Boot Process in RHEL 6 and 7

Boot Process in RHEL 6 and 7.

This will explain about the boot process in RHEL 6 and 7. You can see the difference between RHEL 6 and RHEL 7 boot process.

The boot process explain below.

Stage	RHEL 6	RHEL 7
Boot	·         Stand for Basic Input/Output System. ·         Once the server is powered on Mother Board start executing the BIOS. ·         BIOS performs system integrity checks (POST). ·         Searches, loads and executes the boot loader program. ·         It looks for boot loaded in mass storage by using boot sequence or boot order. ·         Once the boot loader program is detected and loaded into the primary memory (RAM), BIOS gives the control to it. ·         In simple terms BIOS loads and executes the boot loader from MBR (Master Boot Recorder).	·         Stand for Basic Input/Output System. ·         Once the server is powered on Mother Board start executing the BIOS. ·         BIOS performs system integrity checks (POST). ·         Searches, loads and executes the boot loader program. ·         It looks for boot loaded in mass storage by using boot sequence or boot order. ·         Once the boot loader program is detected and loaded into the primary memory (RAM), BIOS gives the control to it. ·         In simple terms BIOS loads and executes the boot loader from MBR (Master Boot Recorder).
MBR	·         Stand for Master Boot Recorder. ·         It is located in the 1st sector (512 bytes) of the bootable disk. Typically /dev/sda. ·         MBR has tree components. §  Primary Boot Loader information  in 1st 446 bytes §  Partition table information in next 64 bytes. §  MBR validation check in the last 2 bytes. ·         It contains information about GRUB. ·         In simple terms MBR loads and executes the GRUB Boot loader	·         Stand for Master Boot Recorder. ·         It is located in the 1st sector (512 bytes) of the bootable disk. Typically /dev/sda. ·         MBR has tree components. §  Primary Boot Loader information  in 1st 446 bytes §  Partition table information in next 64 bytes. §  MBR validation check in the last 2 bytes. ·         It contains information about GRUB2. ·         In simple terms MBR loads and executes the GRUB2 Boot loader
GRUB/GRUB2	·         Stand for Grand Unified Bootloader. ·         If multiple kernel images are installed in server, you can choose which one to be executed. ·         GRUB displays a splash screen, waits for few seconds, if you have not selected anything, it loads the default kernel image as specified in the grub configuration file. ·         GRUB configuration file is /boot/grub/grub.conf (/etc/grub.conf is soft link to this). The following is sample grub.conf of RHEL 6 #boot=/dev/sda default=0 timeout=5 splashimage=(hd0,0)/grub/splash.xpm.gz hiddenmenu title Red Hat Enterprise Linux (2.6.32-431.el6.x86_64)         root (hd0,0)         kernel /vmlinuz-2.6.32-431.el6.x86_64 ro root=/dev/mapper/VolGroup-lv_root rhgb quiet         initrd /initramfs-2.6.32-431.el6.x86_64.img ·         As you notice from the above info, it contains kernel and initrd image. ·         In simple terms GRUB just loads and executes Kernel and initrd images.	·         In RHEL7, GRUB2 will be used. And stands for Grand Unified Bootloader version 2. ·         Supports GPT, additional firmware types, including BIOS, EFI and Open Firmware. ·         Ability to boot on various file systems (xfs,ext4,ntfs,hfs+, raid, etc) ·         GRUB configuration file is /boot/grub2/grub.conf (/etc/grub2.conf is soft link to this). /etc/grub.d/* and /etc/default/grub other grub configuration files.The following is sample grub.conf of RHEL 7 ### BEGIN /etc/grub.d/10_linux ### menuentry 'Red Hat Enterprise Linux Server, with Linux 3.10.0-123.el7.x86_64' --class red --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-3.10.0-123.el7.x86_64-advanced-aa654ec4-5630-4e1e-90a3-c1363239ad09' {         load_video         set gfxpayload=keep         insmod gzio         insmod part_msdos         insmod xfs         set root='hd0,msdos1'         if [ x$feature_platform_search_hint = xy ]; then           search --no-floppy --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1 --hint='hd0,msdos1'  bff6160f-575f-4962-a152-573f76669b74         else           search --no-floppy --fs-uuid --set=root bff6160f-575f-4962-a152-573f76669b74         fi         linux16 /vmlinuz-3.10.0-123.el7.x86_64 root=UUID=aa654ec4-5630-4e1e-90a3-c1363239ad09 ro rd.lvm.lv=rhel/root crashkernel=auto  rd.lvm.lv=rhel/swap vconsole.font=latarcyrheb-sun16 vconsole.keymap=us rhgb quiet LANG=en_US.UTF-8         initrd16 /initramfs-3.10.0-123.el7.x86_64.img } ·         As you notice from the above info, it contains linux16 (kernel) and initrd16 (initramfs) image. ·         In simple terms GRUB2 just loads and executes Kernel and initrd images.
