1. Computer Architecture
2.3.2 Magnetic Disks
2.3.3 IDE
2.3.4 SCSI
By:
Vivek Sthanam
Kamyar Arbabifard
Prudhvi Potu

2. Secondary Storage devices
Two Major types of storage devices:
Direct access storage devices (DASDs)
Magnetic Disks:
hard disks (high capacity, low cost per bit)
Floppy disks ( low capacity, slow, cheap)
Optical Disks:
CD-ROM = ( Compact disc, read only memory)
Serial Devices
Magnetic tapes ( very fast sequential access)

3. 2.3.2 Magnetic Disks
Magnetic disks: long-term, nonvolatile storage for files, and a level of the memory hierarchy below main memory.
Characteristics (as of 2000)
Seagate Cheetah
IBM Travel star
IBM GB Microdrive
Disk diameter (inches)
3.5
2.5
1.0
Formatted data capacity
73.4 GB
32.0 GB
1.0 GB
Cylinders
14,100
21,664
7,167
Disks 12 4 1
Recording surfaces (heads) 24 8 2
Bytes per sector
512-4,096
512
Avg. sectors/track (512B)
424
360
140
Max. areal density (Gb/in2)
6.0
14.0
15.2
Rotation speed (RPM)
10,033
5,411
3,600
Avg. random seek (r/w) (ms)
5.6/6.2
12.0
Min. seek (r/w) (ms)
0.6/0.9
Max. seek (r/w) (ms)
14.0/15.0
23.0
19.0
Data transfer rate (MB/sec)
27-40
11-21

4. Magnetic Disks
Magnetic disks: long-term, nonvolatile storage for files, and a level of the memory hierarchy below main memory.
Average seek time is defined to be:
Where n is the number of all possible seeks.
Average rotation time is defined to be halfway round the disk;

5. Magnetic Disks Cont.
Transfer time is the time it takes to transfer a block of bits, typically a sector, under read/write head, which is a function of the block size, disk size, rotation speed, recording density of the track, and the speed of the electronics connecting the disk to the computer.
Read ahead is used to amortize the long access by reading more than what is simply requested. A buffer of a few MB is used to store the pre fetched data that assumes possible spatial locality.
Areal density is a measure of recording density in terms of bits per square inch:

6. Disk History 1989: 63 Mbit/sq. in 60,000 MBytes 1997: 1450 Mbit/sq. in

7. Spindle
Arm
Head
Actuator
Platters (12) {

8. Disk Device Terminology
Platter
Outer
Track
Inner
Sector
Actuator
Head
Arm
Several platters, with information recorded magnetically on both surfaces (usually)
Bits recorded in tracks, which in turn divided into sectors (e.g., 512 Bytes)
Actuator moves head (end of arm,1/surface) over track (“seek”), select surface, wait for sector rotate under head, then read or write
“Cylinder”: all tracks under heads

9. Disk Device Performance
Arm
Spindle
Platter
Actuator
Disk Latency = Seek Time + Rotation Time + Transfer Time + Controller Overhead
Seek Time? depends no. tracks move arm, seek speed of disk
Rotation Time? depends on speed disk rotates, how far sector is from head
Transfer Time? depends on data rate (bandwidth) of disk (bit density), size of request

10. Disk Device Performance
Average distance sector from head?
1/2 time of a rotation
7200 Revolutions Per Minute  120 Rev/sec
1 revolution = 1/120 sec  8.33 milliseconds
1/2 rotation (revolution)  4.16 ms
Average no. tracks move arm?
Sum all possible seek distances from all possible tracks / # possible
Assumes average seek distance is random
Disk industry standard benchmark

11. Disk Performance Model /Trends
Capacity
+ 100%/year (2X / 1.0 yrs)
Transfer rate (BW)
+ 40%/year (2X / 2.0 yrs)
Rotation + Seek time
– 8%/ year (1/2 in 10 yrs)
MB/$
> 100%/year (2X / <1.5 yrs)
Fewer chips + areal density

12. Floppy Disks Advantages: Disadvantages: Small Easy to carry Cheap
Useful for transferring small files 
Can be used many times 
Security tab to stop data from being overwritten
Disadvantages:
Easy to be damaged
Small storage capacity
Many new computers don’t have floppy disk drives
Can transport a virus from one machine to another
Slow to access and retrieve data when compared to a hard disk
Data can be erased if the disk comes into contact with a magnetic field 

13. Magnetic Tape Advantages: Relatively cheap per megabyte of storage
Can store large amounts of data - over 100 Gb 
Can be set up to do the back-up overnight or over the week 
Disadvantages: 
Needs serial access, so can be quite slow to access data 
Need a special piece of equipment to record and read the data on the tape 

14. Hard Disk Advantages: Disadvantages:
Stores data for operating systems, software and working data
Good for any application requiring very fast access
Good for online and real-time processes
Can handle major procedures such as payroll processing
Disadvantages:
Generally not portable

15. 2.3.3 IDE - Definition
Integrated Drive Electronics or IBM Disc Electronics, IDE is more commonly known as ATA or Parallel ATA (PATA).
It is a standard electronic interface used between a computer motherboard's data paths or bus and the computer's disk storage devices.
The IDE interface is based on the IBM PC Industry Standard Architecture (ISA) 16-bit bus standard.

16. Why IDE? Before IDE, controllers and hard drives were separate.
For example, a controller from one manufacturer might not work with a hard drive from another manufacturer. The distance between the controller and the hard drive could result in poor signal quality and affect performance.
This caused much frustration for computer users.
Thus, IDE was created as a way to standardize the use of hard drives in computers.
The basic concept of IDE is to combine the hard drive and controller.
The controller is a small circuit board with chips that provide guidance as to exactly how the hard drive stores and accesses data.

