The GT.M database subsystem consists of a run-time library and a set of utilities which operate on one or more user-specified Global Directories (GD) and database files. GT.M stores M global variables in database files, which are ordinary UNIX files. Internally, the UNIX files are organized as balanced trees (B-trees) in GT.M Data Structures (GDS). See "GT.M Database Structure" chapter for more information on B-trees and the GDS file structure.

A directory maps global names to a database file. GT.M processes use this mapping when storing and retrieving globals from the database. Multiple global directories can reference a single database file, and a database file can be referenced in multiple global directories, with some exceptions, as discussed in Chapter 4: “Global Directory Editor”. Use the Global Directory Editor (GDE) to create and maintain global directories.

In addition to mapping global variables to database files, global directories also store initial parameters used by the MUPIP CREATE command when creating new database files. GT.M uses environment variables to locate the global directory or, optionally database files.