Boot process in linux

Boot Process (RHEL7 & 6) Booting is the process of to load OS from computers hard disk to primary memory (RAM).                                 In RHEL 6 RHEL 7 BIOS BIOS MBR MBR GRUB GRUB KERNEL KERNEL INIT SYSTEMD RUNLEVELS TARGETS Step 1:  Power ON When you press on power on button SMPS (switch mode power supply) will get an signal to power on, immediate after it PGS (Power on boot signal) will execute to get power to all components. Step 2: POST  (Power-on-Self-Test) is diagnostic testing sequence all the computer parts will diagnose there own. Step 3: BIOS (Basic Input Output System) BIOS is program which verifies all the attached components and identifies device booting order Based on device order BIOS will first boot device, in this case we are considering as HDD as first boot device. Step 4: MBR (Master Boot Record) contains Boot Loader, Partition information and Magic Blocks or validation check MBR Size 512bytes 1.       Boot loader – contains boot loader program which is 446 bytes in size. 2.       64 Bytes of partition information will be located under MBR, which will provide / redirects to actual /boot partition path to find GRUB2 3.       2bytes are magic bytes or validation check to identify errors Step 5: GRUB GRUB displays the splash screen, waits for few seconds, if you dont enter anything it loads the default kernal image as specified in the grub configuration file. GRUB configuration file is in RHEL7 /boot/grub2/grub.cfg GRUB configuration file is in RHEL6 /boot/grub/grub.cfg  (Grand Unified Boot Loader) configuration file located in /boot/grub2/grub.cfg which actually points to initramfs is initial RAM disk, initial root file system will be mounted before real root file system. GRUB has the knowledge of the file system(the old version of Linux Loader LILO didn’t understand filesystem). Basically initramfs will load block device drivers such as SATA, RAID etc.  The initramfs is bound to the kernel and the kernel mounts this initramfs as part of a two-stage boot process. Step 6: KERNEL KERNEL is heart of OS, it is bridge between application and hardware of computer, it is designed in c language. When boot is processed, kernel will load. When kernel loading completes it immediately look forward to start processes / Services. It takes care of process mgmt, memory mgmt, resource mgmt, I/O etc Name of kernel is Maipo 3.10 in RHEL7 & Santiago in RHEL 6 Kernel executes the /sbin/system in RHEL7 & /sbin/init program in RHEL6. Types of kernel 1.Microkernel 2.Monolithic kernel Comparison Chart: Basis Microkernel Monolithic Kernel Basic In microkernel user services and kernel, services are kept in separate address space. In monolithic kernel, both user services and kernel services are kept in the same address space. Size Microkernel are smaller in size. Monolithic kernel is larger than microkernel. Execution Slow execution. Fast execution. Example Windows NT, Symbian, Mac OS Linux,  UNIX, Microsoft Windows (95,98,Me), Solaris, AIX, HP-UX, DOS Step 7 :  Starting Systemd the first system process                 Faster bootup times since daemons start in parallel. As we know before systemd there is no process / service exists. Systemd will be started by a system call fork( ); fork system call have an option to specify PID, that why systemd always hold PID 1. As there is no sequence to start processes / Services, based on default.target will start. Services with system are named something.service Such as For example, To enable a service in RHEL7                 #systemctl enable mariadb.service Then a symbolic link is created from /etc/system/system/multi-user.target.wants/mariadb.service That points to /usr/lib/systemd/system/mariadb.service Step 8 : Targets                 To find which target belongs to which number                                 #ls –l /usr/lib/systemd/system/runlevel0.target #ls –l /usr/lib/systemd/system/runlevel1.target                                                                 ------------------------- #ls –l /usr/lib/systemd/system/runlevel6.target                                 Which shows result as:                                                 lrwxrwxrwx           runlevel0.target ----> poweroff.target lrwxrwxrwx           runlevel1.target ----> rescue.target                                                                 ------------------------- lrwxrwxrwx           runlevel6.target ----> restart.target Targets: RHEL6 RHEL7 #ls –l /etc/inittab #ls –l /etc/systemd/system default.target graphical.target.wants multi-user.target.wants display-manager.service firewalld.service runlevel0 ----- poweroff runlevel0.target --- poweroff.target runlevel1 ----  single user mode runlevel1.target --- rescue.target         setup a rescue shell single user mode runlevel2 --- multi-user without NFS runlevel2.target ---multi-user.target non-graphical, user-defined runlevel runlevel3 ----- multi-user with CLI(networking) runlevel 3.target ---multi-user.target text-console only runlevel4--- Not used/user-defined runlevel 4.target --- multi-user.target non-graphical, user-defined runlevel runlevel5 ----- GUI(X11) runlevel 5.target ---graphical.target graphical mode runlevel6  ------ restart runlevel 6.target --- reboot.target restarts system                                 #systemctl set-default <tab>                                                                                                 reboot.target         printer.target         bluetooth.target                                                 rescue.target         shutdown.target   sleep.target                                                 initrd.target           sound.target          multi-user.target                                                 network.target      suspend.target      default.target                                                        runevel0.target     runlevel6.target    final.target                                                 graphical.target    umount.target      nfs.target Step 9: User Interface (UI)  Once that’s done, the “Wants” entry tells systemd to start the display-manager.service service(/etc/systemd/system/display-manager.service), which runs the GNOME display manager. Your User interface start and prompt you for credential to login. Below are the commands to know time of booting process taken: [root@server ~]# systemd-analyze time Startup finished in 1.895s (kernel) + 2.622s (initrd) + 20.402s (userspace) = 24.919s [root@server ~]# systemd-analyze blame 6.850s firewalld.service 5.714s mariadb.service 5.509s tuned.service 5.350s plymouth-quit-wait.service

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