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Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance SQL database in Microsoft Fabric

This article describes the tempdb system database, a global resource available to all users connected to a Database Engine instance in SQL Server, Azure SQL Database, or Azure SQL Managed Instance.

Overview

The tempdb system database is a global resource that holds:

  • User objects that are explicitly created. They include:

    • Global or local temporary tables and indexes on these tables
    • Temporary stored procedures
    • Table variables
    • Tables returned in table-valued functions
    • Cursors

    User objects that can be created in a user database can also be created in tempdb, however they are created without a durability guarantee, and are dropped when the Database Engine instance restarts.

  • Internal objects that the database engine creates. They include:

    • Work tables to store intermediate results for spools, cursors, sorts, and temporary large object (LOB) storage.
    • Work files for hash join or hash aggregate operations.
    • Intermediate sort results for operations such as creating or rebuilding indexes (if SORT_IN_TEMPDB is specified), or certain GROUP BY, ORDER BY, or UNION queries.

    Each internal object uses a minimum of nine pages: an IAM page and an eight-page extent. For more information about pages and extents, see Pages and extents.

  • Version stores, which are collections of data pages that hold the data rows that support row versioning. There are two types: a common version store and an online index build version store. The version stores contain:

    • Row versions that are generated by data modification transactions in a database that uses row versioning-based READ COMMITTED or SNAPSHOT isolation transactions.
    • Row versions that are generated by data modification transactions for features, such as online index operations, Multiple Active Result Sets (MARS), and AFTER triggers.

Operations within tempdb are minimally logged. tempdb is re-created every time the Database Engine is started so that the system always starts with an empty tempdb database. Temporary stored procedures and local temporary tables are dropped automatically when the session that created them disconnects.

tempdb never has anything to be saved from one uptime period of the Database Engine to another. Backup and restore operations are not allowed on tempdb.

Physical properties of tempdb in SQL Server

The following table lists the initial configuration values of the tempdb data and log files in SQL Server. The values are based on the defaults for the model database. The sizes of these files might vary slightly for different editions of SQL Server.

File Logical name Physical name Initial size File growth
Primary data tempdev tempdb.mdf 8 megabytes Autogrow by 64 MB until the disk is full
Secondary data files temp# tempdb_mssql_#.ndf 8 megabytes Autogrow by 64 MB until the disk is full
Log templog templog.ldf 8 megabytes Autogrow by 64 megabytes to a maximum of 2 terabytes

All tempdb data files should always have the same initial size and growth parameters.

Number of tempdb data files

Depending on the version of the Database Engine, its configuration, and the workload, tempdb might require multiple data files to mitigate allocation contention.

The recommended total number of data files depends on the number of logical processors on the machine. As general guidance:

  • If the number of logical processors is less than or equal to eight, use the same number of data files.
  • If the number of logical processors is greater than eight, use eight data files.
  • If tempdb allocation contention is still observed, increase the number of data files by multiples of four until the contention decreases to acceptable levels, or make changes to the workload.

For more information, see Recommendations to reduce allocation contention in SQL Server tempdb database.

To check current size and growth parameters for tempdb, use the sys.database_files catalog view in tempdb.

Move the tempdb data and log files in SQL Server

To move the tempdb data and log files, see Move system databases.

Database options for tempdb in SQL Server

The following table lists the default value for each database option in the tempdb database and whether the option can be modified. To view the current settings for these options, use the sys.databases catalog view.

Database option Default value Can be modified
ACCELERATED_DATABASE_RECOVERY OFF No
ALLOW_SNAPSHOT_ISOLATION OFF Yes
ANSI_NULL_DEFAULT OFF Yes
ANSI_NULLS OFF Yes
ANSI_PADDING OFF Yes
ANSI_WARNINGS OFF Yes
ARITHABORT OFF Yes
AUTO_CLOSE OFF No
AUTO_CREATE_STATISTICS ON Yes
AUTO_SHRINK OFF No
AUTO_UPDATE_STATISTICS ON Yes
AUTO_UPDATE_STATISTICS_ASYNC OFF Yes
AUTOMATIC_TUNING (FORCE_LAST_GOOD_PLAN) OFF No
CHANGE_TRACKING OFF No
COMPATIBILITY_LEVEL Depends on the Database Engine version.

