PC Hardware in a Nutshell, 3rd Edition

Universal Disc Format (UDF)

ISO-9660 and its variants were designed for duplicating or premastering discs, but were never intended to allow incrementally adding small amounts of data to a disc. Although ISO-9660 allows adding data to a disc (until that disc has been closed), the only way to do so is by opening a new session on that disc. That means that writing even one new file incurs the overhead required for a new session, which ranges from 13 MB to 22 MB.

In part to address these ISO-9660 limitations, OSTA defined a new logical format for optical discs. The official designation of this format is ISO 13346 but the common name is Universal Disc Format (UDF). UDF is an operating system-independent logical formatting standard that defines how data is written to various types of optical discs, including CD-R, CD-(M)RW, DVD-ROM, DVD-Video, and DVD-Audio. UDF uses a redesigned directory structure that allows small amounts of data (called packets) to be written incrementally and individually to disc without incurring the large overhead associated with writing a new session under ISO-9660.

In effect, with UDF each packet is written as a subsession within a standard session, incurring the standard session overhead only when that standard session is closed. Packet-writing software typically closes the session automatically when the disc is ejected using the eject feature of the software. As with ISO-9660, an open session on a UDF-formatted disc can be read only by a CD recorder. Closing ...