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{{Short description|Journaling file system for Linux}}{{For|the gene|EXT3 (gene)}}{{lowercase|title=ext3}}

factoids

Microsoft Windows>Windows (through an IFS)}}ext3, or third extended filesystem, is a journaled file system that is commonly used by the Linux kernel. It used to be the default file system for many popular Linux distributions. Stephen Tweedie first revealed that he was working on extending ext2 in Journaling the Linux ext2fs Filesystem in a 1998 paper, and later in a February 1999 kernel mailing list posting. The filesystem was merged with the mainline Linux kernel in November 2001 from 2.4.15 onward.JOURNAL, Stephen C. Tweedie,e2fsprogs.sourceforge.net/journal-design.pdf, Journaling the Linux ext2fs Filesystem, Proceedings of the 4th Annual LinuxExpo, Durham, NC, May 1998, 2007-06-23, WEB,marc.info/?l=linux-kernel&m=91926557602501&w=2, Re: fsync on large files, Stephen C. Tweedie, February 17, 1999, Linux kernel mailing list, WEB,marc.info/?l=linux-kernel&m=100650331813822&w=2, 2.4.15-final, Rob Radez, November 23, 2001, Linux kernel mailing list, Its main advantage over ext2 is journaling, which improves reliability and eliminates the need to check the file system after an unclean shutdown. Its successor is ext4.WEB,access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Storage_Administration_Guide/ch-ext4.html, Chapter 6. The Ext4 File System Red Hat Enterprise Linux 6,

Advantages

The performance (speed) of ext3 is less attractive than competing Linux filesystems, such as ext4, JFS, ReiserFS, and XFS, but ext3 has a significant advantage in that it allows in-place upgrades from ext2 without having to back up and restore data. Benchmarks suggest that ext3 also uses less CPU power than ReiserFS and XFS.JOURNAL, Justin, Piszcz, Benchmarking Filesystems Part II, Linux Gazette, 122,linuxgazette.net/122/TWDT.html#piszcz, JOURNAL, Hans, Ivers, Filesystems (ext3, reiser, xfs, jfs) comparison on Debian Etch,www.debian-administration.org/articles/388, 2010-11-03, 2008-09-13,www.debian-administration.org/articles/388," title="web.archive.org/web/20080913112251www.debian-administration.org/articles/388,">web.archive.org/web/20080913112251www.debian-administration.org/articles/388, dead, It is also considered safer than the other Linux file systems, due to its relative simplicity and wider testing base.WEB, Roderick W., Smith, 2003-10-09, Introduction to Linux filesystems and files, Linux.com,www.linux.com/archive/feature/31939,www.linux.com/archive/feature/31939," title="web.archive.org/web/20110830122414www.linux.com/archive/feature/31939,">web.archive.org/web/20110830122414www.linux.com/archive/feature/31939, August 30, 2011, WEB, 2010-04-23, James, Trageser, Which Linux filesystem to choose for your PC? Ext2, Ext3, Ext4, ReiserFS (Reiser3), Reiser4, XFS, Btrfs,linuxtweaks.wordpress.com/2010/04/23/which-linux-filesystem-to-choose/, ext3 adds the following features to ext2:

• A journal
• Online file system growth
• HTree indexing for larger directoriesWEB,ext2.sourceforge.net/2005-ols/paper-html/node3.html, Directory indexing, Mingming, Cao, Features found in Linux 2.6, 2009-04-01, 2019-07-18,ext2.sourceforge.net/2005-ols/paper-html/node3.html," title="web.archive.org/web/20190718230754ext2.sourceforge.net/2005-ols/paper-html/node3.html,">web.archive.org/web/20190718230754ext2.sourceforge.net/2005-ols/paper-html/node3.html, dead,

Without these features, any ext3 file system is also a valid ext2 file system. This situation has allowed well-tested and mature file system maintenance utilities for maintaining and repairing ext2 file systems to also be used with ext3 without major changes. The ext2 and ext3 file systems share the same standard set of utilities, e2fsprogs, which includes an fsck tool. The close relationship also makes conversion between the two file systems (both forward to ext3 and backward to ext2) straightforward.ext3 lacks “modern” filesystem features, such as dynamic inode allocation and extents. This situation might sometimes be a disadvantage, but for recoverability, it is a significant advantage. The file system metadata is all in fixed, well-known locations, and data structures have some redundancy. In significant data corruption, ext2 or ext3 may be recoverable, while a tree-based file system may not.

Size limits

The maximum number of blocks for ext3 is 232. The size of a block can vary, affecting the maximum number of files and the maximum size of the file system:WEB,lxr.linux.no/linux+v2.6.29/Documentation/filesystems/ext2.txt, Documentation/filesystems/ext2.txt, Matthew Wilcox, Linux kernel source documentation, {| class=“wikitable”! Block size! Maximumfile size! Maximumfile-system sizeKibibyte>KiBGibibyte>GiBTebibyte>TiB| 2 KiB| 256 GiB| 8 TiB| 4 KiB| 2 TiB| 16 TiBpaging>pages, such as Alpha.| 2 TiB| 32 TiB

Journaling levels

There are three levels of journaling available in the Linux implementation of ext3:

Journal (lowest risk): Both metadata and file contents are written to the journal before being committed to the main file system. Because the journal is relatively continuous on disk, this can improve performance, if the journal has enough space. In other cases, performance gets worse, because the data must be written twice—once to the journal, and once to the main part of the filesystem.