For more information, see ALTER DATABASE (Transact-SQL) compatibility level.		 Yes
CONCAT_NULL_YIELDS_NULL OFF Yes
CONTAINMENT NONE No
CURSOR_CLOSE_ON_COMMIT OFF Yes
CURSOR_DEFAULT GLOBAL Yes
Database state ONLINE No
Database update READ_WRITE No
Database user access MULTI_USER No
DATE_CORRELATION_OPTIMIZATION OFF Yes
DB_CHAINING ON No
DELAYED_DURABILITY DISABLED

Regardless of this option, delayed durability is always enabled on tempdb.		 Yes
ENCRYPTION OFF No
MIXED_PAGE_ALLOCATION OFF No
NUMERIC_ROUNDABORT OFF Yes
PAGE_VERIFY CHECKSUM for new installations of SQL Server

Existing PAGE_VERIFY value might be retained when an instance of SQL Server is upgraded in place.		 Yes
PARAMETERIZATION SIMPLE Yes
QUOTED_IDENTIFIER OFF Yes
READ_COMMITTED_SNAPSHOT OFF No
RECOVERY SIMPLE No
RECURSIVE_TRIGGERS OFF Yes
Service Broker ENABLE_BROKER Yes
TARGET_RECOVERY_TIME 60 Yes
TEMPORAL_HISTORY_RETENTION ON Yes
TRUSTWORTHY OFF No

For a description of these database options, see ALTER DATABASE SET Options (Transact-SQL).

tempdb in Azure SQL Database

In Azure SQL Database, some aspects of tempdb behavior and configuration are different from SQL Server.

For single databases, each database on a logical server has its own tempdb. In an elastic pool, tempdb is a shared resource for all databases in the same pool but temporary objects created by one database are not visible to other databases in the same elastic pool.

Objects in tempdb, including catalog views and dynamic management views (DMVs), are accessible via a cross-database reference to the tempdb database. For example, you can query the sys.database_files view:

SELECT file_id,
       type_desc,
       name,
       size,
       max_size,
       growth
FROM tempdb.sys.database_files;

Global temporary tables in Azure SQL Database are database-scoped. For more information, see Database scoped global temporary tables in Azure SQL Database.

To learn more about tempdb sizes in Azure SQL Database, review:

tempdb in SQL Managed Instance

In Azure SQL Managed Instance, some aspects of tempdb behavior and default configuration are different from SQL Server.

You can configure the number of tempdb files, their growth increments, and their maximum size. For more information on configuring tempdb settings in Azure SQL Managed Instance, see Configure tempdb settings for Azure SQL Managed Instance.

Azure SQL Managed Instance supports temporary objects in the same way as SQL Server, where all global temporary tables and global temporary stored procedures are accessible by all user sessions within the same SQL managed instance.

To learn more about tempdb sizes in Azure SQL Managed Instance, review resource limits.

tempdb in SQL database in Fabric

To learn more about tempdb sizes in SQL database in Microsoft Fabric, review the resource limits section in Feature comparison: Azure SQL Database and SQL database in Microsoft Fabric.

Similarly to Azure SQL Database, global temporary tables in SQL database in Microsoft Fabric are database-scoped. For more information, see Database scoped global temporary tables in Azure SQL Database.

Restrictions

The following operations can't be performed on the tempdb database:

  • Adding filegroups.
  • Backing up or restoring the database.
  • Changing collation. The default collation is the server collation.
  • Changing the database owner. tempdb is owned by sa.
  • Creating a database snapshot.
  • Dropping the database.
  • Dropping the guest user from the database.
  • Enabling Change Data Capture.
  • Participating in database mirroring.
  • Removing the primary filegroup, primary data file, or log file.
  • Renaming the database or primary filegroup.
  • Running DBCC CHECKALLOC.
  • Running DBCC CHECKCATALOG.
  • Setting the database to OFFLINE.
  • Setting the database or primary filegroup to READ_ONLY.

Permissions

Any user can create temporary objects in tempdb.

Users can access only their own non-temporary objects in tempdb, unless they receive additional permissions.

It's possible to revoke the CONNECT permission on tempdb to prevent a database user or role from using tempdb. This isn't recommended because many operations require the use of tempdb.

