ext4

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ext4 filesystem
The ext4 or fourth extended filesystem is a journaling file system for Linux, developed as

the successor to ext3.It limits the 64-bit storage limits and several other performance

improvements are added to ext3. extensions to ext3 is being declined by some developers due

to security concerns.

Features
up to 1 exabyte sizeed volumes can be supported and 16 terabytes sized files can be supported.

Extents
This was introduced in order to replace the traditional block mapping scheme used by ext2/3 filesystems.

Backward compatibility:
This filesystem is ext3 and ext2 reverse compatible and as a result, the ext3 and ext2 filesystems can be mounted as ext4. The performance is slightly improved as new features like block allocation algorithmof ext4 can also be used with ext3 and ext2.

Persistent pre-allocation:
pre-allocation of on-disk space for a file is allwed by ext4. While most most file systems use a block of 0s to reserve the space here a a new fallocate() system call is integrated. The space allocated for files such as these would be guaranteed and would likely be contiguous.

Delayed allocation:
A technique called the allocate-on-flush is used by the ext4 filesystem.The block allocation is delayed until the data is going to be written to the disk. Due to this, the fragmentation is reduced and the performance is increased.

Other features include
-Break 32,000 subdirectory limit
-Journal checksumming
-Faster file system checking
-Multiblock allocator
-Improved timestamps

Disadvantages
Delayed allocation and potential data loss:
Due to the new delayed allocation, risk of data loss during system crashes and power loss before all the data has been written to disk inclreases.

For more details, see:
http://en.wikipediawiki/Ext4
http://twinclingtwincling/slides/ext4fs.pdf
http://scribddoc/7114603/Ext4-File-System