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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 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 Disk s and Flash Drive s. 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:
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. VERSIONS 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. These versions are:
Plain (original) version 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 version 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 version 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'' version 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 version. See also chapter "Why your computer might not read a particular UDF disk" below. Spared (RW) version 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'' version was used. RW disks may be formatted in the ''plain'' UDF version, in the ''VAT'' version and in the ''Spared'' version. The consequences of using these versions are as follows:
Understanding this Sparing Table is necessary to be able to read discs written in this version. 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'' version. 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'' version but not the ''VAT'' or ''Spared'' versions. 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'' version 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
NATIVE OS SUPPORT Note: Unless otherwise noted, ''read'' and ''write'' support means that only the ''plain'' UDF version is supported, but not the ''VAT'' and ''spared'' versions.
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