A data structure in SQL Server that tracks which pages and extents in a database file are in use, available, or allocated to specific objects.
- SQL Server uses several allocation map page types including: GAM (Global Allocation Map), SGAM (Shared Global Allocation Map), and IAM (Index Allocation Map).
- DBAs consult allocation maps when diagnosing: Storage fragmentation, page allocation errors, and database file growth issues.
- Allocation map corruption is a serious integrity problem: It is detectable via DBCC CHECKDB and often indicates hardware or I/O subsystem failures.
- Understanding allocation maps helps DBAs: Optimize file group placement, manage TempDB contention, and plan file layout for large databases.
- Related terms: Extent, Page Split, DBCC CHECKDB, Fragmentation, TempDB.
