View source for Ext2

{{For|the gene|EXT2 (gene)}}
{{short description|File system for the Linux kernel}}
{{lowercase}}
{{Infobox file system
| name = ext2
| full_name = Second extended file system
| developer = [[Rémy Card]]
| introduction_os = [[Linux]]
| introduction_date = January 1993
| preceded_by       = [[extended file system]]
| succeeded_by      = [[ext3]]
| partition_id = <code>[[Basic Data Partition|EBD0A0A2-B9E5-4433-87C0-68B6B72699C7]]</code> ([[GUID Partition Table|GPT]])<br /><code>0x83</code> ([[Master Boot Record]])<br /><code>Apple_UNIX_SVR2</code> ([[Apple Partition Map]])
| directory_struct = Table
| file_struct = bitmap (free space), table (metadata)
| bad_blocks_struct = Table
| max_filename_size = 255 bytes
| max_files_no = 10<sup>18</sup>
| max_volume_size = 2–32 [[tebibyte|TiB]]
| max_file_size = 16 [[gibibyte|GiB]] – 2 [[tebibyte|TiB]]
| filename_character_set = All bytes except [[Null character|NUL]] ('\0') and '/'
| dates_recorded = modification (mtime), attribute modification (ctime), access (atime)
| date_range = December 14, 1901 - [[Year 2038 problem|January 18, 2038]]
| date_resolution = 1&nbsp;s
| forks_streams =
| attributes = 
| file_system_permissions = Unix permissions, [[Access-control_list#POSIX_ACL|POSIX Access Control Lists]] (ACL)
| compression = No (Available through patches)
| encryption = No
| OS = [[Linux]], [[BSD]],<ref>{{Cite web |title=Chapter 8. Disks, File Systems, and Boot Loaders |url=https://www.freebsd.org/doc/en_US.ISO8859-1/books/faq/disks.html#idp59236072 |access-date=11 April 2018 |website=www.freebsd.org}}</ref><ref>{{Cite web |title=ext2fs(5) |url=https://www.freebsd.org/cgi/man.cgi?query=ext2fs&sektion=5&manpath=freebsd-release-ports |access-date=11 April 2018 |website=www.freebsd.org}}</ref> [[ReactOS]],<ref>{{Cite web |title=ReactOS 0.4.2 Released |url=https://reactos.org/project-news/reactos-042-released |access-date=17 August 2016 |website=reactos.org}}</ref> Windows (through an [[Installable File System|IFS]]), [[macOS]] (through [[Filesystem_in_Userspace|FUSE]]), [[HelenOS]],<ref>{{Cite web |title=HelenOS Wiki: Ext4fs |url=https://www.helenos.org/wiki/Fs/Ext4fs |access-date=6 November 2023 |website=www.helenos.com}}</ref> [[RIOT (operating system)|RIOT]],<ref>{{Cite web |title=pkg/lwext4: add lightweight implementation of the ext2/3/4 filesystem |url=https://github.com/RIOT-OS/RIOT/pull/18682 |access-date=6 November 2023 |website=www.github.com}}</ref> [[Zephyr (operating system)|Zephyr]]<ref>{{Cite web |title=subsys: fs: Implementation of ext2 file system |url=https://github.com/zephyrproject-rtos/zephyr/pull/55152 |access-date=6 November 2023 |website=www.github.com}}</ref>

}}

'''ext2''', or '''second extended file system''', is a [[file system]] for the [[Linux]] [[kernel (operating system)|kernel]].  It was initially designed by French software developer [[Rémy Card]] as a replacement for the [[extended file system]] (ext). Having been designed according to the same principles as the [[Berkeley Fast File System]] from [[Berkeley Software Distribution|BSD]], it was the first commercial-grade filesystem for Linux.<ref>{{Cite book |last=Mauerer |first=Wolfgang |url=https://books.google.com/books?id=-6zvRFEfQ24C&pg=PT490 |title=Professional Linux Kernel Architecture |publisher=John Wiley & Sons |year=2010 |isbn=9781118079911}}</ref>

