External memory

One . Principle and mode of magnetic recording

1. Principle of magnetic recording

The basic principle of magnetic recording is to use the residual state of hard magnetic materials to save binary information .

2. Magnetic recording mode

Two . Disk memory （ a key ）

1. The structure of the disk

• The disk is made up of heads , Disc , Driver , Controller, etc .
• Each disc can provide two recording surfaces , Each recording surface is equipped with an independent magnetic head .
• A plurality of narrow concentric circles are distributed on each recording surface of each disc , be called Magnetic track （Track）, Data is recorded on the track .
• The outermost circle of the track is 0 Avenue , The inward track number increases in turn . Each track is equally divided into several segments , Each segment is called a sector （Sector）
• Tracks with the same serial number on all recording surfaces form a cylinder （Cylinder）, Its number is the same as the track number .
  

2. Disk parameters

1. Channel density

Along the radius of the disk , The number of tracks per unit length is called track density . Company ： Avenue /mm, Avenue / Inch

2. Potential density

Bit density refers to the number of binary bits stored in a unit length on a track . Company ： position /mm, position / Inch
Closer to the disc center , The higher the bit density of the track .

3. storage capacity

Storage capacity refers to the total amount of binary information that the entire disk can store . Disk capacity is divided into formatted capacity and unformatted capacity .

Example analysis ：

3. disk array

Concept ：
Independent redundant disk array （Redundant Array of Independent Disk,RADI） By the University of California, Berkeley David A. Patternson Professor at 1988 in , It is called disk array for short .RAID Form a large capacity in the form of disk array , Response speed block , Highly reliable storage subsystem , It has the following advantages ：
（1） High transmission speed （2） Provides fault tolerance （3） The price of the same capacity is lower than that of the traditional disk drive

RAID0

RAID1

RAID2

RAID3