Universal Disk Format

Universal Disk Format (UDF) is a profile of the specification known as ISO/IEC 13346 and ECMA-167 and is an open vendor-neutral file system for computer data storage for a broad range of media. In practice, it has been most widely used for DVDs and newer optical disc formats, supplanting ISO 9660. Due to its design, it is very well suited for incremental updates on both recordable or (re)writable optical media. UDF is developed and maintained by the Optical Storage Technology Association (OSTA).

Normally, authoring software will master a UDF file system in a batch process and write it to optical media in a single pass. But when packet writing to rewriteable media, such as CD-RW, UDF 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, but in that case the space occupied by the deleted files cannot be reclaimed (and instead becomes inaccessible).

Multi-session mastering is also possible in UDF, though some implementations may be unable to read disks with multiple sessions.^[2]

History

The Optical Storage Technology Association standardized the UDF file system to form a common file-system for all optical media. It aimed to make a common file-system: both for read-only media and for re-writable optical media. When first standardized, the UDF file system aimed to replace ISO 9660, allowing support for both read-only and writable media. After the release of the first version of UDF, the DVD Consortium adopted it as the official file-system for DVD Video and DVD Audio.^[3]

Revisions

Multiple revisions of UDF have been released:^[3][4]

• 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 3, 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, easier crash recovery and optional duplication of file system information: All metadata like nodes and directory contents are written on a separate partition which can optionally be mirrored. This format is used by some versions of Blu-ray discs.
• Revision 2.60 (March 1, 2005). Added Pseudo OverWrite method for drives supporting pseudo overwrite capability on sequentially recordable media. (Some versions of Blu-ray discs use this format.)

Packet writing

Packet writing was introduced in Revision 1.50. For write-once media, the entire disk is virtualized, making the write-once nature transparent for the user; the disk can be treated the same way one would treat a rewritable disc, but with steadily decreasing available space. For rewritable media, addresses are virtualized so that defect management (sector relocation) can be performed. However, UDF defect management does not apply to systems that already implement another form of defect management, such as Mount Rainier for optical discs, or a disk controller for a hard drive.

Incorrect or incomplete implementations of Revision 1.50 (or above) may result in compatibility problems due to improper virtualization in reading or writing the disk.^{[citation needed]}

Builds

These are 3 main types of UDF file system on disks:

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

Plain build

This format can be used on any type of disk that allows random read/write access, such as hard disks, DVD+RW and DVD-RAM media. Similarly to other common file system formats, such as FAT, directory entries point directly to the block numbers of their file contents. In 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 build

DVD-R and CD-R media have limitations when being written to, in that each physical block can only be written to once, and the writing must happen incrementally. Thus the plain build of UDF 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 an ISO 9660 filesystem gets written to CD media. To enable CD-R to be used virtually like a hard disk, whereby the user can add and modify files on a CD-R at will (so-called "drive letter access" on Windows), OSTA added the VAT build to the UDF standard.

The VAT is an additional structure on the disk that helps in remapping physical blocks when files or other data on the disk are modified. The write-once nature of the media means that when a file is first added and then deleted on the disk, the file's data still remains on the disk. It does not appear in the directory any more, but special tools can be used to access the previous state of the disc (the state before the delete occurred), making recovery possible. Eventually the disk will be full, as free space cannot be recovered by deleting files. However, this behavior can be used to advantage for the purpose of archiving data.

Understanding the VAT structure is necessary in order to read such discs, but not all UDF file systems support VAT. See also "Why your computer might not read a particular UDF disk", below.

Spared (RW) build

DVD-RW and CD-RW media have fewer limitations than DVD-R and CD-R media. Sectors can be rewritten at random (though in packets at a time). These media can be erased entirely at any time, making the disc blank again, ready for writing a new UDF or other file system (e.g., ISO 9660 or CD Audio) to it. DVD-RW and CD-RW disks may thus be used as a blankable-R media but may also be formatted in the plain, VAT and Spared UDF builds.

However, it is important to understand that sectors of -RW media may "wear out" after a while, meaning that their data becomes unreliable, through having been rewritten too often (typically after a few hundred rewrites, with CD-RW). If the plain build is used on a -RW media, file-system level modification of the data must not be allowed, as this would quickly wear out often-used sectors on the disc (such as those for directory and block allocation data), which would then go unnoticed and lead to data loss. Hence, if software formats -RW media with UDF in the plain build, it should set the "hard write protection" flag on the volume to ensure that no UDF software attempts to overwrite files on the volume as is possible with random-rewritable media such as hard disks.

To allow modification of files on the disc, the media can be used like -R media using the VAT build. This ensures that all blocks get written only once (successively), ensuring that there are no blocks that get rewritten more often than others. This way, a RW disc can be erased and reused many times before it should become unreliable. To get true overwritability (which is not possible with the VAT build) of files on RW media, the disc needs to be formatted using the Spared build which adds an extra Sparing Table. This table keeps track of bad sectors and remaps them to working ones.

Once a -RW disc has been used with the spared UDF build, the disc should never be reused with any other format, as the information about the bad blocks would get lost, potentially leading to the aforementioned unreliability. Since DVD+RW discs can't emulate DVD+R, they can only be formatted in the plain and in the Spared UDF build.

Consequences of using specific builds

The consequences of using these builds are as follows:

• When using the plain build, 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 build, CD-RW/DVD-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 build works basically like the plain build, but uses an extra Sparing Table to remap worn-out sectors.

Understanding Sparing Tables is necessary to be able to read discs written in Sparable build correctly. 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 build. 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.

Compatibility

Many DVD players do not support any UDF revision other than version 1.02. Discs created with a newer revision may still work in these players if the ISO 9660 bridge format is used. As some popular DVD burning programs default to UDF version 1.5 when burning video DVDs, users^[who?] have found it necessary to avoid using the software wizard^[which?] and instead manually configure the burn to version 1.02. Even if an operating system claims to be able to read UDF 1.50, it still may only support the plain build and not necessarily either the VAT or Spared Universal Disk Format builds.

Mac OS X 10.4.5 claims to support Revision 1.50 (see man mount_udf), yet it can only mount disks of the plain build properly and provides no virtualization support at all. It cannot mount UDF disks with VAT, as seen with the Sony Mavica issue.^[5] Releases before 10.4.11 mount disks with Sparing Table but does not read its files correctly. Version 10.4.11 fixes this problem.^[6][7]

Similarly, Microsoft Windows XP Service Pack 2 (SP2) cannot read DVD-RW disks that use the Universal Disk Format (UDF) 2.00 defect management system.^[8] This problem occurs if the UDF defect management system creates a sparing table that spans more than one sector on the DVD-RW disk. Windows XP SP2 can recognize that a DVD is using UDF, but Windows Explorer displays the contents of a DVD as an empty folder. A hotfix is available for this^[9] and is included in Service Pack 3.^[10]

Character set

The specification allows for nine character encodings: one by agreement, one specified by ECMA-6 (also known as ASCII), three subsets of ASCII, a subset of ECMA-94 (Latin-1), and various other graphical characters.^[18][19]

See also

• Comparison of file systems
• DVD authoring
• ISO/IEC 13490

References

Further reading

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

External links

• OSTA home page
• Wenguang Wang's UDF Introduction
• Linux UDF support
• Microsoft Windows UDF Read Troubleshooting
• UDF Revision 2.60 March 1, 2005
• AIX - CD-ROM file system and UDFS