17. Current And Past Configuration
ATA-1:
This is first standard which was developed by Compaq, Control Data Corp. and Western Digital, by using an 8- or 16-bit interface.
It instituted the use of a master/slave configuration, and defined multi-Word Direct Memory Access (DMA)mode and Programmed I/O (PIO) modes 0-2.
ATA-2:
It is known as Enhanced IDE (EIDE).
It has outlined PIO mode 3 and PIO mode 4.

18. Current And Past Configuration
ATA-3:
It improved the reliability of high-speed transfers.
Also, added Self-Monitoring Analysis and Reporting Technology (SMART).
ATA/ATAPI-4:
It increased data transfer rate support to 33 megabytes per second (MBps) known as Ultra DMS/33.
It has the AT Attachment Packet Interface (ATAPI) feature.
AT/ATAPI-5:
Supports data transfer rates of up to 66 MBps.
ATA/ATAPI-6: 
It Supports Ultra DMA/100, which lets drives theoretically reach 100 MBps.
Also includes Automatic Acoustic Management, which allows drives using this feature to automatically adjust access speed and reduce running noise.

19. IDE Interface Components

20. Characteristics of IDE
Speed: The fastest IDE devices are able to transfer data at 133 Megabytes per second (Mbps).
Jumpers: IDE devices have jumpers, the computer uses them to differentiate between multiple devices sharing a cable.
Cable Size: IDE device use a 40-pin connection.
Cable Count: IDE devices require fewer cable count than other devices.

21. 2.3.4 SCSI Howard Shugart (Inventor of the Floppy Drive)
Shugart Associates Systems Interface - SASI (1979)
ANSI modified the SASI and standardized it - SCSI (1986)
SCSI: Small Computer System Interface
SCSI is pronounced scuzzy
Higher version
Fast SCSI
Ultra SCSI
Ultra2 SCSI
Ultra3 SCSI
Ultra4 SCSI
Ultra5 SCSI

22. Characteristics IDE-Like Organization Sylander Track Sector
Different Interface
Higher Transfer Rates
Standard disk in many high-end workstations
Useful for RAID configurations
More than just hard-disk interface
Available in 8 and 16 bit (wide) versions

23. More Than Hard-disk Interface
It is a bus connecting to:
A SCSI Controller (Usually plug-in card)
Up to 7 SCSI devices or 15 in wide versions
SCSI Hard Disks
CD-ROM
CD Recorders
Scanner
Tape Units
SCSI Devices
Unique ID from 0 to 7 or 0 to 15 in wide versions
Two connectors
Input
Output
Cables connect the output of one to the input of the other one in series (chain)
Last device needs termination to prevent reflection

24. SCSI Cable Most common cable for 8-bit SCSI 50 wires
25 of them are ground paired to provide excellent noise immunity
8 for data
1 for parity
9 for control
other for power and reserved for future
The 16-bit devices need a second cable
The cables may several meter long

25. Operation
SCSI controllers and peripherals can operate either as initiator or as targets
Usually the controller act as initiator issuing commands
Other peripherals act as targets
Commands are blocks of up to 16 byte telling the target what to do
Commands and responses occur in phases
using various control signals to:
delineate phases
arbitrate bus access between multiple devices
The arbitration is important because SCSI allows all devices to run at once potentially improving performance

26. SCSI Commands
SCSI commands are sent in a command descriptor blocks (CDP)
CDP consists of
1 byte of operation code (opcode)
5 or more byte containing command specific parameters
After receiving the CDP by the device, it will return a status code byte and other information
In the published SCSI standards command are designated as:
mandatory (required for all devices)
optional
vendor-specific
Due to different device types an opcode may produce different, but usually comparable effects
Ex. opcode 0x01 recalibrates the disk drive by seeking back to physical sector 0, but rewinds the medium in a tape drive

27. IDE Vs. SCSI IDE SCSI Cost Overall, IDE is a much cheaper solution.
When compared with IDE, SCSI is often more expensive to implement and support.
Expansion
IDE/EIDE allows 2 two devices per channel. Most computers have 2 channels.
SCSI is capable of supporting up to 7 or 15 devices.
Ease
IDE is commonly a much easier product to setup than SCSI.
Configuring SCSI can be more difficult for most users when compared to IDE.
Faster
Today, the latest IDE and SCSI drives running at the same RPM are very close.
All the fastest drives are almost always available for SCSI first and in many cases 10,000+ RPM hard drives are only available as SCSI drives.
Resources
All motherboards today have an ATA/IDE interface and unless additional drives are needed no additional resources need to be taken.
Unlike IDE, SCSI requires an interface expansion card in most cases (unless the motherboard already has it). Adding any new hardware means more system resources are going to be required.

28. Credits Kamyar Arbabifard Vivek Sthanam Prudhvi Potu Section 2.3.4
Additional References
SCSI Commands Reference Manual“ Seagate
Vivek Sthanam
Section 2.3.2
Kanellos, Michael (24 August 2006). "A divide over the future of hard drives". CNETNews.com. Retrieved 24 June 2010.
"IBM OEM MR Head | Technology | The era of giant magnetoresistive heads". Hitachigst.com. 27 August Retrieved 4 September 2010
Prudhvi Potu
Section 2.3.3
Frawley, Lucas. "Parallel vs. Serial ATA". What Is? The Information for Your Computer Questions. Directron.com. Retrieved 23 January 2012
"Serial ATA: A Comparison with Ultra ATA Technology". Seagate Technology. Archived from the original on Retrieved 23 January 2012.