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Information about Universal Disk Format

The Universal Disk Format (UDF) is a format specification of a file system for storing files on optical media. It is an implementation of the ISO/IEC 13346 standard (also known as ECMA-167). It is considered to be a replacement of ISO 9660, and today is widely used for (re)writable optical media. UDF is developed and maintained by the Optical Storage Technology Association (OSTA).

Optical disc authoring
Optical media types
Standards

History

The UDF file system was standardized by the Optical Storage Technology Association to form a common file system for all optical media. The goal was to make a common file system for read-only media, and optical media that are re-writable. This is still the main goal for ongoing UDF standardization, although support for the more obscure WORM media is about to be limited, and support for non-optical media may be added.

When it was first standardized, the UDF file system was intended to replace ISO9660, allowing support for both read-only and writable media. Almost directly after the first version of UDF was released, it was adopted by the DVD Consortium as the official file system for DVD Video and DVD Audio. Nowadays, a UDF file system may be found on most authored optical discs in the market, and on almost all recordable DVD media that are used for video recording.

As intended, initially UDF was mainly found on optical media. Most operating systems needed special third-party software to support reading it. Nowadays, almost all operating systems natively support at least reading UDF file systems, and many support some form of writing as well. Because of this increased support, UDF is gaining popularity on non-optical media that mainly need to be exchangeable, such as Iomega REV discs, large flash media, and even on hard disk drives.

Media

DVD-Video media use UDF version 1.02. These discs contain a so-called UDF Bridge format, where both an ISO 9660 (Level 1) as well as a UDF 1.02 filesystem are present on the same disc, describing the same filesystem.

All standard formats for video recording on DVD-style media use some version of the UDF filesystem. A UDF file system is normally mastered by authoring software in a batch process and written to disc in a single pass. However, when packet writing to rewriteable media such as CD-RW, UDF also allows files to be created, deleted and changed on-disc just as a general-purpose filesystem would on removable media like floppy disks and flash drives. This is also possible on write-once media such as CD-R, with the caveat that space cannot be reclaimed from deleted files.

Revisions of the UDF format

UDF has been released in multiple revisions:
  • Revision 1.02 (August 30, 1996). This format is used by DVD-Video discs.
  • Revision 1.50 (February 4, 1997). Added support for (virtual) rewritability on CD-R/DVD-R media by introducing the VAT structure. Added sparing tables for defect management on rewritable media such as CD-RW, and DVD-RW and DVD+RW.
  • Revision 2.00 (April 3rd, 1998). Added support for Stream Files and real-time files (for DVD recording) and simplified directory management. VAT support was extended.
  • Revision 2.01 (March 15, 2000) is mainly a bugfix release to UDF 2.00. Many of the UDF standard's ambiguities were resolved in version 2.01.
  • Revision 2.50 (April 30, 2003). Added the Metadata Partition facilitating metadata clustering and optional duplication of file system information.
  • Revision 2.60 (March 1, 2005). Added Pseudo OverWrite method for drives supporting pseudo overwrite capability on sequentially recordable media.
For next releases of UDF, changes are discussed in relation to using UDF on very large hard disk media, and using UDF on holographic storage media.

Flavors of UDF

While the UDF specification has never been explicit about it, all UDF revisions since 1.5 actually describe three different variations of the format. For lack of an official name from OSTA they are called flavors here.

These flavors are:
  • Plain (Random Read/Write Access). This is the original format supported in all UDF revisions
  • VAT (Incremental Writing). Used specifically for writing to CD-R and DVD-R (write-once) media
  • Spared (Limited Random Write Access). Used specifically for writing to CD-RW and DVD-RW (rewritable) media

Plain (original) flavor

This format can be used on any type of disk that allows random read/write access, such as hard disks and DVD-RAM media. Similar to other common file system formats, such as FAT, directory entries point directly to the block numbers of their file contents. When writing to such a disk in this format, any physical block on the disk may be chosen for allocation of new or updated files.

Since this is the basic format, practically any OS or File System Driver claiming support for UDF should be able to read this format.

VAT flavor

DVD-R and CD-R media has some limitations when being written to: Every physical block can only be written to once, and writing must happen incrementally.

This means that the plain UDF flavor can only be written to CD-Rs by pre-mastering the data and then writing all data in one piece to the media, similar to the way ISO 9660 gets written to CD media.

To allow CD-R to be used virtually like hard disks, meaning that you could add and modify files on a CD-R at will (so-called drive letter access on Windows), OSTA added the VAT flavor to the UDF standard.

