RAID stands for Redundant Array of Inexpensive (Independent) Disks.

Please check following levels of RAIDs.

RAID 0 – Striping
RAID 1 – Mirroring
RAID 5 – striping with parity
RAID 10 (also known as RAID 1+0) – combining mirroring and striping

RAID level 0 – Striping
……………………
Minimum 2 disks.
Excellent performance ( as blocks are striped ).
No redundancy ( no mirror, no parity ).
Don’t use this for any critical system.
RAID 0 offers great performance, both in read and write operations. There is no  overhead caused by parity controls.
All storage capacity is used, there is no overhead.
The technology is easy to implement

Disadvantages

RAID 0 is not fault-tolerant. If one drive fails, all data in the RAID 0 array are lost. It should not be used for mission-critical systems.

Ideal use

RAID 0 is ideal for non-critical storage of data that have to be read/written at a high speed, such as on an image retouching or video editing station.

If you want to use RAID 0 purely to combine the storage capacity of twee drives in a single volume, consider mounting one drive in the folder path of the other drive. This is supported in Linux, OS X as well as Windows and has the advantage that a single drive failure has no impact on the data of the second disk or SSD drive.

RAID level 1 – Mirroring
…………………………………..

Minimum 2 disks.
Good performance ( no striping. no parity ).
Excellent redundancy ( as blocks are mirrored ).
RAID 1 offers excellent read speed and a write-speed that is comparable to that of a single drive.
In case a drive fails, data do not have to be rebuild, they just have to be copied to the replacement drive.
RAID 1 is a very simple technology.

Disadvantages

The main disadvantage is that the effective storage capacity is only half of the total drive capacity because all data get written twice.
Software RAID 1 solutions do not always allow a hot swap of a failed drive. That means the failed drive can only be replaced after powering down the computer it is attached to. For servers that are used simultaneously by many people, this may not be acceptable. Such systems typically use hardware controllers that do support hot swapping.

Ideal use

RAID-1 is ideal for mission critical storage, for instance for accounting systems. It is also suitable for small servers in which only two data drives will be used.

RAID 5 – striping with parity
………………………………………..

Minimum 3 disks.
Good performance ( as blocks are striped ).
Good redundancy ( distributed parity ).
Best cost effective option providing both performance and redundancy. Use this for DB that is heavily read oriented. Write operations will be slow.
Read data transactions are very fast while write data transactions are somewhat slower (due to the parity that has to be calculated).
If a drive fails, you still have access to all data, even while the failed drive is being replaced and the storage controller rebuilds the data on the new drive.

Disadvantages

Drive failures have an effect on throughput, although this is still acceptable.
This is complex technology. If one of the disks in an array using 4TB disks fails and is replaced, restoring the data (the rebuild time) may take a day or longer, depending on the load on the array and the speed of the controller. If another disk goes bad during that time, data are lost forever.

Ideal use

RAID 5 is a good all-round system that combines efficient storage with excellent security and decent performance. It is ideal for file and application servers that have a limited number of data drives.

RAID 10 – combining mirroring and striping
………………………………………………………………
Minimum 4 disks.
This is also called as “stripe of mirrors”
Excellent redundancy ( as blocks are mirrored )
Excellent performance ( as blocks are striped )
If you can afford the dollar, this is the BEST option for any mission critical applications (especially databases).
If something goes wrong with one of the disks in a RAID 10 configuration, the rebuild time is very fast since all that is needed is copying all the data from the surviving mirror to a new drive. This can take as little as 30 minutes for drives of  1 TB.

Disadvantages

Half of the storage capacity goes to mirroring, so compared to large RAID 5  or RAID 6 arrays, this is an expensive way to have redundancy.

What about RAID levels 2, 3, 4 and 7?

These levels do exist but are not that common (RAID 3 is essentially like RAID 5 but with the parity data always written to the same drive). This is just a simple introduction to RAID-systems.

Describing below booting system of Linux, Please check below:

The stages involved in Linux Booting Process are:

BIOS
Boot Loader
– MBR
– GRUB
Kernel
Init
Runlevel scripts

 BIOS

1.)This is the first thing which loads once you power on your machine.

