EXTERNAL MEMORY
A TIMING COMPARISON
With the foregoing parameters defined, two different I/O operations that illustrate the danger of relying on average values. When we consider a disk we would like to estimate the total time for the transfer
ACCESS TIME
•The time it takes to get in position to read or write.
•Seek time + rotational Delay.
ROTATIONAL DELAY
•The time it takes for the disk to rotate so that the required sector is above the disk head.
•On average, half a rotation is required to get in position.
SEEK TIME
•The time it takes to move the disk arm to the required cylinder.
•In old disk - proportional to the distance.
•Newer disks - more complicated - accelerates and then decelerates.
DISK PERFORMANCE PARAMETERS
FLOPPY DISK
• 8”, 5.25”, 3.5”
• Small capacity —Up to 1.44Mbyte (2.88M never popular)
• Slow
• Universal
• Cheap
• Obsolete
WINCHESTER HARD DISK
• Developed by IBM in Winchester (USA)
• Sealed unit
• One or more platters (disks)
• Heads fly on boundary layer of air as disk spins
• Very small head to disk gap
• Getting more robust
•Universal
• Cheap
• Fastest external storage
• Getting larger all the time
HEAD MECHANISM
CLUSTER
Cluster is Group of two or more sector together in one track on single hard drive platter
SECTOR
Sector is Smallest aggregation of data(Clump) on a single track on single hard drive platter
CYLINDER
Cylinder is Group of same tracks(Rings of data) in same position on each hard drive platter together
TRACK
Track is Group of clusters located in same (Rings of data) on a single hard drive platter
SURFACE DATA DETAIL
PLATTER
One or more aluminum, glass, or ceramic disk that is coated in a magnetic media and is located within a hard drive to permanently store all your computer's data. When the computer is turned on, these platters will begin to rotate at so many rotations per minute (RPM). This rate varies depending upon the model of hard drive you have; an example of how fast a hard drive may spin is 7200 RPM. As the disk platters are rotating, the read/write head accesses information on one of the platters. To help store and retrieve the data from the platter, data is stored in tracks, sectors, and cylinders on each platter
Single platter vs Multi platter
These attributes are important for several reasons: larger platter density generally equals higher data throughput performance, but may result in slower seek performance. Meanwhile, less disks and actuator arms mean less moving parts in the drive to eventually break, and can also lower the heat and noise output of a drive
Head Motion
In the table list the major characteristics that differentiate among the various types of magnetic disks. The head May be Either fixed or movable with respect to the radial steering of the platter.
fixed-head disk, there is one read-write head per track. All of the heads are mounted on a rigid arm That extends across all tracks.
movable-head disk, there is only one read-write head per side. Again the head is mounted on an arm. Because the head must be reliable to be Positioned Above Any track, the arm can be extended or retracted for this purpose.
Disk Portability
The disk Itself is mounted in a disk drive, which Consists of the arm, the shaft That Rotates the disk, and the electronics circuitry needed for input and output and the binary data to the control mechanism
non removable disk is Permanently mounted on the disk drive. A removable disk can be removed and with replaced Reviews another disk, Such as Hard disk in personal computer.
Removable disk is media that enables a user to move data between computers without having to open their computer, such as floppy-disk flash-drive.
Sides
For MOST disks, the magnetizable coating is Applied to Both Sides of the platters, which is Then Referred to as double-sided. If the magnetizable coating is Applied to one side only, Then It is termed as single sided disk
double-sided is disk with both sides prepared for recording data. You can store twice as much data on a double-sided disk, but you need to use a double-sided disk drive. All modern disks and disk drives are double-sided.
Physical Characteristics
In a fixed-head disk, there is one read-write head per track. All of the heads are mounted on a rigid arm that extends across all tracks, such systems are rare today. In a movable-head disk, there is only one read-write head. The arm can be extended or retracted for this purpose