UDF is the file format used on optical discs like DVDs.

Universal Disk Format is an open standard file system for computer data storage, developed and maintained by the Optical Storage Technology Association. UDF is most commonly used for DVDs and newer optical disc formats on both recordable and rewritable discs.

Technical Specifications

UDF supports a large partition size with a maximum of 2 terabytes with a 512 byte block size or 8TB with a 2 kilobyte block size. UDF also supports 64-bit file sizes, extended file attributes without size limitation, long file names, Unicode file name encoding, hard and symbolic links, metadata checksums and redundancy and defect management.

Compatibility

The UDF specification is open, unlike the FAT system, which Microsoft owns, so UDF files are cross-platform, meaning that you can copy them between Windows, Mac and Unix-like systems.

Application

UDF files support different types of optical media, including CD-R, DVD-R and DVD+R, which are all “write once,” meaning they can only be recorded to once. UDF also supports rewritable media such as CD-RW, DVD-RW and DVD+RW, since they need defect management. UDF does not support compressed or encrypted files and directories, however.

How UDF Works

Normally, when writing to optical media, the authoring software will master a UDF file system in a batch process and write it to the disc in a single pass. But when packet writing to rewritable media, such as CD-RW, DVD-RW and DVD+RW, UDF allows you to create, delete and modify files on the disc as if it were other removable media like a flash drive or a floppy disk. This same process is used on write-once discs, such as CD-R, DVD-R and DVD+R, but in this instance, the space occupied by the deleted files cannot be reclaimed and becomes inaccessible, preventing any additional recording to the disc.

For each UDF definition there are four interdependent files: the Header File, the Data File, the Virtual Instrument Definition File (VIDF) and the Plot Interface Definition File (PIDF). UDF Data and Header files come in pairs and are time-based files, with each pair containing the UDF measurements from a fixed time period. The VIDF file is also time-based, but typically its contents are valid over very long time periods. It is not uncommon for a single VIDF file to service all the UDF associated Header and Data Files. The PIDF is not time-based and there is only one PIDF per UDF definition.