Understanding the Oracle Linux boot process can help you troubleshoot problems while booting a system. The boot process involves several files and errors in these files are the usual cause of boot problems.

When an Oracle Linux system boots, it performs the following operations:

1. The computer's BIOS performs a power-on self-test (POST), and then locates and initializes any peripheral devices including the hard disk.

2. The BIOS reads the Master Boot Record (MBR) into memory from the boot device. (For GUID Partition Table (GPT) disks, this MBR is the protective MBR on the first sector of the disk.) The MBR stores information about the organization of partitions on that device. On a computer with x86 architecture, the MBR occupies the first 512 bytes of the boot device. The first 446 bytes contain boot code that points to the boot loader program, which can be on the same device or on another device. The next 64 bytes contain the partition table. The final two bytes are the boot signature, which is used for error detection.

   The default boot loader program that is used in Oracle Linux is the GRand Unified Bootloader version 2 (GRUB 2).

3. The boot loader loads the vmlinuz kernel image file into memory and extracts the contents of the initramfs image file into a temporary, memory-based file system (tmpfs).

4. The kernel loads the driver modules from the initramfs file system that are needed to access the root file system.

5. The kernel starts the systemd process with a process ID of 1 (PID 1). systemd is the ancestor of all processes on a system. systemd reads its configuration from files in the /etc/systemd directory. The /etc/systemd/system.conf file controls how systemd handles system initialization.

   systemd reads the file linked by /etc/systemd/system/default.target, for example /usr/lib/systemd/system/multi-user.target, to determine the default system target.

   Note:

   You can use a kernel boot parameter to override the default system target. See Kernel Boot Parameters.

   The system target file defines the services that systemd starts.

   systemd brings the system to the state defined by the system target, performing system initialization tasks such as:

   • Setting the host name.

   • Initializing the network.

   • Initializing SELinux based on its configuration.

   • Printing a welcome banner.

   • Initializing the system hardware based on kernel boot arguments.

   • Mounting the file systems, including virtual file systems such as the /proc file system.

   • Cleaning up directories in /var.

   • Starting swapping.

   See About System-State Targets.

6. If you have made /etc/rc.local executable and you have copied /usr/lib/systemd/system/rc-local.service to /etc/systemd/system, systemd runs any actions that you have defined in /etc/rc.local. However, the preferred way of running such local actions is to define your own systemd unit.

For information about systemd and on how to write systemd units, see the systemd(1), systemd-system.conf(5), and systemd.unit(5) manual pages.

The GRUB 2 bootloader can load many operating systems in addition to Oracle Linux and it can chain-load proprietary operating systems. GRUB 2 understands the formats of file systems and kernel executables, which allows it to load an arbitrary operating system without needing to know the exact location of the kernel on the boot device. GRUB 2 requires only the file name and drive partitions to load a kernel.

There are several kernel boot parameters that you can set. The following table lists some of the more commonly used parameters.

The kernel boot parameters that were last used to boot a system are recorded in /proc/cmdline, as shown in the following example:

cat /proc/cmdline

BOOT_IMAGE=/vmlinuz-3.10.0-123.el7.x86_64 root=UUID=52c1cab6-969f-4872-958d-47f8518267de
ro rootflags=subvol=root vconsole.font=latarcyrheb-sun16 crashkernel=auto vconsole.keymap=uk
rhgb quiet LANG=en_GB.UTF-8

For more information, see the kernel-command-line(7) manual page.

To modify boot parameters before booting a kerne, follow these stepsl:

1. When presented with the GRUB boot menu, use the arrow keys to highlight the required kernel and then press the space bar.

   The following figure shows the GRUB menu with the Unbreakable Enterprise Kernel (UEK) boot entry selected.

   Figure 1-1 GRUB Menu with UEK boot entry selected

   
   
   

2. Press E to edit the boot configuration for the kernel.

3. Using the arrow keys, scroll down the screen until the cursor is at the start of the boot configuration line for the kernel (which starts linux16).

4. Edit this line to change the boot parameters.

   For example, press End to go to the end of the line and enter an additional boot parameter.

   The following figure shows the kernel boot line with the additional parameter systemd.target=runlevel1.target, which starts the rescue shell.

   Figure 1-2 Kernel Boot Line with an Additional Parameter to Select the Rescue Shell

   
   
   

5. Press Ctrl+X to boot the system.

To modify boot parameters in the GRUB 2 configuration so that they are applied by default at every reboot, follow these steps:

1. Edit the /etc/default/grub file and modify the parameters in the GRUB_CMDLINE_LINUX definition, for example:

   GRUB_CMDLINE_LINUX="vconsole.font=latarcyrheb-sun16 vconsole.keymap=uk 
   crashkernel=auto  rd.lvm.lv=ol/swap rd.lvm.lv=ol/root biosdevname=0 
   rhgb quiet systemd.unit=runlevel3.target"

   The previous example adds the systemd.unit=runlevel3.target parameter so that the system boots into multi-user, non-graphical mode by default.

2. Rebuild the /boot/grub2/grub.cfg file as follows:

   sudo grub2-mkconfig -o /boot/grub2/grub.cfg

   This change takes effect for subsequent system reboots of all configured kernels.