The canonical implementation of ext2 is the "ext2fs" filesystem driver in the Linux kernel. Other implementations (of varying quality and completeness) exist in [[GNU Hurd]],<ref>{{Cite web |title=GNU Hurd/ hurd/ translator/ ext2fs |url=https://www.gnu.org/software/hurd/hurd/translator/ext2fs.html |access-date=2024-02-10 |website=GNU Hurd}}</ref> [[MINIX 3]],<ref>{{Cite web |title=MINIX Release 3.2.0 |url=https://wiki.minix3.org/doku.php?id=releases:3.2.0:start |access-date=2024-02-10 |website=MINIX 3}}</ref><ref>{{Cite web |title=git.minix3.org Git - minix.git/tree - minix/fs/ext2/ |url=https://git.minix3.org/index.cgi?p=minix.git;a=tree;f=minix/fs/ext2 |access-date=2024-02-10 |website=MINIX 3 Source Code}}</ref> some [[BSD]] kernels,<ref>{{Cite web |title=NetBSD src/sys/ufs/ext2fs/ |url=http://cvsweb.netbsd.org/bsdweb.cgi/src/sys/ufs/ext2fs/ |access-date=2024-02-10 |website=NetBSD Source Code}}</ref><ref>{{Cite web |title=OpenBSD src/sys/ufs/ext2fs/ |url=https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/sys/ufs/ext2fs/ |access-date=2024-02-10 |website=OpenBSD Source Code}}</ref><ref>{{Cite web |title=ext2fs « fs « sys - src - FreeBSD source tree |url=https://cgit.freebsd.org/src/tree/sys/fs/ext2fs |access-date=2024-02-10 |website=FreeBSD Source Code}}</ref> in [[MiNT]],<ref>{{Cite web |title=freemint/sys/xfs/ext2fs at master · freemint/freemint |url=https://github.com/freemint/freemint/tree/master/sys/xfs/ext2fs |access-date=2024-02-10 |website=FreeMiNT Source Code}}</ref> [[Haiku (operating system)|Haiku]]<ref>{{Cite web |title=ext2 « file_systems « kernel « add-ons « src - haiku - Haiku's main repository |url=https://git.haiku-os.org/haiku/tree/src/add-ons/kernel/file_systems/ext2 |access-date=2024-02-10 |website=Haiku Source Code}}</ref> and as third-party [[Microsoft Windows]]<ref>{{Cite web |title=matt-wu/Ext3Fsd: Ext2/3/4 file system driver for Windows |url=https://github.com/matt-wu/Ext3Fsd |access-date=2024-02-10 |website=matt-wu/Ext3Fsd}}</ref> and [[macOS]] (via [[Filesystem_in_Userspace|FUSE]]) drivers. This driver was deprecated in Linux version 6.9 in favor of the ext4 driver, as the ext4 driver works with ext2 filesystems.<ref name="phoronix-deprecated" />

ext2 was the default filesystem in several [[Linux distribution]]s, including [[Debian]] and [[Red Hat Linux]], until supplanted by [[ext3]], which is almost completely compatible with ext2 and is a [[journaling file system]]. ext2 is still the filesystem of choice for [[Flash memory|flash]]-based storage media (such as [[SD card]]s and [[USB flash drive]]s){{citation needed|date=February 2024}} because its lack of a journal increases performance and minimizes the number of writes, and flash devices can endure a limited number of write cycles. Since 2009,<ref>{{Cite web |title=ext4: Allow ext4 to run without a journal |url=https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0390131ba84fd3f726f9e24fc4553828125700bb |access-date=12 January 2021 |website=Linux kernel source tree}}</ref> the [[Linux kernel]] supports a journal-less mode of [[ext4]] which provides benefits not found with ext2, such as larger file and volume sizes.<ref>{{Cite web |title=Linux File Systems: Ext2 vs Ext3 vs Ext4 |url=https://www.thegeekstuff.com/2011/05/ext2-ext3-ext4/ |access-date=2020-07-13 |website=www.thegeekstuff.com}}</ref>

