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 supports mounting an ext3 filesystem; while there
are some performance gains from the delayed allocation and inode
table readahead, the best performance gains require enabling ext4
features in the filesystem using tune2fs, or formatting a new
filesystem as an ext4 filesystem initially. Without explicit enabling
of ext4 features, the on disk filesystem format stays fully backward
compatible.

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/