What is GAM and SGAM?

What is GAM and SGAM?

When we create a database, the data files will be logically divided into the pages and extents. Later, when user objects are created, the pages are allocated to them to store the data. GAM (Global Allocation Map) and SGAM (Shared Global Allocation Map) pages are used to track the space allocation in SQL Server.

What is a data page in database?

A database page is an 8 KB chunk of data. When you insert any data into a SQL Server database, it saves the data to a series of 8 KB pages inside the data file. If multiple data files exist within a filegroup, SQL Server allocates pages to all data files based on a round-robin mechanism.

What are SQL pages?

The Page is the smallest unit of data storage in Microsoft SQL Server. A page contains the data in the rows. A row can only reside in one page. Each Page can contain 8KB of information, due to this, the maximum size of a Row is 8KB.

What is GAM in SQL?

SQL Server uses a structure called the Global Allocation Map (GAM) to keep track of all free extents in a database file. The GAM is consists of several GAM pages, one for each GAM Interval in each file.

What is SGAM?

Shared Global Allocation Map (SGAM) SGAM pages record which extents are currently being used as mixed extents and also have at least one unused page. Each SGAM covers 64,000 extents, or almost 4-GB of data. The SGAM has 1-bit for each extent in the interval it covers.

What is SQL architecture?

It’s a relational database management system that supports a number of applications, including business intelligence, transaction processing and analytics. Microsoft SQL Server is built on SQL, which is a programming language used to manage databases and query data.

How do you create a data page?

Creating a data page

1. In the header of Dev Studio, click Create Data Model Data Page .
2. On the Create Data Page page, in the Data Page Record Configuration section, in the Label field, enter a name for the top-level page that this rule creates and maintains.
3. Click Create and open.

What is database page size?

The default database page size in InnoDB is 16KB. By recompiling the code, you can set it to values ranging from 8KB to 64KB. Further, to put rows in relation to pages: The maximum row length, except for variable-length columns (VARBINARY, VARCHAR, BLOB and TEXT), is slightly less than half of a database page.

Is Gam a page?

GAM pages are used by SQL Server to track, which extents are in use and which ones are unallocated. Each GAM page contains a 7988-byte bitmap covering all extent in a single GAM interval of 511232 pages. Each bit corresponds to a single extent, and a value of 1 indicates that that extent is still unallocated.

What are mixed extents in SQL Server 2016?

Mixed extents contain pages allocated to different objects. This count does include all the IAM pages in the file. Applies to: SQL Server 2016 (13.x) SP2 and later. Total number of pages modified in allocated extents of the file since last full database backup.

What are the different types of extents in SQL Server?

There are two types of extents: mixed extents and dedicated (or uniform) extents. The first 8 pages that are allocated to any IAM chain (either for an index in SQL Server 2000 or allocation unit in SQL Server 2005) are single-page allocations, which are called mixed pages.

How are pages allocated from uniform extents counted?

Total number of pages allocated from uniform extents for user objects in the database. Unused pages from an allocated extent are included in the count. IAM pages are not included, because they are always allocated from mixed extents. PFS pages are included if they are allocated from a uniform extent.

What are the two types of an extent?

There are two types of extents: mixed extents and dedicated (or uniform) extents. The first 8 pages that are allocated to any IAM chain are single-page allocations, which are called mixed pages. This means that only a single page is allocated to the IAM chain at a time, rather than a whole extent.