==History==
The early development of the Linux kernel was made as a cross-development under the [[MINIX]] operating system. The [[MINIX file system]] was used as Linux's first file system. The Minix file system was mostly free of [[computer bug|bugs]], but used 16-bit offsets internally and thus had a maximum size limit of only 64 [[megabyte]]s, and there was also a filename length limit of 14 characters.{{r|Strobel}} Because of these limitations, work began on a replacement native file system for Linux.<ref name="Strobel">{{Cite book |last=Strobel |first=Stefan |title=Linux—Unleashing the Workstation in Your PC |last2=Uhl |first2=Thomas |publisher=Springer-Verlag |year=1994 |page=54}}</ref>

To ease the addition of new file systems and provide a generic file [[application programming interface|API]], [[Virtual file system|VFS]], a virtual file system layer, was added to the Linux kernel. The extended file system ([[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.<ref name="anatomyExt4">{{Cite web |last=Jones |first=M. Tim |date=17 February 2009 |title=Anatomy of ext4 |url=http://www.ibm.com/developerworks/linux/library/l-anatomy-ext4/ |archive-url=https://web.archive.org/web/20150220020146/http://www.ibm.com/developerworks/linux/library/l-anatomy-ext4/ |archive-date=2015-02-20 |access-date=8 February 2012 |publisher=IBM Developer Works}}</ref> 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 [[gigabyte]]s of data and filenames of up to 255 characters. But it still had problems: there was no support of separate [[time code|timestamp]]s for file access, [[inode]] modification, and data modification.

As a solution for these problems, two new filesystems were developed in January 1993 for Linux kernel 0.99: [[xiafs]] and the '''second extended file system''' ('''ext2'''),<ref name="anatomyExt4" /> 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.{{citation needed|date=November 2022}} 

Since then, ext2 has been a testbed for many of the new extensions to the VFS API. Features such as the withdrawn [[POSIX]] [[access control list|draft ACL proposal]] and the withdrawn [[extended attribute]] proposal were generally implemented first on ext2 because it was relatively simple to extend and its internals were well understood.

On Linux kernels prior to 2.6.17,<ref>[https://archive.today/20120709233348/http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=a0f62ac6362c168754cccb36f196b3dfbddc3bc3 linux/kernel/git/torvalds/linux-2.6.git/commitdiff:], [PATCH] 2TB files: add blkcnt_t,  Author:Takashi Sato, 26 Mar 2006 09:37:52 +0000 (01:37 -0800) — Commit allowing for large files, git.kernel.org</ref> restrictions in the block driver mean that ext2 filesystems have a maximum file size of 2&nbsp;TiB.

ext2 is still recommended over journaling file systems on bootable USB flash drives and other [[solid-state drive]]s{{by whom|date=February 2024}}. ext2 performs fewer writes than ext3 because there is no journaling. As the major aging factor of a flash chip is the number of erase cycles, and as erase cycles happen frequently on writes, decreasing writes increases the life span of the solid-state device.<ref>{{Cite web |title=Solid State Disk Drives |url=http://www.supertalent.com/tools/ssd.php |publisher=Super Talent Technology}}</ref> Another good practice for filesystems on flash devices is the use of the ''no[[atime (Unix)|atime]]'' mount option, for the same reason.

Beginning with Linux kernel 6.9, the ext2fs driver was deprecated and is no longer enabled in the default configuration. The central reason was that it did not support [[Y2038|dates past 2038]].<ref name="phoronix-deprecated">{{Cite web |last=Larabel |first=Michael |date=2024-03-26 |title=Linux 6.9 Deprecates The EXT2 File-System Driver |url=https://www.phoronix.com/news/Linux-6.9-Deprecates-EXT2 |access-date=2024-03-26 |website=Phoronix |language=en}}</ref> Users are recommended to upgrade to ext4.<ref name="bootlin">{{Cite web |last=Opdenacker |first=Michael |date=2024-03-25 |title=ext2 filesystem driver now marked as deprecated - Bootlin's blog |url=https://bootlin.com/blog/ext2-filesystem-driver-now-marked-as-deprecated/ |access-date=2024-03-26 |language=en-US}}</ref>

==ext2 data structures==
{{More citations needed section|date=January 2017}}
The space in ext2 is split up into [[block (data storage)|block]]s.  These blocks are grouped into block groups, analogous to [[cylinder group]]s in the [[Unix File System]]. There are typically thousands of blocks on a large file system.  Data for any given file is typically contained within a single block group where possible.  This is done to minimize the number of disk seeks when reading large amounts of contiguous data.

