sys.dm_os_latch_stats (Transact-SQL)

THIS TOPIC APPLIES TO:yesSQL Server (starting with 2008)noAzure SQL DatabasenoAzure SQL Data Warehouse noParallel Data Warehouse

Returns information about all latch waits organized by class.

Note

To call this from Azure SQL Data Warehouse or Parallel Data Warehouse, use the name sys.dm_pdw_nodes_os_latch_stats.

Column name Data type Description
latch_class nvarchar(120) Name of the latch class.
waiting_requests_count bigint Number of waits on latches in this class. This counter is incremented at the start of a latch wait.
wait_time_ms bigint Total wait time, in milliseconds, on latches in this class.

Note: This column is updated every five minutes during a latch wait and at the end of a latch wait.
max_wait_time_ms bigint Maximum time a memory object has waited on this latch. If this value is unusually high, it might indicate an internal deadlock.
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_latch_stats can be used to identify the source of latch contention by examining the relative wait numbers and wait times for the different latch classes. In some situations, you may be able to resolve or reduce latch contention. However, there might be situations that will require that you to contact Microsoft Customer Support Services.

You can reset the contents of sys.dm_os_latch_stats by using DBCC SQLPERF as follows:

DBCC SQLPERF ('sys.dm_os_latch_stats', CLEAR);  
GO  

This resets all counters to 0.

Note

These statistics are not persisted if SQL Server is restarted. All data is cumulative since the last time the statistics were reset, or since SQL Server was started.

Latches

A latch is a lightweight synchronization object that is used by various SQL Server components. A latch is primarily used to synchronize database pages. Each latch is associated with a single allocation unit.

A latch wait occurs when a latch request cannot be granted immediately, because the latch is held by another thread in a conflicting mode. Unlike locks, a latch is released immediately after the operation, even in write operations.

Latches are grouped into classes based on components and usage. Zero or more latches of a particular class can exist at any point in time in an instance of SQL Server.

Note

sys.dm_os_latch_stats does not track latch requests that were granted immediately, or that failed without waiting.

The following table contains brief descriptions of the various latch classes.

Latch class Description
ALLOC_CREATE_RINGBUF Used internally by SQL Server to initialize the synchronization of the creation of an allocation ring buffer.
ALLOC_CREATE_FREESPACE_CACHE Used to initialize the synchronization of internal freespace caches for heaps.
ALLOC_CACHE_MANAGER Used to synchronize internal coherency tests.
ALLOC_FREESPACE_CACHE Used to synchronize the access to a cache of pages with available space for heaps and binary large objects (BLOBs). Contention on latches of this class can occur when multiple connections try to insert rows into a heap or BLOB at the same time. You can reduce this contention by partitioning the object. Each partition has its own latch. Partitioning will distribute the inserts across multiple latches.
ALLOC_EXTENT_CACHE Used to synchronize the access to a cache of extents that contains pages that are not allocated. Contention on latches of this class can occur when multiple connections try to allocate data pages in the same allocation unit at the same time. This contention can be reduced by partitioning the object of which this allocation unit is a part.
ACCESS_METHODS_DATASET_PARENT Used to synchronize child dataset access to the parent dataset during parallel operations.
ACCESS_METHODS_HOBT_FACTORY Used to synchronize access to an internal hash table.
ACCESS_METHODS_HOBT Used to synchronize access to the in-memory representation of a HoBt.
ACCESS_METHODS_HOBT_COUNT Used to synchronize access to a HoBt page and row counters.
ACCESS_METHODS_HOBT_VIRTUAL_ROOT Used to synchronize access to the root page abstraction of an internal B-tree.
ACCESS_METHODS_CACHE_ONLY_HOBT_ALLOC Used to synchronize worktable access.
ACCESS_METHODS_BULK_ALLOC Used to synchronize access within bulk allocators.
ACCESS_METHODS_SCAN_RANGE_GENERATOR Used to synchronize access to a range generator during parallel scans.
ACCESS_METHODS_KEY_RANGE_GENERATOR Used to synchronize access to read-ahead operations during key range parallel scans.
APPEND_ONLY_STORAGE_INSERT_POINT Used to synchronize inserts in fast append-only storage units.
APPEND_ONLY_STORAGE_FIRST_ALLOC Used to synchronize the first allocation for an append-only storage unit.
APPEND_ONLY_STORAGE_UNIT_MANAGER Used for internal data structure access synchronization within the fast append-only storage unit manager.
APPEND_ONLY_STORAGE_MANAGER Used to synchronize shrink operations in the fast append-only storage unit manager.
BACKUP_RESULT_SET Used to synchronize parallel backup result sets.
BACKUP_TAPE_POOL Used to synchronize backup tape pools.
BACKUP_LOG_REDO Used to synchronize backup log redo operations.
BACKUP_INSTANCE_ID Used to synchronize the generation of instance IDs for backup performance monitor counters.
BACKUP_MANAGER Used to synchronize the internal backup manager.
BACKUP_MANAGER_DIFFERENTIAL Used to synchronize differential backup operations with DBCC.
BACKUP_OPERATION Used for internal data structure synchronization within a backup operation, such as database, log, or file backup.
BACKUP_FILE_HANDLE Used to synchronize file open operations during a restore operation.
BUFFER Used to synchronize short term access to database pages. A buffer latch is required before reading or modifying any database page. Buffer latch contention can indicate several issues, including hot pages and slow I/Os.

