Ext4

From Wikipedia, the free encyclopedia

The correct title of this article is ext4. The initial letter is shown capitalized due to technical restrictions.

ext4
Developer	Mingming Cao, Dave Kleikamp, Alex Tomas, Andrew Morton, others
Full name	Fourth extended file system
Introduced	October 10, 2006 (Linux 2.6.19)
Partition identifier	0x83 (MBR) EBD0A0A2-B9E5-4433-87C0-68B6B72699C7 (GPT)
Structures
Directory contents	Table, Tree
File allocation	bitmap (free space), table (metadata)
Bad blocks	Table
Limits
Max file size
Max number of files
Max filename size
Max volume size	1024 PiB = 1 EiB
Allowed characters in filenames	All bytes except NUL and '/'
Features
Dates recorded
Date range
Forks
Attributes
File system permissions
Transparent compression
Transparent encryption
Supported operating systems	Linux

The ext4, or fourth extended filesystem is a journalled file system that was revealed on October 10, 2006 by Andrew Morton as a compatible improvement to the ext3, featuring support for volumes up to 1 exabyte (1024 petabytes) and added extent support.

It was included in version 2.6.19 of the Linux kernel which was released on November 29, 2006.

The ext4 filesystem is backward compatible with ext3, that is, it can be mounted as an ext3 partition (using “ext3” as the filesystem type when mounting). Similarly, mounting an ext3 filesystem as ext4 is also possible (using the “ext4dev” filesystem type). However, if the ext4 partition uses extents (one of the major new features of ext4), backward compatibility and therefore the ability to mount the filesystem as ext3 is lost. Extents are not used by default; the “extents” option is explicitly required (e.g. mount /dev/sda1 /mnt/point -t ext4dev -o extents).