Packet Writing is method of writing
CDs that are formated using the
UDF standard. The
difference between PW and normal
CD session-based, or
disc-at-once burning method is that PW allows to write small amounts of
data in chunks, without creating or recreating a
TOC for the CD (a time-consuming operation, for allowing
ISO9660-compatible readers to read the disc).
There are two kinds of Packet Writing:
CD-R Packet Writing
A
blank CD is inserted. First, a session is opened and the
TOC of the CD is left blank until the session is closed. During the open session, Each
variable length packet is written right after another, each time specifying the size of the packet so it is possible for the reader to know where the next packet is located. Every
modification of the
file system on the CD that has to be synced with the
CD, requires packet writes which tell about the difference that was introduced. The computer keeps track about the file system in memory for caching purposes. When the disc is ejected, the
cache is emptied, but the session is not closed. If the user specifies to close the session when the disc is ejected, then the system writes an
ISO9660 Level 3
TOC, so that regular CD readers can read the CD. Using
ISO9660 Level 3 along with
UDF is possible because the files are not fragmented on the
disc. CD-R Packet Writing is efficient, making it possible to
accommodate more blocks than there are on a regular 650MB disc.
CD-RW Packet Writing
With
CD-RW discs it is possible to use the same method as with
CD-R discs, but the
UDF standard allows to use the rewritability of
CD-RW in a special way. The
CD maintains a
file systems of
fixed length packets, and these packets can be rewritten. The advantage of this method is that the CD behaves more like a
hard drive or a floppy diskette. There are a few problems: Some
CD-RW discs support a limited amount of
erasures, like around 1000, so after many rewrites some packets will fail to write and the file system
integrity will start to break. Another problem is that files get fragmented, and the slow CD drive's
seek rates make it much slower to read files. A third problem is that because the packets are fixed size, there's a lot of space
wastage. A
UDF CD-RW offers only 494MB of
storage as a result. The last problem with this method is that it is impossible to write an
ISO9660 TOC for the file system because of the fragmented files, which makes it possible only for
UDF supported systems and
MultiRead drives to read the CD.