Difference between (Extended) ext2/3 and ext4 File System

* Ext2
-It was introduced in 1993. Developed by Remy Card.
-ext2 stands for second extended file system.
-This was developed to overcome the limitation of the original ext file system.
-ext2 does not have journaling feature.
-ext2 is recommended for flash drives, usb drives etc
-Maximum individual file size can be from 16GB to 2TB (depends on block size)
-Overall ext2 FS size can be from 2TB to 32TB

* Ext3
-It was introduced in 2001. Developed by Stephen Tweedie.
-ext3 stands for third extended file system.
    -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,file system
corruption chances are less because of journaling.
-Maximum individual file size can be from 16GB to 2TB
-Overall ext3 FS size can be from 2TB to 32TB
-There are three types of journaling available in ext3 file system.
1) Journal – both Metadata and Content are saved in the journal.
2) Ordered – Only metadata is saved in the journal. Metadata are journaled only after writing the content to disk. This is the default.
3) 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
-It was introduced in 2008.
-Ext4 stands for fourth extended file system.
-Starting from Linux Kernel 2.6.19 ext4 was available.
-Maximum individual file size can be from 16 GB to 16TB
-Overall maximum ext4 FS size is 1024PB (petabyte), 1PB = 1024TB (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)
    -ext4 default inode size is 256 bytes.(in ext3 inode size is 128 bytes)
-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”.
-Faster file system checking as Unallocated blocks are skipped during FS checking
-Improved timestamps- Up to the nanosecond. Which will defer the year 2038 problem
-Online Defragmentation

What is Extents?
-Ext3 uses a block mapping scheme (block 4Kb), the bigger the file needs huge block mapping will lead to slower handling.
-Ext4 introduces the concept of Extents. An extent is basically a “Bunch of blocks”.
Basically it say “write the data is in the next N blocks ie extent” instead of mapping each individual block separately.
-Ext4 will support up to 128Mb extents,This improve performance and also help in reducing fragmentation.

Multiblock Allocation-
-Ext3 uses a block allocator that decides which free blocks will be used to write the data. But this allocator
can only allocate one block at a time.
-Ext4 will support multi-block allocation, which allocates many blocks in a single call and avoids a lot of overhead.

Thank you,
Arun Bagul