Ordered (medium risk): Only metadata is journaled; file contents are not, but it’s guaranteed that file contents are written to disk before associated metadata is marked as committed in the journal. This is the default on many Linux distributions. If there is a power outage or kernel panic while a file is being written or appended to, the journal will indicate that the new file or appended data has not been “committed”, so it will be purged by the cleanup process. (Thus appends and new files have the same level of integrity protection as the “journaled” level.) However, files being overwritten can be corrupted because the original version of the file is not stored. Thus it’s possible to end up with a file in an intermediate state between new and old, without enough information to restore either one or the other (the new data never made it to disk completely, and the old data is not stored anywhere). Even worse, the intermediate state might intersperse old and new data, because the order of the write is left up to the disk’s hardware.WEB,www.ibm.com/developerworks/library/l-fs8.html, Common threads: Advanced filesystem implementor’s guide, Part 8, IBM developerWorks, Daniel Robbins, Daniel Robbins (computer programmer), 2001-12-01,www.ibm.com/developerworks/linux/library/l-fs8.html," title="web.archive.org/web/20071013213924www.ibm.com/developerworks/linux/library/l-fs8.html,">web.archive.org/web/20071013213924www.ibm.com/developerworks/linux/library/l-fs8.html, 2007-10-13, curious onloooker: Speeding up ext3 filesystems. Evuraan.blogspot.com (2007-01-09). Retrieved on 2013-06-22.

Writeback (highest risk): Only metadata is journaled; file contents are not. The contents might be written before or after the journal is updated. As a result, files modified right before a crash can become corrupted. For example, a file being appended to may be marked in the journal as being larger than it actually is, causing garbage at the end. Older versions of files could also appear unexpectedly after a journal recovery. The lack of synchronization between data and journal is faster in many cases. JFS uses this level of journaling, but ensures that any “garbage” due to unwritten data is zeroed out on reboot. XFS also uses this form of journaling.

In all three modes, the internal structure of file system is assured to be consistent even after a crash. In any case, only the data content of files or directories which were being modified when the system crashed will be affected; the rest will be intact after recovery.

Disadvantages

Functionality

Because ext3 aims to be backward-compatible with the earlier ext2, many of the on-disk structures are similar to those of ext2. Consequently, ext3 lacks recent features, such as extents, dynamic allocation of inodes, and block sub-allocation.WEB,ext2.sourceforge.net/2005-ols/paper-html/node18.html, Extents, Delayed Allocation, Rob, Radez, 2005, future of ext3, 2008-07-30, 2008-07-08,ext2.sourceforge.net/2005-ols/paper-html/node18.html," title="web.archive.org/web/20080708205131ext2.sourceforge.net/2005-ols/paper-html/node18.html,">web.archive.org/web/20080708205131ext2.sourceforge.net/2005-ols/paper-html/node18.html, dead, A directory can have at most 31998 subdirectories, because an inode can have at most 32,000 links (each direct subdirectory increases their parent folder inode link counter in the ”..” reference).Robert Nichols (2007-04-03) Re: How many sub-directories ? {{Webarchive|url=https://web.archive.org/web/20081006043647linux.derkeiler.com/Newsgroups/comp.os.linux.misc/2007-04/msg00153.html |date=2008-10-06 }} linux.derkeiler.comOn ext3, like for most current Linux filesystems, the system tool “fsck” should not be used while the filesystem is mounted for writing. Attempting to check a filesystem that is already mounted in read/write mode will (very likely) detect inconsistencies in the filesystem metadata. Where filesystem metadata is changing, and fsck applies changes in an attempt to bring the “inconsistent” metadata into a “consistent” state, the attempt to “fix” the inconsistencies will corrupt the filesystem.

Defragmentation

{{Update section|date=January 2020}}There is no online ext3 defragmentation tool that works on the filesystem level. There is an offline ext2 defragmenter, e2defrag. However, e2defrag may destroy data, depending on the feature bits turned on in the filesystem; it does not know how to handle many of the newer ext3 features.WEB,marc.info/?l=ext3-users&m=116231468911590&w=2, Post to the ext3-users mailing list, Andreas Dilger, ext3-users mailing list post, There are userspace defragmentation tools, like ShakeShake. Vleu.net. Retrieved on 2013-06-22. and defrag.Defrag written in shell. Ck.kolivas.org (2012-08-19). Retrieved on 2013-06-22.Defrag written in Python. Bazaar.launchpad.net. Retrieved on 2013-06-22. Shake works by allocating space for the whole file as one operation, which will generally cause the allocator to find contiguous disk space. If there are files which are used at the same time, Shake will try to write them next to one another. Defrag works by copying each file over itself. However, this strategy works only if the file system has enough free space. A true defragmentation tool does not exist for ext3.RE: searching for ext3 defrag/file move program. Redhat.com (2005-03-04). Retrieved on 2013-06-22.However, as the Linux System Administrator Guide states, “Modern Linux filesystem(s) keep fragmentation at a minimum by keeping all blocks in a file close together, even if they can’t be stored in consecutive sectors. Some filesystems, like ext3, effectively allocate the free block that is nearest to other blocks in a file. Therefore it is not necessary to worry about fragmentation in a Linux system.“5.10. Filesystems. Tldp.org (2002-11-09). Retrieved on 2013-06-22.While ext3 is resistant to file fragmentation, ext3 can get fragmented over time or for specific usage patterns, like slowly writing large files.{{cite web|url=http://trac.transmissionbt.com/ticket/849 |quote=The default Ubuntu filesystem (“ext3“) will fragment large (>1GB), slowly growing files (