This is the next generation of the ext3 filesystem.

Unlike the change from ext2 filesystem to ext3 filesystem,
the on-disk format of ext4 is not forwards compatible with
ext3; it is based on extent maps and it supports 48-bit
physical block numbers. The ext4 filesystem also supports delayed
allocation, persistent preallocation, high resolution time stamps,
and a number of other features to improve performance and speed
up fsck time. For more information, please see the web pages at
http://ext4.wiki.kernel.org.

The ext4 filesystem will support mounting an ext3
filesystem; while there will be some performance gains from
the delayed allocation and inode table readahead, the best
performance gains will require enabling ext4 features in the
filesystem, or formating a new filesystem as an ext4
filesystem initially.

To compile this file system support as a module, choose M here. The
module will be called ext4.

If unsure, say N.

source code:

fs/ext4/