Each block group contains a copy of the superblock and block group descriptor table, and all block groups contain a  block bitmap, an inode bitmap, an inode table, and finally the actual data blocks.

The superblock contains important information that is crucial to the booting of the [[operating system]]. Thus backup copies are made in multiple block groups in the file system. However, typically only the first copy of it, which is found at the first block of the file system, is used in the booting.

The group descriptor stores the location of the block bitmap, inode bitmap, and the start of the inode table for every block group. These, in turn, are stored in a group descriptor table.<ref>{{Cite web |title=The Second Extended Filesystem — The Linux Kernel documentation |url=https://docs.kernel.org/filesystems/ext2.html |access-date=2025-02-28 |website=docs.kernel.org}}</ref>

===Inodes===
Every file or directory is represented by an [[inode]]. The term "inode" comes from "index node" (over time, it became i-node and then inode).<ref>"Programmer's Journal", Volume 5, 1987, p. 174</ref> The inode includes data about the size, permission, ownership, and location on disk of the file or directory.

Example of ext2 inode structure:

<div align="center">[[File:Ext2-inode.svg|Estructure]]</div>

Quote from the Linux kernel documentation for ext2:<ref>{{Cite web |title=The Second Extended File System |url=https://www.nongnu.org/ext2-doc/ext2.html |access-date=2019-12-11 |website=www.nongnu.org}}</ref>
{{blockquote|There are pointers to the first 12 blocks which contain the file's data in the inode.  There is a pointer to an indirect block (which contains pointers to the next set of blocks), a pointer to a doubly indirect block and a pointer to a trebly indirect block.}}

Thus, there is a structure in ext2 that has 15 pointers. Pointers 1 to 12 point to direct blocks, pointer 13 points to an indirect block, pointer 14 points to a doubly indirect block, and pointer 15 points to a triply indirect block.

===Directories===
Each directory is a list of directory entries. Each directory entry associates one file name with one inode number, and consists of the inode number, the length of the file name, and the actual text of the file name.  To find a file, the directory is searched front-to-back for the associated filename.  For reasonable directory sizes, this is fine.  But for very large directories this is inefficient, and ext3 offers a second way of storing directories ([[HTree]]) that is more efficient than just a list of filenames.

The root directory is always stored in inode number two, so that the file system code can find it at mount time. Subdirectories are implemented by storing the name of the subdirectory in the name field, and the inode number of the subdirectory in the inode field. Hard links are implemented by storing the same inode number with more than one file name. Accessing the file by either name results in the same inode number, and therefore the same data.

The special directories "." (current directory) and ".." (parent directory) are implemented by storing the names "." and ".." in the directory, and the inode number of the current and parent directories in the inode field. The only special treatment these two entries receive is that they are automatically created when any new directory is made, and they cannot be deleted.

===Allocating data===
When a new file or directory is created, ext2 must decide where to store the data. If the disk is mostly empty, then data can be stored almost anywhere. However, clustering the data with related data will minimize seek times and maximize performance.

ext2 attempts to allocate each new directory in the group containing its parent directory, on the theory that accesses to parent and children directories are likely to be closely related. ext2 also attempts to place files in the same group as their directory entries, because directory accesses often lead to file accesses. However, if the group is full, then the new file or new directory is placed in some other non-full group.

The data blocks needed to store directories and files can be found by looking in the data allocation bitmap. Any needed space in the inode table can be found by looking in the inode allocation bitmap.

==File-system limits==
{| class="wikitable" style="float:right; margin:0.5em 0 0.5em 1em"
|+ Theoretical ext2 limits under Linux<ref>{{Cite web |title=File system guide |url=http://linuxreviews.org/sysadmin/filesystems/ |url-status=dead |archive-url=https://web.archive.org/web/20110917065413/http://linuxreviews.org/sysadmin/filesystems/ |archive-date=2011-09-17 |access-date=2008-01-24 |publisher=Linux Reviews}}</ref>
|-
!align="left" | Block size:
|align="right" | 1 KiB
|align="right" | 2 KiB
|align="right" | 4 KiB
|align="right" | 8 KiB
|-
!align="left" | max. file size:
|align="right" | 16 GiB
|align="right" | 256 GiB
|align="right" | 2 TiB
|align="right" | 2 TiB
|-
!align="left" | max. filesystem size:
|align="right" | 4 TiB
|align="right" | 8 TiB
|align="right" | 16 TiB
|align="right" | 32 TiB
|}

