File System (EXT2,EXT3,EXT4)

1.0             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

2.0             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).

3.0             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”.

Inode

An Inode number points to an Inode. An Inode is a data structure that stores the following information about a file :

• Size of file
• Device ID
• User ID of the file
• Group ID of the file
• The file mode information and access privileges for owner, group and others
• File protection flags
• The timestamps for file creation, modification etc
• link counter to determine the number of hard links
• Pointers to the blocks storing file’s contents

Hard Link

hard link is more of mirror copy….does something to file1 and it appears in file 2
deleting one still keeps the other ok

Soft Link

•  Soft or symbolic is more of a short cut to the original file….if you delete the original the shortcut fails and if you only delete the short cut nothing happens to the original.
• A ‘soft link’ is a symbolic link to another file either on the same file system or a different file system. A ‘soft link’ may span file systems while a ‘hard link’ must point to a file on the same file system.
• A soft link (or symbolic link) is just a way of accessing files (or directories) without moving them. You just have to associate the path of the file with a special type of file (a link) which will lead you directly to it. It is called soft by comparison to a hard link which has different properties.
• A soft link is basically a file that contains the absolute path of the file it is pointing to. The file system tools simply know to follow the link in the file. This allows us to link across the system

Difference between hard link and soft link

Hard links share the same inode. Soft links do not.
Hard links can’t cross file systems. Soft links do
In Linux, as in commercial Unix systems, the ‘ln’ command is used to create a symbolic link in file systems. A ‘hard link’ is a link pointing directly to a specific inode on the same file system.