Basic knowledge of hard disk (head, track, sector, cylinder)

summary

1. Disc （platter）
2. head （head）
3. Magnetic track （track）
4. A sector （sector）
5. cylinder （cylinder）

Disc one-sided and head

There are usually multiple disks in the hard disk , Each disc contains two faces , Each disk has a corresponding read / Write head . Limited by the overall volume and production cost of hard disk , The number of discs is limited , Be in commonly 5 Within one piece . The disc number is from bottom to top 0 Start , If the bottom disc has 0 Surface and 1 Noodles , The last disc will be numbered 2 Surface and 3 Noodles .
Here's the picture ：

 

 

chart 1

A sector and Magnetic track

The following figure shows a disk surface , Circles of gray concentric circles in the disk are tracks , Draw a straight line from the center of the circle , The track can be divided into several arc segments , An arc segment on each track is called a sector （ The green part of the picture ）. A sector is the smallest component of a disk , Usually 512 byte .（ Due to increasing disk size , Some manufacturers set the size of each sector to 4096 byte ）

 

chart 2

head and cylinder

A hard disk usually consists of a set of overlapping disks , Each disk is divided into an equal number of tracks , And from the outer edge “0” Numbered starting , Tracks with the same number form a cylinder , It's called the cylinder of the disk . The number of cylinders on a disk is equal to the number of tracks on a disk . Because each disk has its own head , therefore , The number of disks is equal to the total number of heads . Here's the picture

 

 

chart 3

Disk capacity calculation

storage capacity ＝ Number of heads × Magnetic track ( cylinder ) Count × Sectors per channel × Bytes per sector

chart 3 Medium disk is a 3 Disc 6 A magnetic head ,7 A cylinder （ Each disc 7 Tracks ） Of disks , chart 3 Each track in the has 12 Sectors , So the capacity of this disk is ：

storage capacity 6 * 7 * 12 * 512 = 258048

The number of sectors per track is the same as the old hard disk , The density of the outer ring is small , The density of the inner ring is high , The amount of data that can be stored in each circle is the same . The data density of the new hard disk is the same , In this way, the longer the circumference of the track , The more sectors , The larger the amount of data stored .

Disk read response time

1. Seek time ： The time it takes for the head to move from the beginning to the track where the data is located , The shorter the search time ,I/O The faster the operation , At present, the average seek time of the disk is generally 3－15ms, Be in commonly 10ms about .
2. Rotation delay ： The time it takes for disk rotation to move the sector in which the requested data is located under the read / write head , The rotation delay depends on the disk speed . Ordinary hard disks are generally 7200rpm, Slow 5400rpm.
3. Data transfer time ： The time required to complete the transmission of the requested data .
   To sum up ： From the above indicators 、 In fact, the most important thing is 、 Or say 、 There should be only two things we care about most ： Seek time ; Rotation delay .

The time required to read and write disk information can be divided into ： Seek time 、 Delay time 、 Transmission time . To improve disk transmission efficiency , The software should focus on reducing seek time and delay time .

block / cluster

summary

Disk block / cluster （ Virtual ）. Block is the smallest logical storage unit in the operating system . The smallest unit that the operating system deals with disk is disk block .
In layman's terms , stay Windows The next is like NTFS And other file systems are called clusters ; stay Linux The next is like Ext4 And other file systems are called blocks （block）. Each cluster or block may include 2、4、8、16、32、64…2 Of n Power sector .

Why are there disk blocks ？

Easy to read ： Because the number of sectors is relatively small , A large number is difficult to address , So the operating system combines adjacent sectors , Form a block , Then perform the overall operation on the block .

Separate the dependence on the bottom ： The operating system ignores the design of the underlying physical storage structure . The concept of virtual disk block , In the system, the block is considered to be the smallest unit .

page

The operating system often communicates with memory and hard disk , Be similar to “ block ” The concept of , All need a virtual basic unit . therefore , And memory operation , It's the concept of a virtual page as the smallest unit . Working with hard drives , It's the smallest unit in a block .

A sector 、 block / cluster 、page The relationship between

1. A sector ： The smallest read / write unit of a hard disk
2. block / cluster ： Is the smallest unit of the operating system for hard disk reading and writing
3. page： Is the smallest unit of operation between memory and the operating system .

A sector <= block / cluster <= page