Difference between ext2, ext3 and ext4 File System on Linux.

Linux is a very flexible operating system, Linux can read and write several different file systems that originated with other operating systems much different from linux. 
One main reason is that Linux support so many file systems because Linux works on VFS (Virtual File System) Layer. The VFS layer avoids duplication of common code between all file systems also it provides a fairly universal backward compatible method for programs to access data from almost all type of file systems.

ext2 Filesystem:

• ext2 stands for second extended file system.
• Introduced with kernel 1.0 in 1993.
• Super block feature increase file system performance
• Flexible can handle upto 4 TB
• ext2 filesystem is popular on USB and other solid-state devices.
• ext2 filesystem reserve 5% of disk space for root

ext3 Filesystem:

• ext3 filesystem was introduced in 2001 & Developed by Stephen Tweedie.
• ext3 filesystem stands for third extended file system.
• ext3 filesystem was released with Linux Kernel 2.4.15.
• The main advantage of ext3 filesystem is that it allows journaling.
• By journaling the possibility of file system corruption is less, So its a main advantage.
• In ext3 filesystem Maximum file size can be from 16 GB to 2 TB.
• Overall ext3 filesystem size can be from 2 TB to 32 TB.
• We can convert a ext2 filesystem to ext3 filesystem directly.

Journaling: it has a dedicated area in the file system, Where all changes are tracked and when the system crashes the possibility of data loss is less because of Journaling.

ext4 Filesystem:

• ext4 filesystem was introduced in 2008.
• ext4 filesystem stands for fourth extended file system.
• ext4 filesystem was released with Linux Kernel 2.6.19.
• ext4 filesystem supports huge individual file size.
• In ext4 filesystem Maximum file size can be from 16 GB to 16 TB.
• Overall ext4 filesystem size is 1 EB (exabyte). 1 EB = 1024 PB (petabyte). 1 PB = 1024 TB (terabyte).
• In ext4 filesystem we can mount an existing ext3 filesystem as ext4 filesystem.

Converting ext2 Filesystem to ext3 Filesystem:
For example, if we are converting /dev/sda3 that is mounted as /home, from ext2 to ext3, do the following:

#umount /dev/sda3

#tune2fs -j /dev/sda3

#mount /dev/sda3 /home

Converting ext3 Filesystem to ext4 Filesystem:
For example, if we are converting /dev/sda3 that is mounted as /home, from ext3 to ext4, do the following:

#umount /dev/sda3

#tune2fs -0 extents,uninit_bg,dir_index /dev/sda3

#e2fsck -pf /dev/sda3

#mount /dev/sda3 /home

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