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ext3

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ext3
Developer(s)Stephen Tweedie
Full nameThird extended file system
IntroducedNovember 2001 with Linux 2.4.15
Partition IDs0x83 (MBR)
EBD0A0A2-B9E5-4433-87C0-68B6B72699C7 (GPT)
Structures
Directory contentsTable, h-Tree with dir_index enabled
File allocationbitmap (free space), table (metadata)
Bad blocksTable
Limits
Max volume size2TiB – 32TiB
Max file size16GiB – 2TiB
Max no. of filesVariable[1]
Max filename length255 bytes
Allowed filename
characters		All bytes except NUL and '/'
Features
Dates recordedmodification (mtime), attribute modification (ctime), access (atime)
Date rangeDecember 14, 1901 - January 18, 2038
Date resolution1s
AttributesNo-atime, append-only, synchronous-write, no-dump, h-tree (directory), immutable, journal, secure-delete, top (directory), allow-undelete
File system
permissions		Unix permissions, ACLs and arbitrary security attributes (Linux 2.6 and later)
Transparent
compression		No
Transparent
encryption		No (provided at the block device level)
Other
Supported
operating systems		Linux, BSD, Windows (through an IFS)

The ext3 or third extended filesystem is a journalled file system that is commonly used by the Linux operating system. It is the default file system for many popular Linux distributions. Stephen Tweedie first revealed that he was working on extending ext2 in a February 1999 kernel mailing list posting[2] and the filesystem was merged with the mainline kernel from 2.4.15 onward.[3]

Advantages

Although its performance (speed) is less attractive than competing linux filesystems such as JFS, ReiserFS and XFS, it does have the significant advantage that it allows in-place upgrades from the ext2 file system without having to back up and restore data as well as requiring lower CPU consumption than ReiserFS and XFS.[4]

The ext3 file system adds, over its predecessor:

  • A Journaling file system
  • Online file system growth
  • Htree (Specialized version of a btree) for larger directories

Without these, any ext3 file system is also a valid ext2 file system. This 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 a fsck tool. The close relationship also makes conversion between the two file systems (both forward to ext3 and backward to ext2) straightforward.

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 in some circumstances. 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 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.[5] [6]
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 unexpected after a journal recovery. The lack of syncronicity between data and journal is faster in many cases. XFS and JFS use this level of journaling, but ensures that any "garbage" due to unwritten data is zeroed out on reboot.

While in some contexts the lack of "modern" filesystem features such as dynamic inode allocation and extents could be considered a disadvantage, in terms of recoverability this gives ext3 a significant advantage over file systems with those features. The file system metadata is all in fixed, well-known locations, and there is some redundancy inherent in the data structures that may allow ext2 and ext3 to be recoverable in the face of significant data corruption, where tree-based file systems may not be recoverable.

Disadvantages

Functionality

Since ext3 aims at being mostly compatible with ext2, many of the on-disk structures are similar to those of ext2. Because of that, ext3 lacks a number of features of more recent designs, such as dynamic allocation of i-nodes and variable block sizes (frags or tails).

ext3 filesystems cannot be fscked while the filesystem is mounted for writing. A dump of the filesystem taken while it is mounted read-write may result in corrupt data within the dump file.

ext3 does not support extents, a feature found in other filesystems such as JFS, ext4 and XFS.

Defragmentation

There is no online ext3 defragmentation tool working on the filesystem level. An offline ext2 defragmenter, e2defrag, exists but requires that the ext3 filesystem be converted back to ext2 first. But depending on the feature bits turned on the filesystem, e2defrag may destroy data; it does not know how to treat many of the newer ext3 features.[7]

There are userspace defragmentation tools like Shake[8] and defrag [9]. Shake works by allocating space for the whole file bolt upright and hoping that it will make the newly allocated file less fragmented. It also tries to write files used at the same time next to each others. Defrag works by copying each file over itself.
However they only work if the filesystem is reasonably empty. A true defragmentation tool does not exist for ext3.[10]

That being said, 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."[11]

Irrespective of the above (subjective) statement, file fragmentation can be an important issue in server environments such as in multi-media server applications. While it is true that ext3 is more resistant to file fragmentation than FAT filesystems, and NTFS, nonetheless ext3 filesystems can and do get fragmented over time. Consequently the successor to the ext3 filesystem, ext4, includes a filesystem defragmentation utility and support for extents (contiguous file regions).