Optimize tempdb performance in SQL Server

The size and physical placement of tempdb files can affect performance. For example, if the initial size of tempdb is too small, time and resources might be taken up to autogrow tempdb to the size required to support the workload every time the Database Engine instance is restarted.

  • If possible, use instant file initialization to improve performance of the growth operations for data files.
  • Preallocate space for all tempdb files by setting the file size to a value large enough to accommodate the typical workload in the environment. Preallocation prevents tempdb from autogrowing too often, which can negatively affect performance.
  • The files in the tempdb database should be set to autogrow to provide space during unplanned growth events.
  • Dividing tempdb into multiple data files of equal size can improve efficiency of operations that use tempdb.
    • To avoid data allocation imbalance, data files should have the same initial size and growth parameters because the Database Engine uses a proportional-fill algorithm that favors allocations in files with more free space.
    • Set the file growth increment to a reasonable size, for example 64 MB, and make the growth increment the same for all data files to prevent growth imbalance.

To check current size and growth parameters for tempdb, use the following query:

SELECT name AS file_name,
       type_desc AS file_type,
       size * 8.0 / 1024 AS size_mb,
       max_size * 8.0 / 1024 AS max_size_mb,
       CAST(IIF(max_size = 0, 0, 1) AS bit) AS is_autogrowth_enabled,
       CASE WHEN growth = 0 THEN growth
            WHEN growth > 0 AND is_percent_growth = 0 THEN growth * 8.0 / 1024
            WHEN growth > 0 AND is_percent_growth = 1 THEN growth
       END
       AS growth_increment_value,
       CASE WHEN growth = 0 THEN 'Autogrowth is disabled.'
            WHEN growth > 0 AND is_percent_growth = 0  THEN 'Megabytes'
            WHEN growth > 0 AND is_percent_growth = 1  THEN 'Percent'
       END
       AS growth_increment_value_unit
FROM tempdb.sys.database_files;

Put the tempdb database on a fast I/O subsystem. Individual data files or groups of tempdb data files don't necessarily need to be on different disks unless you're encountering disk-level I/O bottlenecks.

If there is I/O contention between tempdb and user databases, put tempdb files on disks that differ from the disks that user databases use.

Note

To improve performance, delayed durability is always enabled on tempdb even if the database option DELAYED_DURABILITY is set to DISABLED. Because tempdb is recreated at startup, it doesn't go through a recovery process and doesn't provide a durability guarantee.

Performance improvements in tempdb for SQL Server

Introduced in SQL Server 2016 (13.x)

  • Temporary tables and table variables are cached. Caching allows operations that drop and create the temporary objects to run very quickly. Caching also reduces page allocation and metadata contention.
  • The allocation page latching protocol is improved to reduce the number of UP (update) latches that are used.
  • Logging overhead for tempdb is reduced to reduce disk I/O bandwidth consumption on the tempdb log file.
  • SQL Setup adds multiple tempdb data files during a new instance installation. Review the recommendations and configure your tempdb in the Database Engine Configuration page of SQL Setup, or use the command-line parameter /SQLTEMPDBFILECOUNT. By default, SQL Setup adds as many tempdb data files as the number of logical processors or eight, whichever is lower.
  • When there are multiple tempdb data files, all files autogrow at the same time and by the same amount, depending on growth settings. Trace flag 1117 is no longer required. For more information, read -T1117 and -T1118 changes for TEMPDB and user databases.
  • All allocations in tempdb use uniform extents. Trace flag 1118 is no longer required. For more information on performance improvements in tempdb, see the blog article TEMPDB - Files and Trace Flags and Updates, Oh My!.
  • The AUTOGROW_ALL_FILES property is always turned on for the PRIMARY filegroup.

Introduced in SQL Server 2017 (14.x)

  • The SQL Setup experience improves guidance for initial tempdb file allocation. SQL Setup warns customers if the initial file size is set to a value greater than 1 GB and if instant file initialization is not enabled, preventing instance startup delays.
  • The sys.dm_tran_version_store_space_usage dynamic management view tracks version store usage per database. This DMV is useful for DBAs who want to proactively plan tempdb sizing based on the version store usage requirement per database.
  • Intelligent query processing features such as adaptive joins and memory grant feedback reduce memory spills on consecutive executions of a query, reducing tempdb utilization.

