Microsoft® SQL Server 2012 Unleashed

Global and Shared Global Allocation Map Pages

The allocation map pages track whether extents have been allocated to objects and indexes and whether the allocation is for mixed extents or uniform extents. As mentioned in the preceding section, there are two types of GAMs:

Global allocation map (GAM)—The GAM keeps track of all allocated extents in a database, regardless of what it’s allocated to. The structure of the GAM is straightforward: each bit in the page outside the page header represents one extent in the file, where 1 means that the extent is not allocated, and 0 means that the extent is allocated. Nearly 8,000 bytes (64,000 bits) are available ...