Allocation Unit

The internal SQL Server structure that organizes how data pages are stored for a particular partition of a table or index.

• SQL Server uses three types of allocation units: IN_ROW_DATA (for regular row data), LOB_DATA (for large object data such as TEXT, IMAGE, and XML), and ROW_OVERFLOW_DATA (for variable-length columns that exceed row size limits).
• DBAs encountering storage inefficiencies or page split issues often examine allocation unit structures: This helps them understand how data is physically organized.
• Monitoring allocation unit sizes helps: Predict database file growth and plan capacity accordingly.
• Related terms: Allocation Map, Page Split, Extent, Row Overflow.