sys.dm_os_buffer_descriptors (Transact-SQL)

Článek
06/19/2023

Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance Azure Synapse Analytics Analytics Platform System (PDW)

Returns information about all the data pages that are currently in the SQL Server buffer pool. The output of this view can be used to determine the distribution of database pages in the buffer pool according to database, object, or type. In SQL Server, this dynamic management view also returns information about the data pages in the buffer pool extension file. For more information, see Buffer Pool Extension.

When a data page is read from disk, the page is copied into the SQL Server buffer pool and cached for reuse. Each cached data page has one buffer descriptor. Buffer descriptors uniquely identify each data page that is currently cached in an instance of SQL Server. sys.dm_os_buffer_descriptors returns cached pages for all user and system databases. This includes pages that are associated with the Resource database.

Note

To call this from Azure Synapse Analytics or Analytics Platform System (PDW), use the name sys.dm_pdw_nodes_os_buffer_descriptors. This syntax is not supported by serverless SQL pool in Azure Synapse Analytics.

Column name	Data type	Description
database_id	int	ID of database associated with the page in the buffer pool. Is nullable. In Azure SQL Database, the values are unique within a single database or an elastic pool, but not within a logical server.
file_id	int	ID of the file that stores the persisted image of the page. Is nullable.
page_id	int	ID of the page within the file. Is nullable.
page_level	int	Index level of the page. Is nullable.
allocation_unit_id	bigint	ID of the allocation unit of the page. This value can be used to join sys.allocation_units. Is nullable.
page_type	nvarchar(60)	Type of the page, such as: Data page or Index page. Is nullable.
row_count	int	Number of rows on the page. Is nullable.
free_space_in_bytes	int	Amount of available free space, in bytes, on the page. Is nullable.
is_modified	bit	1 = Page has been modified after it was read from the disk. Is nullable.
numa_node	int	Nonuniform Memory Access node for the buffer. Is nullable.
read_microsec	bigint	The actual time (in microseconds) required to read the page into the buffer. This number is reset when the buffer is reused. Is nullable.
is_in_bpool_extension	bit	1 = Page is in buffer pool extension. Is nullable.
pdw_node_id	int	Applies to: Azure Synapse Analytics, Analytics Platform System (PDW) The identifier for the node that this distribution is on.

Permissions

On SQL Server and SQL Managed Instance, requires VIEW SERVER STATE permission.

On SQL Database Basic, S0, and S1 service objectives, and for databases in elastic pools, the server admin account, the Microsoft Entra admin account, or membership in the ##MS_ServerStateReader## server role is required. On all other SQL Database service objectives, either the VIEW DATABASE STATE permission on the database, or membership in the ##MS_ServerStateReader## server role is required.

Permissions for SQL Server 2022 and later

Requires VIEW SERVER PERFORMANCE STATE permission on the server.

Remarks

sys.dm_os_buffer_descriptors returns pages that are being used by the Resource database. sys.dm_os_buffer_descriptors does not return information about free or stolen pages, or about pages that had errors when they were read.

From	To	On	Relationship
sys.dm_os_buffer_descriptors	sys.databases	database_id	many-to-one
sys.dm_os_buffer_descriptors	<userdb>.sys.allocation_units	allocation_unit_id	many-to-one
sys.dm_os_buffer_descriptors	<userdb>.sys.database_files	file_id	many-to-one
sys.dm_os_buffer_descriptors	sys.dm_os_buffer_pool_extension_configuration	file_id	many-to-one

Examples

A. Returning cached page count for each database

The following example returns the count of pages loaded for each database.

SELECT COUNT(*)AS cached_pages_count  
    ,CASE database_id   
        WHEN 32767 THEN 'ResourceDb'   
        ELSE db_name(database_id)   
        END AS database_name  
FROM sys.dm_os_buffer_descriptors  
GROUP BY DB_NAME(database_id) ,database_id  
ORDER BY cached_pages_count DESC;

B. Returning cached page count for each object in the current database

The following example returns the count of pages loaded for each object in the current database.

SELECT COUNT(*)AS cached_pages_count   
    ,name ,index_id   
FROM sys.dm_os_buffer_descriptors AS bd   
    INNER JOIN   
    (  
        SELECT object_name(object_id) AS name   
            ,index_id ,allocation_unit_id  
        FROM sys.allocation_units AS au  
            INNER JOIN sys.partitions AS p   
                ON au.container_id = p.hobt_id   
                    AND (au.type = 1 OR au.type = 3)  
        UNION ALL  
        SELECT object_name(object_id) AS name     
            ,index_id, allocation_unit_id  
        FROM sys.allocation_units AS au  
            INNER JOIN sys.partitions AS p   
                ON au.container_id = p.partition_id   
                    AND au.type = 2  
    ) AS obj   
        ON bd.allocation_unit_id = obj.allocation_unit_id  
WHERE database_id = DB_ID()  
GROUP BY name, index_id   
ORDER BY cached_pages_count DESC;

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