The VAT is an additional structure on the disk that helps remapping physical blocks when files or other data on the disc gets modified. The nature of the write-once media leads to the effect that when a file gets first added and later deleted on the disk, the file's data still remains on the disk. While it does not appear in the directory any more, special tools may be used to access the previous state of the disc before the delete occurred, making recovery possible. Additionally, incremental writing also means that eventually the disk will be full and free space cannot be recovered by deleting files. This behavior can be used to an advantage for archiving data, though.

Understanding this VAT structure's contents is necessary to be able to read such written discs as well, but not all UDF file systems support this flavor. See also chapter "Why your computer might not read a particular UDF disk" below.

Spared (RW) flavor

DVD-RW and CD-RW media have fewer limitations than DVD-R and CD-R media: While sectors can be rewritten at random, these sectors may "wear out" after a while, meaning that their data becomes unreliable after having been rewritten too often (typically after a few hundred rewrites with CD-RW).

RW media can be erased entirely at any time, making it blank again, ready for writing a new UDF or other file system (e.g. ISO 9660 or CD Audio) to it. The wear-out effect is not an issue here unless the disk gets erased more than a few hundred times or the Spared flavor was used.

RW disks may be formatted in the plain UDF flavor, in the VAT flavor and in the Spared flavor.

The consequences of using these flavors are as follows:
  • When using the plain flavor, in theory a disc driver may allow rewriting any disc sector at random, meaning the RW is truly rewritable in the fashion of hard disks. However, because of the wear-out effect, this would soon lead to loss of data. For that reason, if a plain UDF file system is written to RW media, the file system should lock (write protect) the UDF volume to prevent accidental modification by a computer, or better, disc drivers should never even attempt to provide random-write access to RW media unless they can assure that no data loss due to wear-out can happen.
  • When using the VAT flavor, RW media effectively appears as CD-R or DVD+/-R media to the computer. However, the media may be erased again at any time.
  • Finally, the Spared flavor works basically like the plain flavor, but uses an extra Sparing Table to remap worn-out sectors. This allows reclaiming most of the disk's space when deleting a file.
Understanding this Sparing Table is necessary to be able to read discs written in this flavor. The problem is that some existing versions of UDF File System software ignore this extra information and treat such UDF discs as if they had the plain flavor. As long as the media has no worn-out sectors, this does not matter - the files can be read properly. But once sectors are remapped, a File System not paying attention to the Sparing Table will read outdated sectors, leading to retrieval of the wrong data.

Why a computer might not read a particular UDF disk

Even if a computer claims to be able to read UDF 1.50, it still may only support the plain flavor but not the VAT or Spared flavors.

An example is Mac OS X (10.4.5), which claims to support UDF 1.50 (see man mount_udf), yet it can only mount disks of the plain flavor properly (it cannot mount UDF disks with a VAT at all, see Sony Mavica problem, and while it appears to be able to mount CD-RWs written with a Sparing Table, it does not read its files correctly in the case that files are actually remapped).

Standalone DVD player compatibility

Some users have reported that video DVDs burned on their computers in UDF version 1.5 are not compatible with their set-top video DVD players. These players seem to be only compatible with UDF version 1.02. As some popular DVD burning programs default to UDF version 1.5 when burning video DVDs, users have found it necessary to avoid using the software wizard and instead manually configure the burn to version 1.02.

References

  • ISO/IEC 13346 standard, also known as ECMA-167.

Native OS support

This is an incomplete list. Please add to this list if you are aware of an omission.

