Red Hat Enterprise Linux 7 startup process

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This article is my note when I study RHCE7.0, I hope to be able to help you

1. Connect the computer to the power supply. The system firmware (modern UEFI or older BIOS) runs the power-on self-test (post) and begins to initialize some of the hardware.

2. The system firmware searches for bootable devices, may be configured in the UEFI boot firmware, or it may search the master boot Record (MBR) on all disks in the order configured in the BIOS.

3. The system firmware reads the boot loader from disk, and then gives the system control to the boot loader. In RHEL7, it is usually Grub2.

4. The bootloader loads its configuration from disk, and then displays to the user a menu of possible configurations for booting. After the user makes a selection, the bootloader loads the configured kernel and Initramfs from disk and places them in memory. Initramfs is a gzip-cpio archive that contains kernel modules for all necessary hardware at startup, initialization scripts, and so on. In RHEL7, Initramfs contains the entire system that is available to itself.

5. The boot loader transfers system control to the kernel, passing the boot loader's kernel command line with any options specified, and Initramfs in-memory locations

6. For all hardware that the kernel can find the driver in Initramfs, the kernel initializes the hardware and then acts as a PID 1

7. Execute/sbin/init from Initramfs. In RHEL7, Initramfs contains the working copy of Systemd as/sbin/init and contains the Udev daemon.

8. The SYSTEMD instance in Initramfs executes all the cells of the initrd.target target. This includes the root file system that mounts the time on the/sysroot.

9. The kernel root file system switches (turns) from the Initramfs root file system to the system root file system previously mounted on the/sysroot. The SYSTEMD is then automatically re-executed using the SYSTEMD copy installed in the system.

SYSTEMD will find the default targets that are passed from the kernel command line or configured in the system, and then start (or stop) the unit to conform to the target's configuration, which automatically resolves the dependencies between the cells. Essentially, the systemd target is a set of cells that should reach the desired system state after activation. These targets typically contain at least one logon or graphical login screen that generates basic text.

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Red Hat Enterprise Linux 7 startup process