{{original research|bit|reason=There are lots of calculations and numbers, all unsourced. (Can ext2 even have a 512-byte block size?)|date=December 2021}}

The reason for some limits of ext2 are the file format of the data and the operating system's kernel. Mostly these factors will be determined once when the file system is built. They depend on the block size and the ratio of the number of blocks and inodes.{{citation needed|date=December 2021}}
<!-- TODO: research about the following limits in Kernel 2.6 -->
In Linux the block size is limited by the architecture [[page size]].<!--Block sizes of 8nbps;KiB are only possible on alpha-architectures by default. -->

There are also some userspace programs that cannot handle [[Large file support|files larger than 2&nbsp;GiB]].

If ''b'' is the block size, the maximal file size is limited to min( ((''b''/4)<sup>3</sup> + (''b''/4)<sup>2</sup> + ''b''/4 + 12) × ''b'', (2<sup>32</sup> − 1) × 512 ) due to the i_block structure (an array of direct/indirect EXT2_N_BLOCKS) and i_blocks (32-bit integer value) representing the number of 1024 byte (1 kilobyte)<ref>{{Cite web |title=Creating the Ext2 Filesystem - Understanding the Linux kernel, Second edition |url=https://www.oreilly.com/library/view/understanding-the-linux/0596002130/ch17s04.html |access-date=25 July 2023 |website=www.oreilly.com}}</ref> "blocks" in the file.{{or|date=December 2021}}

The maximal number of sublevel-directories is 31998, due to the link-count limit.{{citation needed|date=December 2021}}  Directory indexing is not available in ext2, so there are performance issues for directories with a large number of files (>10,000).  The theoretical limit on the number of files in a directory is 1.3 × 10<sup>20</sup>,{{or|date=December 2021}} although this is not relevant for practical situations.

Note: In Linux 2.4 and earlier, block devices were limited to 2&nbsp;TiB, limiting the maximal size of a partition, regardless of block size.<ref>{{Cite web |title=The Second Extended File System |url=https://cscie28.dce.harvard.edu/lectures/lect04/6_Extras/ext2-struct.html |access-date=2024-12-07 |website=cscie28.dce.harvard.edu}}</ref>

==Compression extension==
'''e2compr''' is a modification to the ext2 driver in the [[Linux kernel]] to support compression and decompression of files by the file system, without any support by user applications. e2compr is a small patch against ext2.

e2compr compresses only regular files; the administrative data (superblock, inodes, [[directory (file systems)|directory]] [[Computer file|files]], etc.) are not compressed (mainly for safety reasons). Access to compressed [[Block size (data storage and transmission)|blocks]] is provided for read and write operations. The [[compression algorithm]] and [[Cluster (file system)|cluster size]] is specified on a per-file basis. Directories can also be marked for compression, in which case every newly created file in the directory will be automatically compressed with the same cluster size and the same algorithm that was specified for the directory.

e2compr is not a new file system. It is only a patch to ext2 made to support the EXT2_COMPR_FL flag. It does not require user to make a new partition, and will continue to read or write existing ext2 file systems. One can consider it as simply a way for the read and write routines to access files that could have been created by a simple utility similar to gzip or compress. Compressed and uncompressed files coexist nicely on ext2 partitions.

The latest e2compr-branch is available for current releases of Linux 2.4, 2.6, and 3.0. The latest patch for Linux 3.0 was released in August 2011 and provides [[Symmetric multiprocessor system|multicore]] and [[High memory]] support. There are also branches for Linux 2.0 and 2.2.

