Getting Acquainted With the ext4 File System
The majority of computer users don't spend much time thinking about the file system their operating system uses. Granted, people installing alternative operating systems might give more consideration to the available file systems than the general population -- unless there's a specific need for a certain file system, many go with their distribution's default option.
While your file system type isn't something you should need to think about regularly, and making an informed choice for machines with specialized, dedicated tasks is important, usually choosing the default file system type for your operating system is fine. Several Linux distributions (including Fedora and Ubuntu) are planning to move their default file systems from ext3 to ext4 in upcoming releases, while a number of distributions already (or will soon) support ext4 as a file system option.
IBM has a (rather surprisingly) interesting look at what's different about the ext4 file system, including details on why some changes were made, and how moving from ext3 to ext4 works.
The ext4 file system was declared stable with the December release of the 2.6.28 Linux kernel, and borrows some of the best features of other file system types. For instance, ext4's delayed allocation function was directly influenced by features found in the XFS and ZFS file systems.
Scalability was a major focus in ext4 development, and that's reflected in one of the file system's most noteworthy features -- supporting file systems up to 1 exabyte in size (as an illustration: this file system can function even if it's large enough take up one million 1 TB hard drives). Files within the ext4 system can be up to 16 TB in size, and the number of subdirectories is essentially unlimited (indexing has been specially optimized, however, so that lookup times remain speedy). It might seem excessive with the technology that currently exists, but the ext4 development team figured looking at the longer term now will return benefits far in to the future.
The ext4 file system, like its predecessor, is a journaling system. In an effort to increase reliability, decrease corruption, and spend less time on file system checks (fsck), an online defragmentation tool helps maintain both the file system and individual files that (inevitably) become fragmented over time.
There are a few resources on how to get the fairly backwards-compatible ext4 running on your system, and some further information from the Fedora Project, which plans to roll out ext4 as its default file system in May, when Fedora 11 becomes available. Ubuntu has seen some good results with the ext4 file system when tested in the Jaunty alpha, but so far plans on holding back on making ext4 the default file system until October's Karmic release.