Linux Cloud Automation Operations Basics 22 (System Recovery)

Overview of the system boot process:

PowerUp--bios Initialize--GRUB2 disk boot phase--grub2 file Boot phase--Specify boot partition location-boot kernel, read-only/Mount device-start init program into initialization phase-start SYSTEMD initialization process-take/etc/ systemd/file-Start program-Start the landing environment.

System Startup process:

I. Power-ON

BIOS (in read-only storage on the motherboard, basic input or output system)
Role: Hardware detection, activating hardware

Two. Grub System boot (grub boot divided into two stages)

1. Stage 1 MBR (Master boot Record)

   Master Boot Record 0 tracks on hard disk, one sector, 446 bytes
   DD If=/dev/zero Of=/dev/vda bs=446 count=1 can empty MBR
   Go to rescue mode, perform chroot/mnt/sysimage switch to real/environment,
   and execute Grub2-install/dev/vda
   

experiment: MBR master boot partition damaged after repair method .

(1) Before the experiment, we must close SELinux.

Vim/etc/sysconfig/selinux

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Change it to disable, and then restart the system

(2) Destroy MBR master boot partition

DD If=/dev/zero Of=/dev/vda bs=446 count=1

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Failed to start after rebooting system

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(3) Set the optical drive to boot into rescue mode

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(4) If you enter 1, you will get a shell, then execute the DF command to view the root partition of your system, and then execute

chroot/mnt/sysimage/

Convert the root partition, then switch to the real root environment, and then execute

Grub2-install/dev/vda

Re-restore the MBR partition, and then exit two exits, after shutting down the virtual machine will start back to the hard drive boot, boot up and normal start.

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2. Phase 2 Grub File boot phase
Locate the/boot partition
Read/boot/grub2/grub.cfg
File lost, Grub2-mkconfig >/boot/grub2/grub.cfg

Experiment: How to fix grub file after corruption

(1) Experiment before executing the DF command to view your own root and boot partitions

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(2) Delete grub file boot

Rm-fr/boot/grub2/grub.cfg

(3) Restart Discovery failed to start

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(4) Manually enter the boot file

Where the set root= ' hd0,msdos1 ' boot partition is located

linux16/vmlinuz-3.10.0-327.el7.x86_64 ro Root=/dev/vda3 root partition is located

Initrd16/initramfs-3.10.0-327.el7.x86_64.img execution

Boot boot

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At this point can be turned on, but this is manual boot, for temporary effect, boot should be restored/boot/grub2/grub.cfg file

Grub2-mkconfig >/boot/grub2/grub.cfg

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After recovering the file, the system recovers successfully.

Three. Boot kernel, read-only mount/device
Testing equipment
Initialize the device driver
Enter the system initialization phase
Kernel is missing, the new kernel installation package can be resolved
RPM-IVH kernel-xxxxx.rpm--force

Experiment: Fix kernel file after missing

(1) First check which files are needed for startup

Ls/boot viewing the file for the boot partition

(2) Reboot the kernel file after deletion

rm-fr/boot/vmlinuz-3.10.0-327.el7.x86_64 Deleting kernel files

Reboot

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(3) Restart after the discovery of a normal boot, the lack of kernel files

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(4) Set the optical drive to boot, enter rescue mode, execute the DF command can see the root of the real system is/mnt/sysimage, and here the root is the ISO image root.

(5) Decompose kernel packages in ISO image and copy kernel files to/mnt/sysimage/boot/

rpm2cpio/run/install/repo/packages/kernel-3.10.0-327.el7.x86_64.rpm |cpio-id

cp/boot/vmlinuz-3.10.0-327.el7.x86_64/mnt/sysimage/boot/

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(6) Exit and set the hard drive to start, reboot. Repair Complete Entry System

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Four. System initialization phase

System initialization phase Load INITRD mirror
Open initialization Process Systemd
Start SELinux
Load Kernel parameters
Initialize the system clock, keyboard, host name
Re-read/write mount/device
Activating RAID,LVM
Activate quotas
Start all services in the multi-user.target.wants
Turn on the Virtual Console
Start graphics

initramfs-' Uname-r '. IMG Lost with: mkinitrd initramfs-' uname-r '. img ' uname-r ' recovery

Experiment: System initialization loading INITRD image corruption after recovery

(1) Delete initrd image file initramfs-3.10.0-327.x86_64.img

Rm-fr/boot/initramfs-3.10.0-327.x86_64.img deleting initrd image files

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(2) Restart found unable to enter system

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(3) Set the optical drive to boot, enter rescue mode, perform chroot/mnt/sysimage switch to the real root environment

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(4) mkinitrd/boot/initramfs-$ (UNAME-R). IMG $ (UNAME-R)

(5) ls/boot/Check whether the file is restored, exit two times after setting up the hard drive, the system is back to normal

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This article is from the "13122425" blog, please be sure to keep this source http://13132425.blog.51cto.com/13122425/1954643

Linux Cloud Automation Operations Basics 22 (System Recovery)