Note: Unless otherwise noted, read and write support means that only the plain UDF flavor is supported, but not the VAT and spared flavors.
  • AIX 5.1, 5.2, 5.3 (Read+write)
  • AmigaOS 4.0 (Read+write) 1.02, 1.50 (partially)
  • eComStation [all] (Read+write) 2.0x
  • FreeBSD 5.X (Read) 1.02, 1.50, 2.0x
  • FreeBSD 6.X (Read) 1.02, 1.50, 2.0x, 2.50
  • FreeBSD 7.X (Read) 1.02, 1.50, 2.0x, 2.50, 2.60
  • Linux 2.4.X (Read) 1.02, 1.50
  • Linux 2.6.X (Read+write) 1.02, 1.50, 2.0x (write supported on HDD, DVD-RAM, DVD+RW and CD-RW from 2.6.10 onward, earlier versions support less media), 2.50 (read, via patch) NOTE: before 2.6.17, there's a bug in writing files bigger than 1GB; between 2.6.17 and 2.6.21 (inclusive), this is disabled, as a workaround; and in 2.6.22 this bug is fixed.
  • Mac OS 9 (Read+write) 1.02, 1.50
  • Mac OS X 10.4 (Read+write) 1.02, 1.50
  • magnussoft ZETA 1.2.1 (Read+write) 1.02, 1.50, 2.0x, 2.50, 2.60
  • NetBSD 4.0 (Read) 1.02, 1.50, 2.0x, 2.50, 2.60. Reading multi-session VAT, spared and metapartition variants from all CD, DVD, HD DVD and BD variants as well as HDD and Flash media. Write support is work in progress.
  • Novell NetWare 5.1 (Read)
  • Novell NetWare 6.x (Read)
  • OpenBSD 3.8 (Read) 1.02, 1.50, 2.0x, 2.50, 2.60
  • OS/2 (with additional fee drivers) (Read+write) {Fact} 2.0x
  • Solaris 7 11/99+ (Read) 1.02, 1.50
  • Solaris 8,9,10 (Read+Write) 1.02, 1.50. Write supported "on RAM media" [udfs(7FS) man page: Solaris 9]; works on HDD partition and DVD-RAM (with suitable DVD writer).
  • Windows 98/Me (Read) 1.02
  • Windows 2000 (Read) 1.02, 1.50
  • Windows XP/2003 (Read) 1.02, 1.50, 2.01
  • Windows Vista (Read+write) 1.02, 1.50, 2.0x, 2.50, (Read) 2.60

See also

External links

file system (often also written as filesystem) is a method for storing and organizing computer files and the data they contain to make it easy to find and access them.
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Ecma International is an international, private (membership-based) standards organization for information and communication systems. It acquired its name in 1994, when the European Computer Manufacturers Association
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ISO 9660, a standard published by the International Organization for Standardization (ISO), defines a file system for CD-ROM media. It aims at supporting different computer operating systems such as Unix, Windows and Mac OS, so that data may be exchanged.
..... Click the link for more information.
The Optical Storage Technology Association (OSTA) is an international trade association which promotes the use of recordable optical technologies and products. Representing more than 85 percent of worldwide writable optical product shipment's manufacturers and resellers, it
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optical disc authoring, including DVD authoring, known often as burning, is the process of recording source material—video, audio or other data—onto an optical disc (compact disc or DVD).
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optical disc is a flat, circular, usually polycarbonate disc where data is stored in the form of pits (or bumps) within a flat surface, usually along a single spiral groove that covers the entire recorded surface of the disc.
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disk image is a computer file containing the complete contents and structure of a data storage medium or device, such as a Hard drive, CD or DVD. The term has been generalized to cover any such file, whether originated from an actual physical storage device or not.
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optical disk drive (ODD) is a disk drive that uses electromagnetic waves as part of the process of reading and writing data. It is a computer's peripheral device, that stores data on optical discs.
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Optical disc authoring software is computer software for authoring optical discs including CD-ROMs and DVDs. They are also known by synonyms such as CD burning application or DVD authoring software. Such software is required to use an optical disc recorder.
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Disc At Once, Track At Once, Session at Once (i.e. multiple burning sessions for one disc), or packet writing modes. Each mode serves different purposes:
  • Disc At Once: writes the entire disc in one pass; preferred for duplication masters

..... Click the link for more information.
Disc-At-Once, Track-At-Once, and Session-At-Once.

CD Disc-At-Once

Disc-At-Once or DAO for CD-R media is a mode that masters the disc contents in one pass, rather than a track at a time as in Track At Once.
..... Click the link for more information.
Packet writing is an optical disc recording technology used to allow writeable CD and DVD media to be used in a similar manner to a floppy disk. Packet writing allows the user to access the contents of a CD-R or CD-RW disc directly through a mounted filesystem (Unix, Linux, Mac OS
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Laserdisc

Laserdisc (left) compared to a DVD (right).
Media type: Optical disc
Encoding: Various
Developed by: MCA
Usage: Video storage

Optical disc authoring
  • Optical disc
  • Optical disc image
  • Recorder hardware
  • Authoring software

