Linux can be booted by mbr. In a Linux system, when the system is started from the local hard disk, system control is first transferred to the partition containing the operating system boot file according to the setting of the MBR (Master Boot Record) in the first sector of the hard disk; or directly based on The boot information in the MBR record calls the boot menu (such as GRUB).
The operating environment of this tutorial: CentOS 7 system, Dell G3 computer.
Power-on self-test
When the server is turned on When the system starts, a power-on self-test will be performed to detect whether hardware devices such as CPU, memory, graphics card, keyboard, etc. are faulty. If there are no faults, system control will be handed over according to the preset boot sequence. Under normal circumstances, it is the local hard disk.
Summary: Detect the first device that can boot the system, such as a hard disk or optical drive
MBR boot
When booting from the local hard disk system, first transfer system control to the partition containing the operating system boot file based on the settings of the MBR (Master Boot Record) in the first sector of the hard disk; or directly call the boot menu (such as GRUB) based on the boot information in the MBR record ).
MBR size is 512 bytes, which stores pre-boot information, partition table and other information.
Summary: Run the startup GRUB boot program placed in the MBR sector
GRUB menu
For the Linux operating system, GRUB (unified Boot loader) is the most widely used multi-system boot program. After the system control is transferred to GRUB, the boot menu will be displayed for the user to choose, and the Linux kernel file will be loaded according to the selected option (or the default value), and then the system control will be transferred to the kernel.
CentOS 7 uses the GRUB2 boot bootloader.
GRUB is the first 446 bytes of the MBR program.
Summary: The GRUB boot program obtains the settings and path locations of the kernel and image file system by reading the GRUB configuration file /boot/grub2/grub.cfg
Loading the Linux kernel
The Linux kernel is a pre-compiled special binary file, which is between various hardware resources and system programs and is responsible for resource allocation and scheduling. After the kernel takes over system control, it will fully control the running process of the entire Linux operating system.
In CentOS 7 system, the default kernel file is located at "/boot/vmlinuz-3.10.0-514.el7.x86_64".
Summary: Load the kernel and image file system into memory
init process initialization
After the kernel is loaded, the first program to run It is "/sbin/init" (the running program is called a process). The init process is responsible for completing the initialization of the entire system and finally waiting for the user to log in.
Summary: Load the hardware driver, and the kernel loads the init process into the memory to run
Extended knowledge: MBR sector failure analysis
Cause of failure
Damage caused by viruses, Trojans, etc.
Incorrect partition operation, disk read and write misoperation
Fault phenomenon
The boot program cannot be found and the startup is interrupted
The operating system cannot be loaded and the screen goes black after booting
Solution ideas
Should be done in advance Make a backup file
Boot into the rescue mode with the installation CD
Restore from the backup file
Simulate MBR sector failure and repair it
The MBR is located at the first position of the first hard disk (/dev/sda) physical sectors, a total of 512 bytes
Steps
1. Create a disk partition
2. Format and mount
3. Back up the MBR sector data to the /dev/sdb1 disk. At this time, the MBR sector in the sda disk has been backed up to In the mbr.bak file of sdb1.
4. Simulate the destruction of the MBR sector of sda and overwrite the first 512 bytes of sda with null characters.
5. Restart init6
Establish the mount point/backupdir, mount sdb1 for use, copy the backed up mbr.bak file back to sda, and finally enter exit to exit. After exiting, the system will automatically restart!
Extract from backupdir for overwriting
exit exit repair successfully
Related recommendations:《Linux video tutorial》
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