In this article I am going to show the CDROM boot process for RHEL 7.
Booting process in Linux is complex. Lot of things happen between pressing the button and getting the login prompt.
1) As soon as we poweron the server POST (Power on self test) will happen. Different bootloader are used depending upon the use of legacy BIOS or UEFI on the system.
2) If the BIOS is configured correctly ISOLINUX environment loads and will show the below screen.
ISOLINUX will be loaded from DVD path in DVD is /isolinux/isolinux.bin. I will show the presence of it in DVD later in this article.
Shows the below Welcome screen of Step 2.
Press Enter and you will get the below screen.
Press Tab to see the full configuration options
Currently Quiet mode is set. I will remove this quiet option to see the all messages during booting process.
For the time being to show the other options as well. Just press Esc and then down cursor. Now Rescue a redhat enterprise Linux System will be high lightened. We can again press tab to see the configuration options.
I came back to install Red Hat Enterprise Linux 7.0 Option and pressed tab. I have removed the quiet from end of configuration option.
3) As in previous step we have removed the quiet we can see that vmlinuz and initramfs are loaded from /isolinux directory into kernel.
Below is the image showing it.
4) Kernel loaded from the CDROM will unpack the initramfs (initrd.img) on the CDROM into an memory “rootfs”
5) Kernel starts from systemd from the initramfs. Subsequently systemd starts dracut scripts and then start udev to probe for devices.
udev probe all devices here like video cards, CD drives.
6) anaconda-dract finds “inst.stage2=hd:LABEL=RHEL7.0 ****” in boot arguments and tell udev to look for the disk label the same label. As soon as udev finds the disk, anaconda-dracut will mount the CDROM to /run/install/repo as shown below.
You will get below screen.
Now we can switch the screen using Cltr+Alt+F1 or Cltr+Alt+F2
I have pressed Cltr+Alt+F2 will come to below screen i issued df -Ph to see the mounted file systems.
I have taken the output of various paths in below screenshot. our DVD is mounted on /run/install/repo.
I came back to main screen by pressing Cltr+ALT+F1
7) Once the CDROM is mounted it will start check if it was selected at booting time.
8) Anaconda-dracut mounts the /LIVEOS/squashfs.img from the newly mounted CDROM at /sysroot
systemd then does the switch root into /sysroot.
9) After the systemd does the switch-root, systemd restarts. udev restarts and creating symbolic link for DVD at /dev.
10) systemd starts network Manager and anaconda.
11) Anaconda starts xorg and probe for disks.
12) Here comes our Welcome screen for proceeding with installation.
During graphical installation simultaneously on other windows like Cltr+Alt+F3 or F2 or F5 we can see what is exactly happening at background.