Linux下：磁盘管理和文件系统管理

446bytes: boot loader 主引导记录
64bytes: 分区表
16bytes: 标识一个分区　　　　　　【所以一个机械硬盘最多只能够分4个主分区，或者3个主分区+扩展分区(N个逻辑分区)】
2bytes: 55AA用来标识结尾

# 备份MBR
dd if=/dev/sda1 of=/root/mbr bs=512 count=1

# 恢复MBR：
dd if=/root/mbr of=/dev/sda1 bs=512 count=1

# 恢复分区表：
dd if=/root/mbr of=/dev/sda1 bs=512 skip=446 count=66
　　注：mbr大小512 所以bs=512 count=1 bs=512 skip=446 count=66 用来确定分区表在备份文件中的位置

在使用fdisk命令的时候注意：
# fdisk -u /dev/sda　　　　【正确】
# fdisk -u /dev/sda1　　　 【错误】

[root@oldboy ~]# fdisk -l　　　　【查看当前系统里面的所有分区情况】
　　
Disk /dev/sda: 10.7 GB, 10737418240 bytes
255 heads, 63 sectors/track, 1305 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0006ac8d

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1          26      204800   83  Linux
Partition 1 does not end on cylinder boundary.
/dev/sda2              26         113      694272   82  Linux swap / Solaris
Partition 2 does not end on cylinder boundary.
/dev/sda3             113        1306     9585664   83  Linux

[root@oldboy ~]# fdisk -u /dev/sda　　　　　　【在磁盘sda上面创建新的分区】

WARNING: DOS-compatible mode is deprecated. It‘s strongly recommended to
         switch off the mode (command ‘c‘).

Command (m for help): n
Command action
   e   extended
   p   primary partition (1-4)
e　　　　　　　　　　【扩展分区】
Selected partition 4
First sector (63-20971519, default 63): 
Using default value 63
Last sector, +sectors or +size{K,M,G} (63-2047, default 2047): 1000

Command (m for help): p　　　　【查看分区信息】

Disk /dev/sda: 10.7 GB, 10737418240 bytes
255 heads, 63 sectors/track, 1305 cylinders, total 20971520 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0006ac8d

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *        2048      411647      204800   83  Linux
Partition 1 does not end on cylinder boundary.
/dev/sda2          411648     1800191      694272   82  Linux swap / Solaris
Partition 2 does not end on cylinder boundary.
/dev/sda3         1800192    20971519     9585664   83  Linux
/dev/sda4              63        1000         469    5  Extended
Partition 4 does not end on cylinder boundary.

Partition table entries are not in disk order

Command (m for help): w　　　　【保存分区信息并退出】
The partition table has been altered!

Calling ioctl() to re-read partition table.

WARNING: Re-reading the partition table failed with error 16: Device or resource busy.
The kernel still uses the old table. The new table will be used at
the next reboot or after you run partprobe(8) or kpartx(8)
Syncing disks.
[root@oldboy ~]# cat /proc/partitions 　　【查看内核是否已经识别了分区，这里内核没有识别分区】
major minor  #blocks  name

   8        0   10485760 sda
   8        1     204800 sda1
   8        2     694272 sda2
   8        3    9585664 sda3
[root@oldboy ~]# partx -a /dev/sda　　　　【通知内核重新读取磁盘分区信息】
BLKPG: Device or resource busy
error adding partition 1
BLKPG: Device or resource busy
error adding partition 2
BLKPG: Device or resource busy
error adding partition 3
[root@oldboy ~]# cat /proc/partitions 　　【再次查看，内核已经识别了分区】
major minor  #blocks  name

   8        0   10485760 sda
   8        1     204800 sda1
   8        2     694272 sda2
   8        3    9585664 sda3
   8        4        469 sda4