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Compact Disc

The closely spaced tracks on the readable surface of a Compact Disc cause light to diffract into a full visible colour spectrum
Media type: Optical disc
Encoding: Various
Capacity: Typically up to 700 MB
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CD-ROM (an abbreviation of "Compact Disc read-only media") is a Compact Disc that contains data accessible by a computer. While the Compact Disc format was originally designed for music storage and playback, the format was later adapted to hold any form of binary data.
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CD-R (Compact Disc-Recordable) is a variation of the Compact Disc invented by Philips and Sony. CD-R is a Write Once, Read Many optical medium (though the whole disk does not have to be entirely written in the same session) and retains a high level of
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Compact Disc ReWritable (CD-RW) is a rewritable optical disc format. Known as CD-Erasable (CD-E) during its development, CD-RW was introduced in 1997, and was preceded by the never officially released CD-MO in 1988.
..... Click the link for more information.
MiniDisc

The Sony MZ1 MiniDisc player, the first to hit the market in 1992.
Media type: Magneto-optical disc
Encoding: ATRAC, linear PCM (with Hi-MD)
Capacity: 80 min (standard MiniDisc), up to 45 hours of audio (1 GB capacity) (with Hi-MD)
..... Click the link for more information.
DVD

Media type: Optical disc
Capacity: 4.7 GB (single layer), 8.5 GB (dual layer)
Usage: Data storage, audio, video, games

Optical disc authoring
  • Optical disc
  • Optical disc image
  • Recorder hardware
  • Authoring software

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DVD-R is a DVD recordable format. A DVD-R has a larger storage capacity than its optical predecessor, the 700 MB CD-R, typically storing 4.71 GB (or 4.382 GiB), although the capacity of the original standard developed by Pioneer was 3.95 GB (3.68 GiB).
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DVD-D is a self-destructing disposable DVD format. Like the EZ-D, it is sold in a cardboard sleeve, and begins to destroy itself after several hours.

DVD-D now exists as one time play only for movies, limited time play for video games, and recordable DVD-D.
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DVD-R DL (DL stands for Dual Layer), also called DVD-R9, is a derivative of the DVD-R format standard. DVD-R DL discs employ two recordable dye layers, each capable of storing nearly the 4.7 GB (4.
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DVD+R is a once-writable optical disc with 4.7 GB (4.377 GiB) of storage capacity (more precisely, 2295104 sectors of 2048 bytes each). It has slightly less storage capacity than the DVD-R (4.382 GiB).
..... Click the link for more information.
DVD+R DL (DL stands for Dual Layer, commonly mistaken as Double Layer), also called DVD+R9, is a derivative of the DVD+R format created by the DVD+RW Alliance. Its use was first demonstrated in October 2003.
..... Click the link for more information.
DVD-RW disc ("DVD dash RW", sometimes nicknamed "DVD minus RW") is a rewritable optical disc with equal storage capacity to a DVD-R, typically 4.7 GB. The format was developed by Pioneer in November 1999 and has been approved by the DVD Forum.
..... Click the link for more information.
DVD+RW is the name of a standard for optical discs: one of several types of DVD, which hold up to about 4.7GB per disc (interpreted as approximately 4.7 × 109 bytes; actually 2295104 sectors of 2048 bytes each) and are used for storing films, music or other data.
..... Click the link for more information.
DVD-RW DL is a rewritable optical disc standard with storage capacity of 8.5 GB. DVD-RW DL discs employ two rewritable dye layers. It's approved by DVD Forum as "DVD Specifications for Re-recordable Disc for Dual Layer (DVD-RW for DL) Physical Specifications, Version 2.0".
..... Click the link for more information.
DVD+RW DL is a rewritable optical disc with storage capacity of 8.5 GB. DVD+RW DL discs employ two rewritable dye layers.

In March 2006, the DVD+RW Alliance approved DVD+RW part 2: Dual Layer, volume 1; DVD+RW 8.5 Gbytes, Basic Format Specifications, version 1.
..... Click the link for more information.
DVD-RAM (DVD–Random Access Memory) is a disc specification presented in 1996 by the DVD Forum, which specifies rewritable DVD-RAM media and the appropriate DVD writers. DVD-RAM media have been used in computers as well as camcorders and personal video recorders since 1998.
..... Click the link for more information.
Blu-ray Disc

Media type: High-density optical disc
Encoding: MPEG-2, MPEG-4 AVC (H.264), and VC-1
Capacity: 25 GB (single layer), 50 GB (dual layer)
Read mechanism: 1x@36 Mbit/s & 2x@72 Mbit/s
Developed by: Blu-ray Disc Association
..... Click the link for more information.


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