Thursday, 24 October 2013

Troubleshooting Insufficient Disk Space in tempdb

Tempdb is a SQL Server system database that's a global resource available to all users. SQL Server uses tempdb to complete internal database operations. Tempdb is used for temporary user objects and internal database engine operations, including joins, aggregations, cursors, sorting, hashing, and row versioning. Unlike the data in a typical user database, the data in tempdb isn't persisted after the SQL Server instance is shut down.

Troubleshooting Insufficient Disk Space in tempdb :

Error messages that indicate insufficient disk space in the tempdb database. 

Error

1101 or 1105 Any session must allocate space in tempdb.
3959        The version store is full. This error usually appears after a 1105 or 1101 error in the log.
3967        The version store is forced to shrink because tempdb is full.
3958 or 3966 A transaction cannot find the required version record in tempdb.


The following examples show how to determine the amount of space available in tempdb, and the space used by the version store and internal and user objects.

Determining the Amount of Free Space in tempdb :

The following query returns the total number of free pages and total free space in megabytes (MB) available in all files in tempdb.

SELECT SUM(unallocated_extent_page_count) AS [free pages], 
(SUM(unallocated_extent_page_count)*1.0/128) AS [free space in MB]
FROM sys.dm_db_file_space_usage;

Determining the Amount Space Used by the Version Store :

The following query returns the total number of pages used by the version store and the total space in MB used by the version store in tempdb.

SELECT SUM(version_store_reserved_page_count) AS [version store pages used],
(SUM(version_store_reserved_page_count)*1.0/128) AS [version store space in MB]
FROM sys.dm_db_file_space_usage;

Determining the Longest Running Transaction:

If the version store is using a lot of space in tempdb, you must determine what is the longest running transaction. Use this query to list the active transactions in order, by longest running transaction.

SELECT transaction_id
FROM sys.dm_tran_active_snapshot_database_transactions 
ORDER BY elapsed_time_seconds DESC;

A long running transaction that is not related to an online index operation requires a large version store. This version store keeps all the versions generated since the transaction started. Online index build transactions can take a long time to finish, but a separate version store dedicated to online index operations is used. Therefore, these operations do not prevent the versions from other transactions from being removed. For more information, see Row Versioning Resource Usage.

Determining the Amount of Space Used by Internal Objects :

The following query returns the total number of pages used by internal objects and the total space in MB used by internal objects in tempdb.

SELECT SUM(internal_object_reserved_page_count) AS [internal object pages used],
(SUM(internal_object_reserved_page_count)*1.0/128) AS [internal object space in MB]
FROM sys.dm_db_file_space_usage;

Determining the Amount of Space Used by User Objects:

The following query returns the total number of pages used by user objects and the total space used by user objects in tempdb.

SELECT SUM(user_object_reserved_page_count) AS [user object pages used],
(SUM(user_object_reserved_page_count)*1.0/128) AS [user object space in MB]
FROM sys.dm_db_file_space_usage;

Determining the Total Amount of Space (Free and Used):

The following query returns the total amount of disk space used by all files in tempdb.

SELECT SUM(size)*1.0/128 AS [size in MB]
FROM tempdb.sys.database_files

Monitoring Space Used by Temp Tables and Table Variables :


You can use an approach similar to polling queries for monitoring the space used by temp tables and temp variables. Applications that acquire a large amount of user data inside temp tables or temp variables can cause space use problems in tempdb. These tables or variables belong to the user objects. You can use the user_objects_alloc_page_count and user_objects_dealloc_page_count columns in the sys.dm_db_session_space_usage dynamic management view and follow the methods described earlier.