Universal Disk Format (UDF) is a file system specification defined by OSTA. One objective of UDF is to replace the ISO9660 file system on optical media (CDs, DVDs, etc). It is also a good file system to replace FAT on removable media.

Any removable media (CD, DVD, flash drive, external hard drive, etc) needs a file system format. Ideally, this format should have these characteristics:

  • Can be understood by different platforms. This makes it possible to copy files between Windows, Mac, and Unix systems. FAT and ISO9660 are two formats that can be understood by most systems. However, they have many limitations.
  • Its specification is open. ISO9660 is an open standard, while FAT belongs to Microsoft.
  • Has rich features (preferably a super set of all common file systems) so information won't be lost when files are copied to this file system.
  • Can support different kinds of physical media. Optical media is very different from hard drives. Some media is write once (CD-R, DVD-R, DVD+R, BD-R), some needs defect-management (CD-RW, DVD-RW, DVD+RW, BD-RE, etc), some needs to be expanded sequentially before being overwritten (most RW media).
  • Its format should be as simple as possible. This is important when this format is implemented in embedded devices (DVD player, Camcorder, Camera, etc). Complex data structures such as B-tree are not good candidates for this purpose.
  • Its format should evolve in a compatible way so old media can still be accessed by new systems.

    UDF is the only file system that meets all these standards, since it was designed for the information exchange purpose.

  • UDF is an open standard.
  • The design and evolution of UDF keeps compatibility in mind.
  • UDF natively supports many modern file systems features:
    • Large partition size (maximum 2TB with 512B block size, or 8TB with 2KB block size)
    • 64-bit file size
    • Extended attributes (e.g., named streams, or forks) without size limitation
    • Long file names (maximum 254 bytes, any character can appear in the name)
    • Unicode encoding of file names
    • Sparse file
    • Hard links
    • Symbolic links
    • Metadata checksum
    • Metadata redundancy (optional in UDF 2.50 or later in metadata partition)
    • Defect management (for media that does not manage defect internally, such as CD-RW, DVD-RW, and DVD+RW)
  • UDF defines how different platforms interact with each other. For example, it defines how to store Mac Finder Info and Resource Fork, NTFS ACL, UNIX ACL, OS/2 EA, etc. It also requires platforms to preserve the information that they don't understand.
  • UDF is a truly universal file system. It can be used on all kinds of optical media, including read only (CD-ROM, DVD-ROM, BD-ROM (Blu-ray Disc Read-Only)), write once (CD-R, DVD-R, DVD+R, BD-R), rewritable (CD-RW, DVD-RW, DVD+RW, DVD-RAM, CD-MRW, DVD+MRW, BD-RE), and of course block device (hard drives). Even write-once media appears as a big overwritable floppy under UDF.
  • Drawbacks of the current revision of UDF as of 2.60:
  • Limited partition size. 32-bit block number limits the partition size to 2TB for 512 sector size. Although it is not a problem for the current optical media, it may become a problem later.
  • Does not provide a fast crash recovery mechanism. As the size of the media increases, crash recovery becomes more and more important. Full disc scan before mount becomes less feasible on slow optical media with tens of gigabytes space. Although an implementation may use a journal to protect metadata integrity, this does not guarantee interoperability between platforms since it is not part of the standard.
  • Does not support compressed/encrypted file and directories. As device gets bigger and bigger, compression is not that important. However, encryption may become more compelling since UDF is mainly used on removable media.
  • Becomes more and more complex. UDF 2.50 adds the metadata partition in order to improve performance. File system metadata are clustered within the metadata partition so that they can be accessed quickly. Optionally, a mirror of the metadata partition could be duplicated to provide better robustness in a big cost of performance. This adds non-trivial complexity to the file system. Does the benefit of metadata partition warrant its complexity? If the UDF implementation organizes metadata properly, it may achieve similar (or better) performance to what the metadata partition can provide. Unfortunately, in UDF 2.50 or later, the use of metadata partition is mandatory on overwrite media like CD-RW and hard drive. UDF 2.60 even requires the use of metadata partition on write-once media using pseudo-overwrite partition. If a UDF implementation wants to avoid the complexity of metadata partition, it should use UDF 2.00/2.01.
  • Is not as popular as FAT and ISO9660 now. As more and more systems implement UDF, this problem will go away.

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