Using bitmaps, GT.M efficiently locates free space in the database. A master bitmap has one bit per local bitmap which indicates whether the corresponding local bitmap is full or has free space. When there is no free space in a group of 512 blocks, GT.M clears the associated bit in the master map to show whether the local bitmap is completely busy. Otherwise, GT.M maintains the bit set.

There is only one Master Bitmap per database. You can neither see the contents of the master bitmap directly or can change the size of the master bitmap. The maximum size of a GT.M database is over 7TB (terabytes, assuming 32K blocks).

The size of the master bitmap constrains the size of the database. The size of the master maps reflects current expectations for the maximum operational size of a single database file. Note: In addition to the limit imposed by the size of the master map, GT.M currently limits a tree to a maximum number of 7 levels. This means if a database holds only one global, depending on the density and size of the data, it might reach the level limit before the master map limit.