Getting Started with OpenVMS: A Guide for New Users (HP Technologies)

The INDEXF.SYS file is the central location for information about the files on a volume. Part of INDEXF.SYS is stored at a known location on disk so that a system just starting up can find certain key information, which is then used to locate the remainder of the file system data. Within this file are the following elements:

• The Boot Block This block is used only on disks that contain a copy of the OpenVMS operating system. It is stored at the first logical block on the volume and contains information needed to locate OpenVMS during system startup.

• The Home Block This block is stored at the second logical block and contains such things as the volume name, creation date, cluster factor, and other volumewide information.

• Backup Blocks INDEXF.SYS contains backup copies of important blocks such as the home block and the header describing INDEXF.SYS. These backup copies provide the ability to access the volume should the primary copies be lost due to media defects.

• Index File Bitmap A bitmap (not to be confused with BITMAP.SYS) containing information about which file headers within INDEXF.SYS are unused. This is used to quickly locate a free header when a file is created.

• File Headers Most of INDEXF.SYS contains file headers, which are blocks describing each file on the volume. Among the header contents for a file are the file identification (FID), record attributes, file characteristics, protection codes, file owner, file name, creation date, modification date, and retrieval pointers.

Most files are represented by a single header. However, since each extent (fragment) of a file needs one retrieval pointer in the header and headers are of a fixed size, severely fragmented files may require additional headers.