Kernel	·         initrd stands for Initial RAM Disk. ·         Initrd is used by kernel as temporary root file system until the kernel is booted and the real root file system is mounted (ready-only format). It also contains necessary drivers compiled inside, which helps it to access the hard drive partitions, and other hardware. ·         Kernel re-mounts the root file system as specified in the “root=” in grub.conf as read-write. ·         Kernel executes the /sbin/init program ·         Since init was the 1st program to be executed by Linux kernel, it has the process id (PID) of 1. ·         In simple terms Kernel just executes init program.	·         initrd stands for Initial RAM Disk. In RHEL 7, initrd16 will be used. ·         Initrd16 is used by kernel as temporary root file system until the kernel is booted and the real root file system is mounted (ready-only format) on /sysroot. It also contains necessary drivers compiled inside, which helps it to access the hard drive partitions, and other hardware. ·         Kernel re-mounts the root file system as specified in the “root=” in grub.conf as read-write. ·         Kernel executes the /usr/lib/systemd/systemd ·         Since systemd was the 1st program to be executed by Linux kernel, it has the process id (PID) of 1. ·         In simple terms Kernel just executes systemd program.
Init/Systemd	·         Looks at the /etc/inittab file to decide the Linux run level. ·         Following are the available run levels. §   0 – halt §   1 – Single user mode §   2 – Multiuser, without NFS §   3 -  Full Multiuser Mode §   4 – unused §   5 – X11 §   6 – reboot ·         Init identified the default initlevel from /etc/inittab and uses that to load all appropriate programs. ·         Execute ‘grep initdefault /etc/inittab’ on your system to identify the default run level. ·         Typically you would set the default run level to either 3 or 5. ·         In simple terms Init program executes the programs under run level which is set as a default.	·         The run levels are called as “target”. ·         Looks for a default target, either passed in from the kernel command line or configured on the system. ·         Based on default the target, it starts (and stops) units. Units contains the services and its dependencies. ·         Generally /etc/systemd/system/default.target is soft link and will be pointing to target. ·         Execute ‘systemctl get-default’ on your system to identify the default target ·          Following are the available targets. §  Rescue.target §  Multi-user.target §  Graphical.target ·         Run levels and target comparison. §  runlevel0.target -> poweroff.target §  runlevel1.target -> rescue.target §  runlevel2.target -> multi-user.target §  runlevel3.target -> multi-user.target §  runlevel4.target -> multi-user.target §  runlevel5.target -> graphical.target §  runlevel6.target -> reboot.target ·         Typically you would set the default target to either multi-user.target or graphical.target. ·         Execute ‘systemctl set-default graphical.target’ on your computer to change the default target. ·         In simple terms systemd program executes the programs under target which is set as a default.
