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Concept
Ext4
Summary
ext4 (fourth extended filesystem) is a for Linux, developed as the successor to ext3.
ext4 was initially a series of backward-compatible extensions to ext3, many of them originally developed by Cluster File Systems for the between 2003 and 2006, meant to extend storage limits and add other performance improvements. However, other Linux kernel developers opposed accepting extensions to ext3 for stability reasons, and proposed to fork the source code of ext3, rename it as ext4, and perform all the development there, without affecting existing ext3 users. This proposal was accepted, and on 28 June 2006, Theodore Ts'o, the ext3 maintainer, announced the new plan of development for ext4.
A preliminary development version of ext4 was included in version 2.6.19 of the Linux kernel. On 11 October 2008, the patches that mark ext4 as stable code were merged in the Linux 2.6.28 source code repositories, denoting the end of the development phase and recommending ext4 adoption. Kernel 2.6.28, containing the ext4 filesystem, was finally released on 25 December 2008. On 15 January 2010, Google announced that it would upgrade its storage infrastructure from ext2 to ext4. On 14 December 2010, Google also announced it would use ext4, instead of YAFFS, on Android 2.3.
ext4 is the default file system for many Linux distributions including Debian and Ubuntu.
Large file system
The ext4 filesystem can support volumes with sizes in theory up to 64 zebibyte (ZiB) and single files with sizes up to 16 tebibytes (TiB) with the standard 4 KiB block size, and volumes with sizes up to 1 yobibyte (YiB) with 64 KiB clusters, though a limitation in the extent format makes 1 exbibyte (EiB) the practical limit. The maximum file, directory, and filesystem size limits grow at least proportionately with the filesystem block size up to the maximum 64 KiB block size available on ARM and PowerPC/Power ISA CPUs.
Extents
replace the traditional block mapping scheme used by ext2 and ext3. An extent is a range of contiguous physical blocks, improving large-file performance and reducing fragmentation.
Official source
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