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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.

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