Introduced in SQL Server 2019 (15.x)

  • Database Engine doesn't use the FILE_FLAG_WRITE_THROUGH option when opening tempdb files to allow for maximum disk throughput. Since tempdb is recreated on startup, this option isn't needed to provide data durability. For more information on FILE_FLAG_WRITE_THROUGH, see Logging and data storage algorithms that extend data reliability in SQL Server.
  • Memory-optimized TempDB metadata removes temporary object metadata contention in tempdb.
  • Concurrent Page Free Space (PFS) page updates reduce page latch contention in all databases, an issue most commonly seen in tempdb. This improvement changes the concurrency management of PFS page updates so that they can be updated under a shared latch, rather than an exclusive latch. This behavior is on by default in all databases (including tempdb) starting with SQL Server 2019 (15.x). For more information on PFS pages, read Under the covers: GAM, SGAM, and PFS pages.
  • By default, a new installation of SQL Server on Linux creates multiple tempdb data files, based on the number of logical cores (with up to eight data files). This doesn't apply to in-place minor or major version upgrades. Each tempdb data file is 8 MB, with an auto growth of 64 MB. This behavior is similar to the default SQL Server installation on Windows.

Introduced in SQL Server 2022 (16.x)

Memory-optimized TempDB metadata

Temporary object metadata contention has historically been a bottleneck to scalability for many SQL Server workloads. To address that, SQL Server 2019 (15.x) introduced a feature that's part of the in-memory database feature family: Memory-optimized TempDB metadata.

Enabling the Memory-optimized TempDB metadata feature removes this bottleneck for workloads previously limited by temporary object metadata contention inside tempdb. Starting with SQL Server 2019 (15.x), the system tables involved in managing temporary object metadata can become latch-free, non-durable, memory-optimized tables.

Tip

Because of current limitations, we recommend enabling Memory-optimized TempDB metadata only when object metadata contention occurs and significantly impacts your workloads.

The following diagnostic query returns one or more rows if temporary object metadata contention is occurring. Each row represents a system table, and returns the number of sessions contending for access to that table at the time when this diagnostic query is executed.

SELECT OBJECT_NAME(dpi.object_id, dpi.database_id) AS system_table_name,
       COUNT(DISTINCT(r.session_id)) AS session_count
FROM sys.dm_exec_requests AS r
CROSS APPLY sys.fn_PageResCracker(r.page_resource) AS prc
CROSS APPLY sys.dm_db_page_info(prc.db_id, prc.file_id, prc.page_id, 'LIMITED') AS dpi
WHERE dpi.database_id = 2
      AND dpi.object_id IN (3, 9, 34, 40, 41, 54, 55, 60, 74, 75)
      AND UPPER(r.wait_type) LIKE N'PAGELATCH[_]%'
GROUP BY dpi.object_id, dpi.database_id;

Watch this seven-minute video for an overview of how and when to use Memory-optimized TempDB metadata feature:

Note

Currently, the Memory-optimized TempDB metadata feature is not available in Azure SQL Database, SQL database in Microsoft Fabric, and Azure SQL Managed Instance.

Configure and use Memory-optimized TempDB metadata

The following sections include steps to enable, configure, verify, and disable the Memory-optimized TempDB metadata feature.

Enable

To enable this feature, use the following script:

ALTER SERVER CONFIGURATION SET MEMORY_OPTIMIZED TEMPDB_METADATA = ON;

For more information, see ALTER SERVER. This configuration change requires a restart of the service to take effect.

You can verify whether or not tempdb is memory-optimized by using the following T-SQL command:

SELECT SERVERPROPERTY('IsTempdbMetadataMemoryOptimized');

If the returned value is 1 and a restart has occurred after enabling the feature, then the feature is enabled.

If the server fails to start for any reason after you enable Memory-optimized TempDB metadata, you can bypass the feature by starting the Database Engine instance with minimal configuration using the -f startup option. You can then disable the feature and remove the -f option to restart the Database Engine in normal mode.

Bind to resource pool to limit memory usage

To protect the server from potential out-of-memory conditions, we recommend that you bind tempdb to a resource governor resource pool that limits the memory consumed by Memory-optimized TempDB metadata. The following sample script creates a resource pool and sets its maximum memory to 20%, enables resource governor, and binds tempdb to the resource pool.

