Sunday, 17 January 2016

GAM VS SGAM IN SQL SERVER | DIFFERENCE BETWEEN GAM AND SGAM

Allocation Maps: SQL Server uses special kind of pages, called Allocation Maps, to track extent(collection of 8 pages known as extent) and page usage in a file. There are
several different types of allocation maps pages in SQL Server.

Global Allocation Map (GAM): pages track if extents have been allocated by any objects. The data is represented as bitmaps where each bit indicates the allocation status of an extent. Zero bits indicate that the corresponding extents are in use. The bits with a value of one indicate that the corresponding extents are free. Every GAM page covers about 64,000 extents, or almost 4GB of data. This means that every database file has one GAM page for about 4GB of file size.

Shared Global Allocation Map (SGAM): pages track information about mixed extents. Similar to GAM pages, it is a bitmap with one bit per extent. The bit has a value of one if the corresponding extent is a mixed extent and has at least one free page available. Otherwise, the bit is set to zero. Like a GAM page, SGAM page tracks about 64,000 extents, or almost 4GB of data.
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