SQL Server stores all data in 8192-byte sized blocks called pages. Several types of pages are in use within a typical database. One particularly interesting group is formed by the type-3 pages or Large Object Pages.

LOB_DATA Allocation Units

Most data types in SQL Server take up no more than 8000 bytes of storage. However, there are a few data types, which allow for larger pieces of information to be stored. Examples include the VARCHAR(MAX), VARBINARY(MAX) or XML data types.

Normal data pages that belong to a table are grouped in IN_ROW_DATA allocation units. However, if a value that is larger than 8000 bytes needs to be stored, SQL Server does not attempt to store it in those data pages anymore. It does not even store those values in the same allocation unit. Instead, Large Object data or LOB data is stored in special LOB_DATA allocation units.