0 – 54.0

Optical Disk Storage 80.0

Magnetic Disk Storage 240.0 – 2560.0

Disk Organization

Platter

Track

Platters

Sectors

Tracks

Professor Tod Amon, Lecture Slides, Southern Utah University, Morgan-Kaufmann, 2005

Disk Architecture Magnetic media : Ferris Oxide coating Rotational speed : 5400 – 15,000 RPM Diameter

1 to 3.5 currently Larger in past

Tracks – 10,000 to 50,000 per surface Sectors

100 to 500 per track 512 bytes per sector

ZBR (Zone Bit Recording) : evenly space bits Cylinder : same track on each platter

Disk Access

Components Seek Time – time to position the read/write heads Rotational Latency – time to wait on data to rotate

under the read/write head Transfer Time – time to transfer a block of bits Controller Time : disk controller overhead

Average Rotational Latency

Slowest

Fastest

msRPM

rotation

RPM

rotation6.5

)minsec

60/(5400

5.0

5400

5.0

msRPM

rotation

RPM

rotation0.2

)minsec

60/(000,15

5.0

000,15

5.0

Time to Read Disk

Suppose 512 byte sector 10,000 RPM Average seek time 6ms Transfer Rate of 50 MB/sec Controller overhead of 0.2ms

Average disk time =avg seek time + avg rot lat + trans time + controller time

msmsMB

MBbytes

bytes

RPM

rotms 2.92.0

sec50

10241024

512

minsec60

000,10

5.00.6

RAID

Redundant Array of Inexpensive Disk Goal: Improve performance and reliability by

using an array of many small disks as opposed to a few large expensive disks.

Introduces redundancy in data storage

RAID Levels

RAID Level 0 No redundancy Striping – writes data across multiple disks Parallel access to data

RAID Level 1 Mirroring Writes same data “mirror disks” Requires twice as many disks On disk failure, system just reads backup disk

RAID Levels

RAID Level 2 Detecting and Correcting Code Not used today

RAID Level 3 Bit-Interleaved Parity Keep only enough redundant data to restore lost

data Reads and writes go to all disks in the group, with

one extra disk to hold check information Parity – sum of data modulo 2 used

Parity

XOR () Operation is useful A B – gives sum A B B = A

Recovers A if B and the sum is known

Data blocks D1, D2, D3, D4

P = D1 D2 D3 D4

Any 3 and P allows the missing block to be recovered

X1 X2 X1 X2

0 0 1

0 1 0

1 0 0

1 1 1

Parity reference: Parity, http://www.pcguide.com/ref/hdd/perf/raid/concepts/genParity-c.html, Nov. 16, 2005.

RAID Levels

RAID Level 4 Block-interleave parity Parity stored as blocks and associated with a set

of data blocks One check disk associated with group data disks Allows access to “small reads and writes” RAID 3 read or write access each disk RAID 4 read or write may access a single disk Reads check error detection data stored with

each sector

RAID 3 vs. RAID 4 Parity

RAID 3 RAID 4

RAID Levels

RAID Level 5 Distributed Block-

Interleaved Parity Parity disk is bottle neck

for RAID 4 RAID Level 5 removes

by distributing parity across disk set

RAID Level Summary

RAID 1 and RAID 5 widely used in servers “hot swapping” allows disks to be replaced

w/o shutting the disk array down standby spares – includes extra disks in

system that can be used if disk goes down

I/O Buses

Shared communication link (one or more wires)

Difficult design:— may be bottleneck— length of the bus— number of devices— tradeoffs (buffers for higher

bandwidth increases latency)— support for many different devices— cost

Types of Buses

processor-memory (short high speed, custom design)

backplane (high speed, often standardized, e.g., PCI)

I/O (lengthy, different devices, e.g., USB, Firewire)

Transmission Modes

Synchronous vs. Asynchronous Synchronous

Uses Clock and a synchronous protocol Fast and small All devices must operate at same rate Bus must be short

Asynchronous Does not use a clock Requires a handshaking protocol

Contemporary Bus Standards

Comparison of two contemporary buses

Buses – Other Issues

Bus Arbitration: daisy chain arbitration (not very fair)

centralized arbitration (requires an arbiter), e.g., PCI

collision detection, e.g., Ethernet

Operating system: Polling

Interrupts

direct memory access (DMA)

I/O Options

Parallel ATA(100 MB/sec)

Parallel ATA(100 MB/sec)

(20 MB/sec)

PCI bus(132 MB/sec)

CSA(0.266 GB/sec)

AGP 8X(2.1 GB/sec)

Serial ATA(150 MB/sec)

Disk

Pentium 4processor

1 Gbit Ethernet

Memorycontroller

hub(north bridge)

82875P

MainmemoryDIMMs

DDR 400(3.2 GB/sec)

DDR 400(3.2 GB/sec)

Serial ATA(150 MB/sec)

Disk

AC/97(1 MB/sec)

Stereo(surround-

sound) USB 2.0(60 MB/sec)

. . .

I/Ocontroller

hub(south bridge)

82801EB

Graphicsoutput

(266 MB/sec)

System bus (800 MHz, 604 GB/sec)

CD/DVD

Tape

10/100 Mbit Ethernet