File systems:


The ext2 or second extended file system is a file system for the Linux kernel. It was initially designed by Rémy Card as a replacement for the extended file system (ext). It is fast enough that it is used as the benchmarking standard. Its main drawback is that it is not a journaling file system. Its successor, ext3, is a journaling file system and is almost completely compatible with ext2.

The canonical implementation of ext2 is the ext2fs filesystem driver in the Linux kernel. Other implementations (of varying quality and completeness) exist in HURD, Mac OS X (third-party), Darwin (same third-party as Mac OS X but untested), some BSD kernels and as third-party Microsoft Windows drivers. ext2 was the default filesystem in the Red Hat Linux, Fedora Core and Debian Linux distributions until supplanted more recently by ext3.

The early development of the Linux kernel was made as a cross-development under the Minix operating system. Naturally, it was obvious that the Minix file system would be used as Linux's first file system. The Minix file system was mostly free of bugs, but used 16-bit offsets internally and thus only had a maximum size limit of 64 megabytes. There was also a filename length limit of 14 characters. Because of these limitations, work began on a replacement native file system for Linux.

To ease the addition of new file systems and provide a generic file API, VFS, a virtual file system layer was added to the Linux kernel. The extended file system (ext), was released in April 1992 as the first file system using the VFS API and was included in Linux version 0.96c. The ext file system solved the two major problems in the Minix file system (maximum partition size and filename length limitation to 14 characters), and allowed 2 gigabytes of data and filenames of up to 255 characters. But it still had problems: there was no support for separate access, inode modification and data modification timestamps.

As a solution for these problems, two new filesystems were developed in January 1993: xiafs and the second extended file system (ext2), which was an overhaul of the extended file system incorporating many ideas from the Berkeley Fast File System. ext2 was also designed with extensibility in mind, with space left in many of its on-disk data structures for use by future versions.

Since then, ext2 has been a testbed for many of the new extensions to the VFS API. Features such as POSIX ACLs and extended attributes were generally implemented first on ext2 because it was relatively simple to extend and its internals were well-understood.

The ext2 file system has a maximum data size of 4 terabytes, maximum filename length of 255 characters, and has variable length block size. However, other operating system considerations may mean that this full size is often not realizable on any particular operating system. On Linux, for example, restrictions in the block driver mean that ext2 filesystems have a maximum data size of 2047 gigabytes.

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