Runlevel/Target	·         When the Linux system is booting up, you might see various services getting started. For example, it might say “starting ssh  ….. OK”. Those are the run level programs, executed from the run level directory as defined by your run level. ·         Depending on your default init level setting, the system will execute the programs from one of the following the directories. §  Run level 0 - /etc/rc.d/rc0.d/ §  Run level 1 - /etc/rc.d/rc1.d/ §  Run level 2 - /etc/rc.d/rc2.d/ §  Run level 3 - /etc/rc.d/rc3.d/ §  Run level 4 - /etc/rc.d/rc4.d/ §  Run level 5 - /etc/rc.d/rc5.d/ §  Run level 6 - /etc/rc.d/rc6.d/ ·         Please note that there are also symbolic links available for these directory under /etc directly. So /etc/rc0.d is linked to /etc/rc.d/rc0.d. ·         Under the /etc/rc.d/rc*.d/ directories, you would see scripts that are start with S and K. ·         The scripts start with S are used during startup. S for startup. ·         The scripts start with K are used during shutdown. K for kill. ·         There are numbers right next to S and K in the scripts names. Those are the sequence number in which the scripts should be started or killed. ·         For example, S12rsyslog is to start the rsyslog deamon, which has the sequence number of 12. S55sshd is to start the ssh deamon, which has the sequence number 55. So, rsyslog will be started before the sshd.	·         The systemd start all the system services based on the default target. ·         If the default target pointing to “graphical.target”. Typically the graphical.target file includes these lines.  # cat /usr/lib/systemd/system/graphical.target       Requires=multi-user.target       After=multi-user.target      Wants=display-manager.service ·         This tells systemd to start everything in the multi-user.target before starting the graphical target. 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 (/usr/sbin/gdm). ·         The multi-user.target file includes these. # cat /usr/lib/systemd/system/multi-user.target       Requires=basic.target ·         This tells systemd to start everything in the /usr/lib/systemd/system/basic.target target before starting the other multi-user services. After that, for the multi-user.target, all units (services, targets, etc.) in the /etc/systemd/system/multi-user.target.wants and /usr/lib/systemd/system/multi-user.target.wants directories are started. ·         When you enable a service, a symbolic link is placed in the /etc/systemd/system/multi-user.target.wants directory. That directory is where you will find links to most of the services you think of as starting in multi-user mode (printing, cron, auditing, SSH, and so on). ·         Typical multi-user.target.wants directory contains the soft links of services and/or targets from /usr/lib/systemd/system/*. ·         The basic.target file includes these. # cat /usr/lib/systemd/system/basic.target       Requires=sysinit.target ·         This points systemd to the /usr/lib/systemd/system/sysinit.target, which must start before the basic.target can continue. The basic.target target file starts the firewalld and microcode services from the /etc/systemd/system/basic.target.wants directory and services for SELinux, kernel messages, and loading modules from the /usr/lib/systemd/system/basic.target.wants directory. ·         The sysinit.target file includes these. # cat /usr/lib/systemd/system/sysinit.target       Wants=local-fs.target swap.target ·         Besides mounting file systems and enabling swap devices, the sysinit.target starts targets, services, and mounts based on units contained in the /usr/lib/systemd/system/sysinit.target.wants directory. These units enable logging, set kernel options, start the udevd daemon to detect hardware, and allow file system decryption, among other things. ·         The /etc/systemd/system/sysinit.target.wants directory contains services that start iSCSI, multipath, LVM monitoring and RAID services. ·         The local-fs.target file includes these. # cat /usr/lib/systemd/system/local-fs.target       After=local-fs-pre.target ·         There are no services associated with the local-fs-pre.target target (you could add some to a "wants" directory if you like). However, units in the /usr/lib/systemd/system/local-fs.target.wants directory import the network configuration from the initramfs, run a file system check (fsck) on the root file system when necessary, and remounting the root file system (and special kernel file systems) based on the contents of the /etc/fstab file. ·         To see what services are associated with a particular target. Execute “systemctl list-dependencies multi-user.target”