Just after Linus Torvalds has released Linux 2.6.24 kernel the development on 2.6.25 has started. With 2.6.25 lots of changes concerning ext4 filesystem are suggested.

The ext4 development ist not yet finished (it comes with the 2.6.19 kernel) - that’s the reason why Theodore Ts’o (tytso), leading developer on ext4, discourages from the usage in this early state for productive use. Nevertheless some distros soon will activate it in the kernel options. That’s why the developers of ext4 are going to implement new mount options to reveal that the filesystem is not ready for productive use but for test runs.

The major question is what the advantages of ext4 are.

• Bigger data can be stored:
  Volumes up to 1024 petabytes (equiv. to 1,000,000 terabytes) are supported. With ext3 just 32 terabytes are possible.
• Multiblock allocation:
  Addressing of data can be done with so-called extents, it merges data into blocks. This reduces time of access greatly. Moreover it’s more robust concerning hard disc crashes.
• Number of subfolders increases:
  In comparison to ext3 it can store more subfolders (ext3 can hold 32,768 subdirs). This number will be broken with ext4.
• More precise timestamps:
  Timestamps are more precise - they’d be set in nanoseconds.
• Usage of checksums:
  A great feature will be that checksumming is used for journaling.

Besides those main features they still have some more features up their sleeve (e.g. increase the allowed filesize, online defragmentation, …).

I think those are great features that gives us lots of new possibilites - but some of them are not yet relevant (like 1024 petabytes storage). I’m really anxious how fast time will come when we think in those dimension of quantity.

Mounting ext4 can be done just the common way:

mount -t ext4dev -o extents /dev/sdc1 /mnt/test

It’s not necessary to convert ext3 partitions for the usage of ext4, they can be used by activating ext4 in the kernel.

If you don’t have a filesystem large enough and want to test 64bit feature, there is a script that creates a sparse file to simulate a large device (See explanation by Stephen Tweedie).
An other script is used to remove this sparse (./sparse_remove $map_name).

Ext4 is mostly based on parts of ext3 – similar to ext3 which is mainly based on ext2. Kernel developers think about deleting ext3 from the kernel source ’cause ext4 can manage ext3 perfectly and it would save affort to care for two sources the ext3 and ext4 one.

In the end some performace facts about ext4:
First benchmarks of the ext4 file system