4.3 Disk organization and management

4.3.1 The structure of the disk

一个扇区为一个磁盘块，各个扇区存放数据量相同。

最内侧磁道的扇区面积最小，密度最大。

可用 (柱面号、盘面号、扇区号)Locate a disk block

4.3.2 Disk scheduling algorithm Only affect the seek time

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（1）先来先服务算法

优点：Fair, performance is acceptable if concentrated.
缺点：A large number of processes compete, and the track distribution is relatively scattered, and the performance is poor

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（2）最短寻道时间优先算法 how are you Choose the best track at hand

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Hunger

（3）扫描（SCAN）算法 Also called (elevator scheduling algorithm)

磁盘移到最外侧才能往内移动 No hunger

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（4）循环扫描算法（C-SCAN）

Move to the outermost,Return to the outermost side of the other side

移到最外侧的请求即可，不需要访问到最外侧磁道


LOOK和SCAN 都是为了方便处理接近最礼物和最外的磁道请求

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In addition to reducing the search time, reducing the delay time, it can be performed on the disk sector 交替编号, and the different disk surfaces in the disk group错位命名

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4.3.3 Disk management

1) Initialization
of the disk Carry out low-level formatting and logical formatting of the disk.

A new disk is just a blank disk containing magnetic recording material. Before the disk can store data, itMust be divided into sectorsSo that the disk controller can read and write operations, this process is calledLow-level format (physical partition)（分扇区）。

OS将自己的数据结构记录在磁盘上

• The first step is to divide the disk into 由一个或多个柱面组成的分区(that is, the familiar C disk, D disk, etc. partitions);
• The second step of the physical partition逻辑格式化(创建文件系统) Create a root directory to initialize the storage of free disk block information

2) Boot block When the computer starts, it needs to run one by one , which initializes the CPU, registers, device controller and memory.存放自举程序
初始化程序(自举程序)
The bootloader is usually stored in ROM

• 先运行自举装入程序，通过该程序就可以找到引导块，并将完整的 Loader Load the memory.

3) Bad block

One or more sectors are easily damaged
1、对于简单磁盘 Bad sectors are marked on FAT

2、对于复杂磁盘 Control equipment available 备用块Come 逻辑替换坏块 Sector spare