Further examples in which lack of defragmentation in some Linux filesystems (such as ext3) is a serious issue, includes server applications where rapid, concurrent and random file creation, update or access occurs. Such systems include large-scale carrier grade voice mail systems, Media-Messaging Service Centers (MMSCs) and SMS/SMSCs (Short Message Service Centers) servers. Media servers such as large scale voice mail and UMS servers are required to stream hundreds of voice or video streams concurrently to hundreds of users in near real-time conditions. These types of applications are particularly susceptible to file fragmentation; access delays during playback of a voice (e.g. voice mail) or video file, due to multiple fragmentation in the media file, can lead to playback interruption or distortion. As fragmentation increases over time, service capacity of these systems degrades because of increased CPU and I/O overhead resulting from fragmentation induced disk thrashing.

Compression

Support for transparent compression is available as an unofficial patch for ext3. This patch is a direct port of e2compr and still needs further development, it compiles and boots well with upstream kernels but journaling is not implemented yet. The current patch is named e3compr and can be found here: http://sourceforge.net/projects/e3compr/

Size limits

Template:Disputedsection ext3 has a maximum size for both individual files and the entire filesystem. These limits are dependent on the block size of the filesystem; the following chart summarizes the limits[12]:

Block size Max file size Max filesystem size
1KiB 16GiB 2TiB
2KiB 256GiB 8TiB
4KiB 2TiB 16TiB
8KiB 2TiB 32TiB

It should be noted that the 8 KiB block size is only available on architectures which allow 8 KiB pages (such as Alpha).

No checksumming in journal

Ext3 does not do checksumming when writing to the journal. If barrier=1 is not enabled as a mount option (in /etc/fstab), and if the hardware is doing out-of-order write caching, one runs the risk of severe filesystem corruption during a crash.[13][14] (This option is not enabled by default on almost all popular Linux distributions, and thus most distributions are at risk.)

Consider the following scenario: If hard disk writes are done out-of-order (due to modern hard disks caching writes in order to amortize write speeds), it is likely that one will write a commit block of a transaction before the other relevant blocks are written. If a power failure or kernel panic should occur before the other blocks get written, the system will have to be rebooted. Upon reboot, the file system will replay the log as normal, and replay the "winners" (transactions with a commit block, including the invalid transaction above which happened to be tagged with a valid commit block). The unfinished disk write above will thus proceed, but using corrupt journal data. The file system will thus mistakenly overwrite normal data with corrupt data while replaying the journal. If checksums had been used (if the blocks of the "fake winner" transaction were tagged with mutual checksum), the file system could have known better and not replayed the corrupt data onto the disk.

Ext4dev / Ext4

Main article: ext4

An enhanced version of the filesystem is under development; it was announced by Theodore Ts'o [15] on June 28, 2006 under the name of ext4.

References

  1. ^ The maximum number of inodes (and hence the maximum number of files and directories) is set when the file system is created. If V is the volume size in bytes, then the default number of inodes is given by V/213 (or the number of blocks, whichever is less), and the minimum by V/223. The default was deemed sufficient for most applications.
  2. ^ Stephen C. Tweedie (February 17 1999). "Re: fsync on large files". Linux kernel mailing list. {{cite web}}: Check date values in: |year= (help)
  3. ^ Rob Radez (November 23 2001). "2.4.15-final". Linux kernel mailing list. {{cite web}}: Check date values in: |year= (help)
  4. ^ Linux Gazette : January 2006 (#122)
  5. ^ curious onloooker: Speeding up ext3 filesystems
  6. ^ Common threads: Advanced filesystem implementor's guide, Part 8
  7. ^ Andreas Dilger. "Post to the ext3-users mailing list". ext3-users mailing list post.
  8. ^ Vleu.net: Shake
  9. ^ Index of /apps/defrag
  10. ^ RE: searching for ext3 defrag/file move program
  11. ^ http://www.tldp.org/LDP/sag/html/filesystems.html
  12. ^ Matthew Wilcox. "Documentation/filesystems/ext2.txt". Linux kernel source documentation.
  13. ^ Re: Frequent metadata corruption with ext3 + hard power-off
  14. ^ Re: Frequent metadata corruption with ext3 + hard power-off
  15. ^ LKML: "Theodore Ts'o": Proposal and plan for ext2/3 future development work

See also

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