ext4

The online defragmenter is a simple tool that copies files into a new ext4 inode that refers to contiguous extents.

One of the first visible differences in ext4 is the increased support for file system volumes, file sizes, and subdirectory limits.

Delayed allocation Ext4 can delay allocating disk space until the last moment, which can improve performance.

Ext4 implements this through a new system call that preallocates and initializes a file of a given size.

Extents in ext4 adopt a layered approach to efficiently represent small files as well as extent trees to efficiently represent large files.

The time range has also be extended with two additional bits to increase the lifetime by another 500 years.

For this reason, ext4 was designed to be both forward and backward (to an extent) compatible (see Figure 1).

Ext4 marks unused groups of blocks within the inode table to allow the fsck process to skip them entirely to speed the check process.

But even as ext4 addresses scalability and reliability, it provides a number of enhancements for improved performance.

Ext4 introduces numerous new advancements for performance, scalability, and reliability.

Ext4 has spent a great deal of time addressing possible scenarios with journal corruption that can lead to data corruption.

As file systems scale to the massive sizes possible with ext4, greater reliability concerns will certainly follow.

Ext4 adds support for online defragmentation, which should improve overall performance.

Ext4 replaces ext3's mechanism with extents to improve allocation and support a more efficient storage structure.

Ext4 is so scalable that the maximum file system would consume one million 1-terabyte (TB) disks.

Ext4 supports file systems of up to 1 exabyte in size (1000 petabytes).

Amazingly, timestamps in the extended file system arena prior to ext4 were seconds based.

Finally, the subdirectory limit was extended with ext4, from 32KB directories deep to virtually unlimited.

So if you restore a backup created when you were on EXT4, it will restore as EXT4.

Ext4 permits exabyte file systems that can support larger numbers of files, larger files, and deeper directory structures.

The fourth extended file system (ext4) file system was renamed from ext4dev to ext4, which means that it's stable enough for regular use.

Ext4 is forward compatible in that you can mount an ext3 file system as an ext4 file system.

To combat this, ext4 implements checksumming of the journal to ensure that valid changes make their way to the underlying file system.

Ext4 permits preallocation without doing this, which can improve the performance of some database and multimedia tools.

Btfs has been scrapped leaving Ext4 as the default filesystem, Upstart has seen some tweaks, and i686 is the lowest CPU denominator.

Fast-forward to today. We now have the fourth extended file system (ext4).

For this reason, the minimum timestamp resolution in ext4 is 1 nanosecond.

Ext4 was implemented by a team of developers, led by Theodore Tso (the ext3 maintainer), and was introduced in the 2. 6. 19 kernel.

You can also gradually migrate a file system from ext3 to ext4 online with a mass change.

Most notably, ext4 supports file systems of 1 exabyte in size.

For example, a single ext4 inode has sufficient space to reference four extents (where each extent represents a set of contiguous blocks).

In this way, you can migrate an ext3 file system online to an ext4 file system.

Some sources also indicate that the ext4 timestamps support dates through April 25, 2514, versus January 18, 2038, for ext3.

Two use the ext3 filesystem, while the other one uses ext4.