Compact Disk File System

The Compact Disk File System (CDFS) of Garfinkel (1986) is a system independent hierarchical file system for WORM optical disks. The goals of the CDFS are to be completely transportable across a variety of modern operating systems, to make efficient use of storage space, and to have a relatively high retrieval performance.

Unlike the write-once and time-split B-trees, the CDFS does not provide a structure for the organization of records, but rather a structure for the organization of groups of complete files. The application for which it is primarily intended is to organize files that experience few modifications, such as those

belonging to a source code archive. The smallest unit of registration in the CDFS organization is the file. The basic unit of organization in the CDFS is called a "transaction." A transaction results from the process of writing a complete group of files on the optical disk. All the files in a transaction group are placed on the disk immediately adjacent to the position of the previous transaction. Each individual file is stored contiguously. At the end of a transaction, an updated directory list for the entire file system is stored along with an "End of Transaction" (EOT) record. The EOT record contains a link to the EOT record of the previous transaction allowing access to historical versions of the organization (a dummy EOT record is stored at the start of an empty disk). The last transaction on the disk is the starting point for all accesses and the directory list it contains represents the current version of the file hierarchy.

The CDFS contains three types of "files": regular files, directories, and links. Each file is given a unique sequence number for the file system and a version number. If a file is updated by writing a new copy of it at some later time, the new copy retains the sequence number, but receives a new version number.

Each stored file entry consists of two parts, a file header and a file body. The header, which is invisible to a user, stores a large amount of explicit information about the file. This is an attempt by the CDFS to span the entire space of file charecteristics that any given operating system might record or require. For example, the file header contains the name of the owner of the file; on some systems (e.g., UNIX) this information must be derived by consulting a system database. This explicit information allows the contents of a single disk employing a CDFS to appear to be a native file system on more than one operating system (with appropriate drivers for each system).

A directory is a special type of file that contains entries identifying other files known as members. These entries include pointers to the disk positions of the members and other information such as file sizes and modification times. This extra information is redundant since it is also stored in the file header, but it serves to improve the performance of directory list operations by eliminating the seeks required to access each member.

A link entry is simply a pointer to a file or a directory and allows a file to be a member of more than one directory.

The directory list stored at the end of the files in the transaction is an optimization to reduce seeks. It is a list of the positions of all current directories and subdirectories in the hierarchical file system. Using the directory list improves performance by reducing the seeks needed to traverse the file directory tree. The diagram in Figure 6.4 illustrates how an instance of a CDFS is stored in a series of disk sectors. The example is for two transactions for a CDFS consisting of three files. The second transaction is used to store a second expanded version of the second file in the file system. The arrows in the diagram

represent the pointers which link the various constituents of the CDFS together. For example,

(backward) pointers exist between the EOT records, between each EOT record and its directory list, between the directory list and the directories (in this case just one, the root), and between the root and the three files.

Figure 6.4: State of compact disk file system after two transactions

The CDFS is an efficient means of organizing an archive of a hierarchical file system. Its main drawback is that it does not allow efficient updates to files. Any change to a single file requires the entire file to be rewritten along with a new copy of the directory list. Extra information could be added to the file header to allow files to be stored noncontiguously. This would allow portions of the file to be changed while other parts remained intact.

The robustness of the CDFS inherent in the degree of redundancy found in the organization, coupled with the access it allows to all previous versions of a file, makes it ideal for use in storing file archives.

Being relatively system independent, the CDFS is also an excellent organization for data interchange. It would be possible, for example, to copy a complete UNIX file system to an optical disk employing the CDFS and then transport it to an appropriate VMS installation and access the archived UNIX file system as if it were a native VMS file system.