Differences between Ext2,Ext3 and Ext4 file systems in linux.

EXT2 : Ext2 was the standard file system for linux until the introduction of ext3.It was introduced with the 1.0 kernel in 1993.Ext2 is flexible,can handle file system up to 4 TB,and supports long filenames up to 1012 characters,it has sparse super blocks feature which increase file system performance.In case any user processes fill up a file system,ext2 normally reserves about 5% of disk blocks for exclusive use by root so that root can easily recover from that situation.Now our rescue diskettes use ext2 instead of minix.

EXT3 : Ext3 (Extended 3 file system) provides all the features of ext2,and also features journaling and backward compatibility with ext2.The backward compatibility enables you to still run kernals that are only ext2-aware with ext3 partitions.we can also use all of the ext2 file system tuning,repair and recovery tools with ext3 also you can upgrade an ext2 file system to an ext3 file system without losing any of your data.
Ext3’s journaling feature speeds up the amount of time it takes to bring the file system back to a sane state if it’s not been cleanly unmounted (that is,in the event of a power outage or a system crash). Under ext2,when a file system is uncleanly mounted ,the whole file system must be checked.This takes a long time on large file systems.On an ext3 system ,the system keeps a record of uncommitted file transactions and applies only those transactions when the system is brought back up.So a complete system check is not required and the system will come back up much faster.
A cleanly unmounted ext3 file system can be mounted and used as an ext2 file system,this capability can come in handy if you need to revert back to an older kernel that is not aware of ext3.The kernel sees the ext3 filesystem as an ext2 file system.

Ext4 : Ext4 is part of the Linux 2.6.28 kernel,Ext4 is the evolution of the most used Linux file system, Ext3. In many ways, Ext4 is a deeper improvement over Ext3 than Ext3 was over Ext2. Ext3 was mostly about adding journaling to Ext2, but Ext4 modifies important data structures of the file system such as the ones destined to store the file data. The result is a filesystem with an improved design, better performance, reliability and features.

Features of Ext4 file system : 1. Compatibility
2. Bigger filesystem/file sizes
3. Subdirectory scalability
4. Extents
5. Multiblock allocation
6. Delayed allocation
7. Fast fsck
8. Journal checksumming
9. Online defragmentation
10. Inode-related features
11. Persistent preallocation
12. Barriers on by default


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