Universal Disk Format builds - AbsoluteAstronomy.com

Builds of UDF

These are 3 main types of Universal Disk Format

Universal Disk Format

Universal Disk Format is an implementation of the specification known as ISO/IEC 13346 and ECMA-167 and is an open vendor-neutral file system for computer data storage for a broad range of media. In practice, it has been most widely used for DVDs and newer optical disc formats, supplanting ISO 9660...

file system on disks:

Plain (Random Read/Write Access). This is the original format supported in all UDF revisions
Virtual Allocation Table a.k.a. VAT (Incremental Writing). Used specifically for writing to CD-R and (write-once) media
Spared (Limited Random Write Access). Used specifically for writing to CD-RW and DVD-RW (rewritable) media

Plain build

This format can be used on any type of disk that allows random read/write access, such as hard disks, DVD+RW and DVD-RAM media. Similarly to other common file system formats, such as FAT

File Allocation Table

File Allocation Table is a computer file system architecture now widely used on many computer systems and most memory cards, such as those used with digital cameras. FAT file systems are commonly found on floppy disks, flash memory cards, digital cameras, and many other portable devices because of...

, directory entries point directly to the block numbers of their file contents. 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 OS or File System Driver claiming support for UDF should be able to read this format.

VAT build

DVD-R and CD-R media 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 filesystem gets written to CD media.
To enable 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.
The VAT is an additional structure on the disk that helps in remapping physical blocks when files or other data on the disk are modified. The write-once nature of the media means that when a file is first added and then deleted on the disk, the file's data still remains on the disk. It does not appear in the directory any more, but special tools can be used to access the previous state of the disc (the state before the delete occurred), making recovery possible. Eventually the disk will be full, as free space cannot be recovered by deleting files. However, this behavior can be used to advantage for the purpose of archiving data.
Understanding the VAT structure is necessary in order to read such discs, but not all UDF file systems support VAT. See also "Why your computer might not read a particular UDF disk", below.

Spared (RW) build

DVD-RW and CD-RW media 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 Audio) to it.
DVD-RW and CD-RW disks may thus be used as a blankable-R media but may also be formatted in the plain, VAT and Spared UDF builds.
However, it is important to understand that 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).
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. Hence, if software formats -RW media with UDF in the plain build, it should set the "hard write protection" flag on the volume to ensure that no UDF software attempts to overwrite files on the volume as is possible with random-rewritable media such as hard disks
To allow modification of files on the disc, the media 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.
To get true overwritability (which is not possible with the VAT build) of files on RW media, the disc needs to be formatted using the Spared build which adds an extra Sparing Table. This table keeps track of bad sectors and remaps them to working ones.
Once a -RW disc has been used with the spared UDF build, the disc should never be reused with any other format, as the information about the bad blocks would get lost, potentially leading to the aforementioned unreliability.
Since DVD+RW discs can't emulate DVD+R, they can only be formatted in the plain and in the Spared UDF build.

Consequences of using specific builds

The consequences of using these builds are as follows:

When using the plain build, in theory a disc driver may allow rewriting any disc sector at random, meaning the RW is truly rewritable in the fashion of hard disks. However, because of the wear-out effect, this would soon lead to loss of data. For that reason, if a plain UDF file system is written to RW media, the file system should lock (write protect) the UDF volume to prevent accidental modification by a computer, or better, disc drivers should never even attempt to provide random-write access to RW media unless they can assure that no data loss due to wear-out can happen.
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.
Finally, the Spared build works basically like the plain build, but uses an extra Sparing Table to remap worn-out sectors.

Understanding Sparing Tables is necessary to be able to read discs written in Sparable build correctly. 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 build. 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.