This example uses 20% as the memory limit for demonstration purposes. The optimal value in your environment might be larger or smaller depending on your workload, and can change over time if the workload changes.

CREATE RESOURCE POOL tempdb_resource_pool
WITH (MAX_MEMORY_PERCENT = 20);

ALTER RESOURCE GOVERNOR RECONFIGURE;

ALTER SERVER CONFIGURATION 
SET MEMORY_OPTIMIZED TEMPDB_METADATA = ON 
(RESOURCE_POOL = 'tempdb_resource_pool');

This change also requires a service restart to take effect, even if Memory-optimized TempDB metadata is already enabled.

Verify resource pool binding and monitor memory usage

To verify that tempdb is bound to a resource pool and to monitor memory usage statistics for the pool, use the following query:

WITH resource_pool AS
(
SELECT p.pool_id,
       p.name,
       p.max_memory_percent,
       dp.max_memory_kb,
       dp.target_memory_kb,
       dp.used_memory_kb,
       dp.out_of_memory_count
FROM sys.resource_governor_resource_pools AS p
INNER JOIN sys.dm_resource_governor_resource_pools AS dp
ON p.pool_id = dp.pool_id
)
SELECT SERVERPROPERTY('IsTempdbMetadataMemoryOptimized') AS is_tempdb_memory_optimized_metadata_enabled,
       rp.name AS resource_pool_name,
       rp.max_memory_percent,
       rp.max_memory_kb,
       rp.target_memory_kb,
       rp.used_memory_kb,
       rp.out_of_memory_count
FROM sys.databases AS d
LEFT JOIN resource_pool AS rp
ON d.resource_pool_id = rp.pool_id
WHERE d.name = 'tempdb';

Remove resource pool binding

To remove the resource pool binding while keeping Memory-optimized TempDB metadata enabled, execute the following command and restart the service:

ALTER SERVER CONFIGURATION 
SET MEMORY_OPTIMIZED TEMPDB_METADATA = ON;

Disable

To disable Memory-optimized TempDB metadata, execute the following command and restart the service:

ALTER SERVER CONFIGURATION 
SET MEMORY_OPTIMIZED TEMPDB_METADATA = OFF;

Limitations of Memory-optimized TempDB metadata

  • Enabling or disabling the Memory-optimized TempDB metadata feature requires a restart.

  • In certain cases, you might observe high memory usage by the MEMORYCLERK_XTP memory clerk causing out-of-memory errors in your workload.

    To see memory usage by the MEMORYCLERK_XTP clerk relative to all other memory clerks and relative to the target server memory, execute the following query:

    SELECT SUM(IIF(type = 'MEMORYCLERK_XTP', pages_kb, NULL)) / 1024. AS memoryclerk_xtp_pages_mb,
       SUM(IIF(type <> 'MEMORYCLERK_XTP', pages_kb, NULL)) / 1024. AS other_pages_mb,
       SUM(committed_target_kb) / 1024. AS committed_target_memory_mb
FROM sys.dm_os_memory_clerks
CROSS JOIN sys.dm_os_sys_info;

    If MEMORYCLERK_XTP memory is high, you can mitigate the problem as follows:

    For more information, see memory-optimized tempdb metadata (HkTempDB) out of memory errors.

  • When you use In-Memory OLTP, a single transaction is not allowed to access memory-optimized tables in more than one database. Because of this, any transaction that involves a memory-optimized table in a user database can't also access tempdb system views in the same transaction. If this occurs, you receive the following error:

    A user transaction that accesses memory optimized tables or natively compiled modules cannot access more than one user database or databases model and msdb, and it cannot write to master.

    The following example script produces this error when Memory-optimized TempDB metadata is enabled:

    BEGIN TRAN;

-- Create an In-memory OLTP transaction that accesses a system view in tempdb
SELECT name
FROM tempdb.sys.tables;

-- An attempt to create an In-memory OLTP transaction in the user database fails
INSERT INTO <user database>.<schema>.<memory-optimized table>
VALUES (1);

COMMIT TRAN;

    Note

    This limitation does not apply to temporary tables. You can create a temporary table in the same transaction that accesses a memory-optimized table in a user database.

