In the previous article, I shared information about the SQL Server Extent which is storing eight data pages.

The SQL Server manages two important pages to store information about occupied and free space of each extent.

GAM: Stands for Global Allocation Map.
SGAM: Stands for Shared Global Allocation Map.

Global Allocation Map (GAM):

GAM has a bit for every extent, and it records what extents have been allocated for any use.

It stores information about both Uniform and Mixed extents.

If the bit value is 0, extent is in use, and If the bit value is 1, extent is free.
It can hold information about the total 64000 extents, and approximately size is 4000 MB.

When a new page needs to be allocated, SQL Server finds available extent into GAM pages. When an extent is de-allocated, the bit for this extent is reset in GAM and it makes an available for future allocations.

Shared Global Allocation Map (SGAM):

SGAM has a bit for every extent, and it records what extents are currently being used as mixed extent and have at least one unused page.

If the bit value is 0, either extent is not a mixed extent, or it is a mixed extent and all its pages being used.
If the bit value is 1, it is a mixed extent and having an at least one unused page.

It can hold information about the total 64000 extents and approximately size is 4000 MB.

When almost all extents are full and SQL Server unable to find free extent information from the GAM page, it uses the SGAM page to find the information about the free extent.

DBCC PAGE to check Allocation Status:

/*
FileID: 1
PageID: 296
*/
DBCC TRACEON(3604)
GO
DBCC PAGE('database_name',1,296,3)

In the previous articles, I explained basic about the architecture of Data Files and Data Pages of the SQL Server.

The SQL Server Extent is one type of storage unit, and it stores eight physical contiguous 8KB pages.

Each extent size is 64KB and it can store 16 extents per megabyte.

The eight data pages are logically grouped into extents and extents is responsible to manage storage space for data pages.

As per my previous discussion, a single data page can never merge data from multiple tables.
But SQL Server does not allocate whole extents to the particular table.

There are two types of Extent, Uniform Extent and Mixed Extent.

Uniform Extent: This Extent basically for single user object and it stores all 8 data pages for a single user object. If our table has a large set of records, internally, it stores all 8 data pages into one Extent.

Mixed Extent: This Extent owned by multiple user object and it stores 8 data pages may be for multiple user object. It can also combine different type of data pages into one extent.

When you first create a table SQL Server starts by allocating a data page to it from a Mixed Extent. Once the table has enough data to warrant a full extent, SQL Server will allocate a Uniform Extent to it.

Let me explain with the simple example:

First, create a table with sample records:

CREATE TABLE tbl_TestPages(
   Name CHAR(8000)
)
GO

-- INSERT 50 Records
INSERT INTO tbl_TestPages VALUES('dbrnd.com')
GO 50

DBCC IND to check the information about allocated pages:

DBCC IND('database_name','tbl_TestPages',1)

The Result:

You can find first 8 pages don’t have sequential PagePID because the first time it stored in a Mixed Extent mode. The second 8 pages have a sequential PagePID because the same table requires more 8KB pages.