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Anatomy of ext4

Feb 18, 2009, 17:04 (0 Talkback[s])
(Other stories by M. Tim Jones)

"The first supported file system for Linux was the Minix file system. This file system had some significant performance issues, so another file system was created specifically for Linux called the extended file system. The first extended file system (ext1) was designed by Remy Card and introduced into Linux in April 1992. The ext1 file system was the first to use the virtual file system (VFS) switch implemented in the 0.96c kernel and supported file systems up to 2 gigabytes (GB) in size.

"The second extended file system (ext2), also implemented by Remy Card, was introduced in January 1993. It adopted advanced ideas from other file systems of the day (such as the Berkeley Fast File System [FFS]). Ext2 extended supported file systems of 2TB in size, although 2.6 kernels extended the maximum size of ext2 file systems to 32TB.

"The third extended file system (ext3) was a major advance in Linux file systems, even though its performance was less than some of its competitors. The ext3 file system introduced the concept of journaling to improve the reliability of the file system when the system is abruptly halted. And although competing file systems had better performance (such as Silicon Graphics' XFS and the IBM® Journaled File System [JFS]), ext3 supported in-place upgrades from systems already using ext2. Ext3 was introduced in November 2001 and implemented by Stephen Tweedie.

"Fast-forward to today. We now have the fourth extended file system (ext4). Ext4 introduces numerous new advancements for performance, scalability, and reliability. Most notably, ext4 supports file systems of 1 exabyte in size. Ext4 was implemented by a team of developers, led by Theodore Tso (the ext3 maintainer), and was introduced in the 2.6.19 kernel. It is now stable in the 2.6.28 kernel (as of December 2008).

"Ext4 borrows many useful concepts from a variety of competing file systems. For example, the extent approach to block management had been implemented in JFS. Another block management-related feature (delayed allocation) was implemented in both XFS and in Sun Microsystems' ZFS.

"You'll find various improvements and innovations in the new ext4 file system. The improvements cover a number of feature angles from new functionality (new features), scalability (scaling beyond current file system constraints), reliability (in the face of failures), and of course, performance."

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