2.)  When you press the power button of the machine, CPU looks out into ROM for further instruction.

3.) The ROM contains JUMP function in the form of instrucion which tells the CPU to bring up the BIOS.

4.) BIOS determines all the list of bootable devices available in the system.

5.) Prompts to select bootable device which can be Hard Disk, CD/DVD-ROM, Floppy Drive, USB Flash Memory Stick etc (optional).

6.) Operating System tries to boot from Hard Disk where the MBR contains primary boot loader.

 Boot Loader
To be very brief this phase includes loading of the boot loader (MBR and GRUB/LILO) into memory to bring up the kernel.

MBR (Master Boot Record)
-> It is the first sector of the Hard Disk with a size of 512 bytes.
-> The first 434 – 446 bytes are the primary boot loader, 64 bytes for partition table and 6 bytes for MBR validation timestamp.

NOTE: Now MBR directly cannot load the kernel as it is unaware of the filesystem concept and requires a boot loader with file system driver for each supported file systems, so that they can be understood and accessed by the boot loader itself.

To overcome this situation GRUB is used with the details of the filesystem in /boot/grub.conf and file system drivers.

GRUB

GRUB stands for Grand Unified Bootloader.

This loads the kernel in 3 stages

GRUB stage 1:

The primary boot loader takes up less than 512 bytes of disk space in the MBR – too small a space to contain the instructions necessary to load a complex operating system.
Instead the primary boot loader performs the function of loading either the stage 1.5 or stage 2 boot loader.

GRUB Stage 1.5:

Stage 1 can load the stage 2 directly, but it is normally set up to load the stage 1.5.
This can happen when the /boot partition is situated beyond the 1024 cylinder head of the hard drive.
GRUB Stage 1.5 is located in the first 30 KB of Hard Disk immediately after MBR and before the first partition.
This space is utilized to store file system drivers and modules.
This enabled stage 1.5 to load stage 2 to load from any known location on the file system i.e. /boot/grub

GRUB Stage 2:

This is responsible for loading kernel from /boot/grub/grub.conf and any other modules needed
Loads a GUI interface i.e. splash image located at /grub/splash.xpm.gz with list of available kernels where you can manually select the kernel or else after the default timeout value the selected kernel will boot

The original file is /etc/grub.conf of which you can observe a symlink file at /boot/grub/grub.conf

Kernel

This can be considered the heart of operating system responsible for handling all system processes.

Kernel is loaded in the following stages:

-> Kernel as soon as it is loaded configures hardware and memory allocated to the system.
-> Next it uncompresses the initrd image (compressed using zlib into  zImage or bzImage formats) and mounts it and loads all the necessary drivers.
-> Loading and unloading of kernel modules is done with the help of programs like insmod, and rmmod present in the initrd image.
-> Looks out for hard disk types be it a LVM or RAID.
-> Unmounts initrd image and frees up all the memory occupied by the disk image.
-> Then kernel mounts the root partition as specified in grub.conf as read-only.
-> Next it runs the init process

 Init Process

Executes the system to boot into the run level as specified in /etc/inittab

Runlevel scripts
A no. of runlevel scripts are defined inside /etc/rc.d/rcx.d
Runlevel Directory
0 /etc/rc.d/rc0.d
1 /etc/rc.d/rc1.d
2 /etc/rc.d/rc2.d
3 /etc/rc.d/rc3.d
4 /etc/rc.d/rc4.d
5 /etc/rc.d/rc5.d
6 /etc/rc.d/rc6.d

Based on the selected runlevel, the init process then executes startup scripts located in subdirectories of the /etc/rc.d directory.
Scripts used for runlevels 0 to 6 are located in subdirectories /etc/rc.d/rc0.d through /etc/rc.d/rc6.d, respectively.
For more details on scripts inside /etc/rc.d follow the below link
What are the s and k scripts in the etc rcx.d directories
Lastly, init runs whatever it finds in /etc/rc.d/rc.local (regardless of run level). rc.local is rather special in that it is executed every time that you change run levels.