Linux Important Concepts

6 high level stages of a typical Linux boot process.

1. BIOS

§  BIOS stands for Basic Input/Output System

§  Performs some system integrity checks

§  Searches, loads, and executes the boot loader program.

§  It looks for boot loader in floppy, cd-rom, or hard drive. You can press a key (typically F12 of F2, but it depends on your system) during the BIOS startup to change the boot sequence.

§  Once the boot loader program is detected and loaded into the memory, BIOS gives the control to it.

§  So, in simple terms BIOS loads and executes the MBR boot loader.

2. MBR

§  MBR stands for Master Boot Record.

§  It is located in the 1st sector of the bootable disk. Typically /dev/hda, or /dev/sda

§  MBR is less than 512 bytes in size. This has three components 1) primary boot loader info in 1st 446 bytes 2) partition table info in next 64 bytes 3) mbr validation check in last 2 bytes.

§  It contains information about GRUB (or LILO in old systems).

§  So, in simple terms MBR loads and executes the GRUB boot loader.

3. GRUB

§  GRUB stands for Grand Unified Bootloader.

§  If you have multiple kernel images installed on your system, you can choose which one to be executed.

§  GRUB displays a splash screen, waits for few seconds, if you don’t enter anything, it loads the default kernel image as specified in the grub configuration file.

§  GRUB has the knowledge of the filesystem (the older Linux loader LILO didn’t understand filesystem).

§  Grub configuration file is /boot/grub/grub.conf (/etc/grub.conf is a link to this). The following is sample grub.conf of CentOS.

§  As you notice from the above info, it contains kernel and initrd image.

§  So, in simple terms GRUB just loads and executes Kernel and initrd images.

4. Kernel

§  Mounts the root file system as specified in the “root=” in grub.conf

§  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. Do a ‘ps -ef | grep init’ and check the pid.

§  initrd stands for Initial RAM Disk.

§  initrd is used by kernel as temporary root file system until kernel is booted and the real root file system is mounted. It also contains necessary drivers compiled inside, which helps it to access the hard drive partitions, and other hardware.

5. Init

§  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 identifies the default initlevel from /etc/inittab and uses that to load all appropriate program.

§  Execute ‘grep initdefault /etc/inittab’ on your system to identify the default run level

§  If you want to get into trouble, you can set the default run level to 0 or 6. Since you know what 0 and 6 means, probably you might not do that.

§  Typically you would set the default run level to either 3 or 5.

6. Runlevel programs

§  When the Linux system is booting up, you might see various services getting started. For example, it might say “starting sendmail …. OK”. Those are the runlevel 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 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 programs that start with S and K.

§  Programs starts with S are used during startup. S for startup.

§  Programs starts with K are used during shutdown. K for kill.

§  There are numbers right next to S and K in the program names. Those are the sequence number in which the programs should be started or killed.

§  For example, S12syslog is to start the syslog deamon, which has the sequence number of 12. S80sendmail is to start the sendmail daemon, which has the sequence number of 80. So, syslog program will be started before sendmail.

Linux File System

Ext2

§  Ext2 stands for second extended file system.

§  It was introduced in 1993. Developed by Rémy Card.

§  This was developed to overcome the limitation of the original ext file system.

§  Ext2 does not have journaling feature.

§  On flash drives, usb drives, ext2 is recommended, as it doesn’t need to do the over head of journaling.

§  Maximum individual file size can be from 16 GB to 2 TB

§  Overall ext2 file system size can be from 2 TB to 32 TB

Ext3

§  Ext3 stands for third extended file system.

§  It was introduced in 2001. Developed by Stephen Tweedie.

§  Starting from Linux Kernel 2.4.15 ext3 was available.

§  The main benefit of ext3 is that it allows journaling.

§  Journaling has a dedicated area in the file system, where all the changes are tracked. When the system crashes, the possibility of file system corruption is less because of journaling.

§  Maximum individual file size can be from 16 GB to 2 TB

§  Overall ext3 file system size can be from 2 TB to 32 TB

§  There are three types of journaling available in ext3 file system.

§  Journal – Metadata and content are saved in the journal.

§  Ordered – Only metadata is saved in the journal. Metadata are journaled only after writing the content to disk. This is the default.

§  Writeback – Only metadata is saved in the journal. Metadata might be journaled either before or after the content is written to the disk.

§  You can convert a ext2 file system to ext3 file system directly (without backup/restore).

Ext4

§  Ext4 stands for fourth extended file system.

§  It was introduced in 2008.

§  Starting from Linux Kernel 2.6.19 ext4 was available.

§  Supports huge individual file size and overall file system size.

§  Maximum individual file size can be from 16 GB to 16 TB

§  Overall maximum ext4 file system size is 1 EB (exabyte). 1 EB = 1024 PB (petabyte). 1 PB = 1024 TB (terabyte).

§  Directory can contain a maximum of 64,000 subdirectories (as opposed to 32,000 in ext3)

§  You can also mount an existing ext3 fs as ext4 fs (without having to upgrade it).

§  Several other new features are introduced in ext4: multiblock allocation, delayed allocation, journal checksum. fast fsck, etc. All you need to know is that these new features have improved the performance and reliability of the filesystem when compared to ext3.

§  In ext4, you also have the option of turning the journaling feature “off”.