A file ID (FID) is a unique file identifier on an OpenVMS system. An example of a file ID is 277058,4,0.
It consists of three numbers:
- index to INDEXF.SYS (where the file header is stored)
- sequence number indicating how many times this index has been used
- Relative Volume Number (RVN), which is 0 except for bound volume sets
Because there is only a limited amount of file headers, when a new file is created, it is often assigned a header that has already been used. The sequence number indicates how many times it has been used. This may come in handy when you look for a particular file by its header: by the sequence number, you can tell that it is no longer the same file. For multiple versions of a file, only one file name is stored along with pairs of version numbers and FIDs.
To see the file ID of a particular file, use DIRECTORY/FILE_ID or DIRECTORY/FULL.