  • Queries against system catalog views always use the READ COMMITTED isolation level. When the Memory-optimized TempDB metadata is enabled, queries against system catalog views in tempdb use the SNAPSHOT isolation level. In either case, locking hints are not honored.

  • Columnstore indexes can't be created on temporary tables when Memory-optimized TempDB metadata is enabled.

    • As a consequence, the use of the sp_estimate_data_compression_savings system stored procedure with the COLUMNSTORE or COLUMNSTORE_ARCHIVE data compression parameter is not supported when Memory-optimized TempDB metadata is enabled.

Capacity planning for tempdb in SQL Server

Determining the appropriate size for tempdb depends on many factors. These factors include the workload and the Database Engine features that are used.

We recommend that you analyze tempdb space consumption by performing the following tasks in a test environment where you can reproduce your typical workload:

  • Enable autogrow for tempdb files. All tempdb data files should have the same initial size and autogrow configuration.
  • Reproduce the workload and monitor tempdb space use.
  • If you use periodic index maintenance, execute your maintenance jobs and monitor tempdb space.
  • Use the maximum space used values from the previous steps to predict your total workload usage. Adjust this value for projected concurrent activity, and then set the size of tempdb accordingly.

Monitor tempdb use

Running out of disk space in tempdb can cause significant disruptions and application downtime. You can use the sys.dm_db_file_space_usage dynamic management view to monitor the space used in the tempdb files.

For example, the following example script finds:

  • Free space in tempdb (not considering free disk space that might be available for tempdb growth)
  • Space used by the version store
  • Space used by internal objects
  • Space used by user objects
SELECT SUM(unallocated_extent_page_count) * 8.0 / 1024 AS tempdb_free_data_space_mb,
       SUM(version_store_reserved_page_count) * 8.0 / 1024 AS tempdb_version_store_space_mb,
       SUM(internal_object_reserved_page_count) * 8.0 / 1024 AS tempdb_internal_object_space_mb,
       SUM(user_object_reserved_page_count) * 8.0 / 1024 AS tempdb_user_object_space_mb
FROM tempdb.sys.dm_db_file_space_usage;

To monitor page allocation or deallocation activity in tempdb at the session or task level, you can use the sys.dm_db_session_space_usage and sys.dm_db_task_space_usage dynamic management views. These views can help you identify queries, temporary tables, or table variables that are using large amounts of tempdb space.

For example, use the following example script to obtain the tempdb space allocated and deallocated by internal objects in all currently running tasks in each session:

SELECT session_id,
       SUM(internal_objects_alloc_page_count) AS task_internal_objects_alloc_page_count,
       SUM(internal_objects_dealloc_page_count) AS task_internal_objects_dealloc_page_count
FROM sys.dm_db_task_space_usage
GROUP BY session_id;

Use the following example script to find the tempdb allocated and currently consumed space by internal and user objects for each session and request, for both running and completed tasks:

WITH tempdb_space_usage AS
(
SELECT session_id,
       request_id,
       user_objects_alloc_page_count + internal_objects_alloc_page_count AS tempdb_allocations_page_count,
       user_objects_alloc_page_count + internal_objects_alloc_page_count - user_objects_dealloc_page_count - internal_objects_dealloc_page_count AS tempdb_current_page_count
FROM sys.dm_db_task_space_usage
UNION ALL
SELECT session_id,
       NULL AS request_id,
       user_objects_alloc_page_count + internal_objects_alloc_page_count AS tempdb_allocations_page_count,
       user_objects_alloc_page_count + internal_objects_alloc_page_count - user_objects_dealloc_page_count - user_objects_deferred_dealloc_page_count - internal_objects_dealloc_page_count AS tempdb_current_page_count
FROM sys.dm_db_session_space_usage
)
SELECT session_id,
       COALESCE(request_id, 0) AS request_id,
       SUM(tempdb_allocations_page_count * 8) AS tempdb_allocations_kb,
       SUM(IIF (tempdb_current_page_count >= 0, tempdb_current_page_count, 0) * 8) AS tempdb_current_kb
FROM tempdb_space_usage
GROUP BY session_id, COALESCE (request_id, 0)
ORDER BY session_id, request_id;

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