sys.dm_os_buffer_descriptors (Transact-SQL)

THIS TOPIC APPLIES TO: yesSQL Server (starting with 2008)yesAzure SQL DatabaseyesAzure SQL Data Warehouse yesParallel Data Warehouse

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 2017, 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 SQL Data Warehouse or Parallel Data Warehouse, use the name sys.dm_pdw_nodes_os_buffer_descriptors.

Column name Data type Description
database_id int ID of database associated with the page in the buffer pool. Is nullable.
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 SQL Data Warehouse, Parallel Data Warehouse

The identifier for the node that this distribution is on.

Permissions

On SQL Server, requires VIEW SERVER STATE permission.
On SQL Database Premium Tiers, requires the VIEW DATABASE STATE permission in the database. On SQL Database Standard and Basic Tiers, requires the Server admin or an Azure Active Directory admin account.

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;  

See Also

sys.allocation_units (Transact-SQL)

SQL Server Operating System Related Dynamic Management Views (Transact-SQL)
Resource Database
sys.dm_os_buffer_pool_extension_configuration (Transact-SQL)