==Under other operating systems==
Access to ext2 partitions under Microsoft Windows is possible through an [[Installable File System]], such as ext2ifs<ref>{{Cite magazine |last=Randall |first=Neil |date=September 18, 2007 |title=Linux: You Can Do It! |url=https://books.google.com/books?id=ecwM7EB9SPMC&pg=PA76 |magazine=[[PC Magazine]] |page=76 |volume=26 |issue=18}}</ref> or [[ext2Fsd]].<ref>{{Cite web |last=Wallen |first=Jack |date=September 17, 2012 |title=Enable the mounting of ext2/3 file systems on a Windows machine |url=http://www.techrepublic.com/blog/tr-dojo/enable-the-mounting-of-ext2-3-file-systems-on-a-windows-machine/ |access-date=January 17, 2016 |website=[[TechRepublic]]}}</ref>  [[Filesystem in Userspace]] can be used on macOS.<ref>{{Cite web |last=Kessler |first=Topher |date=June 22, 2012 |title=How to manage Ext2/Ext3 disks in OS X |url=http://www.cnet.com/how-to/how-to-manage-ext2ext3-disks-in-os-x/ |access-date=January 17, 2016 |website=[[CNET]]}}</ref>

==See also==
*[[e2fsprogs]]
*[[StegFS]] – a [[steganographic file system]] based on ext2
*[[cloop]]
*[[List of file systems]]
*[[Comparison of file systems]]
*[[Orlov block allocator]], [[Linux Kernel]]-[[Determinism|determined]] default block allocator for ext2.<ref>{{Cite web |title=The Second Extended Filesystem — the Linux Kernel documentation |url=https://www.kernel.org/doc/html/latest/filesystems/ext2.html#options}}</ref>

==References==
{{Reflist}}

===Notes===
{{refbegin}}
*{{Cite journal |last=Card, Rémy |last2=Ts’o, Theodore |last3=Tweedie, Stephen |year=1994 |title=Design and implementation of the second extended filesystem |url=https://web.mit.edu/tytso/www/linux/ext2intro.html |journal=Proceedings of the First Dutch International Symposium on Linux. |isbn=90-367-0385-9}}
*[https://sourceforge.net/projects/e2compr Sourceforge e2compr project]
*[https://e2compr.sourceforge.net/ Sourceforge e2compr documentation]
*[https://sourceforge.net/projects/e3compr Sourceforge e3compr project page, ext3 compression, alpha]
*[http://www.ddj.com/showArticle.jhtml?articleID=184405431&queryText=seen Dr. Dobb's Data Compression Newsletter Issue #46 - September 2003]
{{refend}}

==Further reading==
*{{cite web |last=Newbigin |first=John |title=John's spec of the second extended filesystem |url=https://uranus.chrysocome.net/explore2fs/es2fs.htm}}
*{{cite web |last=Poirier |first=Dave |year=2009 |title=The Second Extended File System: Internal Layout |url=https://www.nongnu.org/ext2-doc/index.html}}
*{{cite conference |last1=Ts'o |first1=Theodore |last2=Tweedie |first2=Stephen |date=June 2002 |title=Planned Extensions to the Linux Ext2/Ext3 Filesystem |url=https://e2fsprogs.sourceforge.net/extensions-ext23/ |book-title=USENIX 2002 Annual Technical Conference}}
*{{cite magazine |last=Ayers |first=Larry |date=June 1997 |title=E2compr: Transparent File Compression for Linux |url=https://linuxgazette.net/issue18/e2compr.html |magazine=[[Linux Gazette]] |issue=18}}
*{{cite CiteSeerX |last1=Cong |first1=Charles |last2=Brown |first2=Jeremy H. |date=1997-12-26 |title=A System for Transparent File Compression With Caching Under Linux |citeseerx=10.1.1.51.4780}}
*{{cite CiteSeerX |last1=Cong |first1=Charles |last2=Brown |first2=Jeremy H. |date=1997-10-22 |title=A Survey of Modern File Compression Techniques |citeseerx=10.1.1.50.9847}}

==External links==
*[https://e2fsprogs.sourceforge.net/ext2.html ext2fs user-space tools]
*[https://github.com/matt-wu/Ext3Fsd Ext3Fsd] GPL ext2/ext3 file system driver for Windows 2000/XP/2003/VISTA/2008 (opensource, supports read & write, works with FreeOTFE)

{{Filesystem}}

[[Category:1993 software]]
[[Category:Disk file systems]]
[[Category:File systems supported by the Linux kernel]]