Editing Universal Disk Format (section)

== Specifications ==
The UDF standard defines three [[file system]] variations, called "builds". These are:
* [[#Plain_build|''Plain'']] (Random Read/Write Access). This is the original format supported in all UDF revisions
* [[#VAT_build|''Virtual Allocation Table'']], also known as  ''VAT'' (Incremental Writing). Used specifically for writing to write-once media
* [[#Spared_(RW)_build|''Spared'']] (Limited Random Write Access). Used specifically for writing to rewritable media

=== Plain build ===
Introduced in the first version of the standard, this format can be used on any type of disk that allows random read/write access, such as [[hard disk]]s, [[DVD+RW]] and [[DVD-RAM]] media.  Metadata (up to v2.50) and file data is addressed more or less directly.  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 operating system or file system driver claiming support for UDF should be able to read this format.

=== VAT build ===
Write-once media such as [[DVD-R]] and [[CD-R]] 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]] file system gets written to CD media.

To enable a 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 in its revision 1.5. The VAT is an additional structure on the disc that allows [[packet writing]]; that is, remapping physical blocks when files or other data on the disc are modified or deleted. For write-once media, the entire disc is virtualized, making the write-once nature transparent for the user; the disc can be treated the same way one would treat a rewritable disc.

The write-once nature of CD-R or DVD-R media means that when a file is deleted on the disc, the file's data still remains on the disc. It does not appear in the directory any more, but it still occupies the original space where it was stored. Eventually, after using this scheme for some time, the disc will be full, as free space cannot be recovered by deleting files. Special tools can be used to access the previous state of the disc (the state before the delete occurred), making recovery possible.

Not all drives fully implement version 1.5 or higher of the UDF, and some may therefore be unable to handle VAT builds.

=== Spared (RW) build ===
Rewriteable media such as [[DVD-RW]] and [[CD-RW]] 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-DA|CD Audio]]) to it. However, 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).

The plain and VAT builds of the UDF format can be used on rewriteable media, with some limitations. 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. To allow modification of files on the disc, rewriteable discs 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. However, it will eventually become unreliable with no easy way of detecting it. 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.

The ''spared'' build was added in revision 1.5 to address the particularities of rewriteable media. This build adds an extra ''Sparing Table'' in order to manage the defects that will eventually occur on parts of the disc that have been rewritten too many times. This table keeps track of worn-out sectors and remaps them to working ones. UDF defect management does not apply to systems that already implement another form of defect management, such as [[Mount Rainier (packet writing)|Mount Rainier]] (MRW) for optical discs, or a disk controller for a hard drive.

The tools and drives that do not fully support revision 1.5 of UDF will ignore the sparing table, which would lead them to read the outdated worn-out sectors, leading to retrieval of corrupted data.

An [[overhead (computing)|overhead]] that is spread over the entire disc reserves a portion of the data storage space, limiting the usable capacity of a [[CD-RW]] with e.g. 650 MB of original capacity to around 500 MB.<ref name=PC-Thompson>{{cite book| url = https://books.google.com/books?id=kG8LcWfruOAC&pg=PT242 |title = ''"PC Hardware in a Nutshell: A Desktop Quick Reference"'' by Robert Bruce Thompson, Barbara Fritchman Thompson (2003) −– Chapter 11 (starting page 340): CD writers| isbn = 9780596552343| last1 = Thompson| first1 = Robert Bruce| last2 = Thompson| first2 = Barbara Fritchman| date = 24 July 2003| publisher="O'Reilly Media, Inc." }}</ref>