File Layout on Disk

A new disk must be initialized before being used as a Files-11 volume. Initialization establishes INDEXF.SYS and the other reserved files (see "reserved files", further on).

On a new disk, the bulk of INDEXF.SYS is located near the middle of the disk (around the middle-numbered logical blocks) by default. Some small portions of INDEXF.SYS containing backup copies of critical information are placed at other locations on disk, selected for their redundancy against certain types of hardware failures. The portion of INDEXF.SYS containing the boot block and the home block is located at the beginning of the disk, starting at logical block zero. The remaining reserved files are created, and all of the remaining logical blocks are made available for use.

As files are created, they are generally placed at the lowest available logical block numbers, although certain other factors can cause different locations to be selected. The system maintains a cache of block numbers from recently deleted files in memory, so that space for subsequent creations can be found without examining the storage bitmap. The first available spaces will also be skipped if they are not of sufficient size, and certain options are used while creating the file.

The actions of creating, extending, and deleting files eventually result in files being broken into pieces (properly known as extents, but commonly called fragments), each of which may be stored at a different location on disk. File fragmentation is also caused by the process of extending a file when it is not known how large the file will eventually be. This prevents the proper amount of space from being allocated in the first place. If too little is allocated and the next block of free space is taken by another file, the first file must continue at another location. If too much space is allocated, there will be a range of free space between two existing files. An example of this process is presented further on.

Fragmentation degrades performance for two reasons: processing overhead and disk seeks. The processing overhead involves handling lists of pointers to various parts of a file and accounts for a small amount of the lost performance. The bulk of the damage comes from the extra seeks caused when the disk heads must physically move to a different part of the disk, given that the mechanical motion is extremely slow compared with processing speed.

Special programs called defragmenters collect the various pieces of fragmented files together to make them contiguous. Defragmenters for OpenVMS are available from Hewlett-Packard and third-party vendors.