GT.M accesses every GDS file using one of two methods:
Mapped Memory (MM)
Buffered Globals (BG)
With the MM method the entire database is mapped into virtual address space and disk updates are left to the operating system.
This method may vary with platform.
MM databases do not support before-image journaling and therefore, are not eligible for rollback (backward) journal recovery. MM databases do not currently extend automatically. For more information about journaling and types of recovery, refer to the "GT.M Journaling" chapter.
MM databases may provide a performance advantage for files that have high rates of activity and the activity either contains a small proportion of updates or represents temporary work space. MM databases take up large amounts of virtual address space and are, therefore, generally limited to relatively small files.
When using the BG method the data is placed in caches managed directly by GT.M.
The structure of the database is identical for MM and BG access methods.