This latch class covers all possible uses of page latches. sys.dm_os_wait_stats makes a difference between page latch waits that are caused by I/O operations and read and write operations on the page.
BUFFER_POOL_GROW Used for internal buffer manager synchronization during buffer pool grow operations.
DATABASE_CHECKPOINT Used to serialize checkpoints within a database.
CLR_PROCEDURE_HASHTABLE Internal use only.
CLR_UDX_STORE Internal use only.
CLR_DATAT_ACCESS Internal use only.
CLR_XVAR_PROXY_LIST Internal use only.
DBCC_CHECK_AGGREGATE Internal use only.
DBCC_CHECK_RESULTSET Internal use only.
DBCC_CHECK_TABLE Internal use only.
DBCC_CHECK_TABLE_INIT Internal use only.
DBCC_CHECK_TRACE_LIST Internal use only.
DBCC_FILE_CHECK_OBJECT Internal use only.
DBCC_PERF Used to synchronize internal performance monitor counters.
DBCC_PFS_STATUS Internal use only.
DBCC_OBJECT_METADATA Internal use only.
DBCC_HASH_DLL Internal use only.
EVENTING_CACHE Internal use only.
FCB Used to synchronize access to the file control block.
FCB_REPLICA Internal use only.
FGCB_ALLOC Use to synchronize access to round robin allocation information within a filegroup.
FGCB_ADD_REMOVE Use to synchronize access to filegroups for ADD and DROP file operations.
FILEGROUP_MANAGER Internal use only.
FILE_MANAGER Internal use only.
FILESTREAM_FCB Internal use only.
FILESTREAM_FILE_MANAGER Internal use only.
FILESTREAM_GHOST_FILES Internal use only.
FILESTREAM_DFS_ROOT Internal use only.
LOG_MANAGER Internal use only.
FULLTEXT_DOCUMENT_ID Internal use only.
FULLTEXT_DOCUMENT_ID_TRANSACTION Internal use only.
FULLTEXT_DOCUMENT_ID_NOTIFY Internal use only.
FULLTEXT_LOGS Internal use only.
FULLTEXT_CRAWL_LOG Internal use only.
FULLTEXT_ADMIN Internal use only.
FULLTEXT_AMDIN_COMMAND_CACHE Internal use only.
FULLTEXT_LANGUAGE_TABLE Internal use only.
FULLTEXT_CRAWL_DM_LIST Internal use only.
FULLTEXT_CRAWL_CATALOG Internal use only.
FULLTEXT_FILE_MANAGER Internal use only.
DATABASE_MIRRORING_REDO Internal use only.
DATABASE_MIRRORING_SERVER Internal use only.
DATABASE_MIRRORING_CONNECTION Internal use only.
DATABASE_MIRRORING_STREAM Internal use only.
QUERY_OPTIMIZER_VD_MANAGER Internal use only.
QUERY_OPTIMIZER_ID_MANAGER Internal use only.
QUERY_OPTIMIZER_VIEW_REP Internal use only.
RECOVERY_BAD_PAGE_TABLE Internal use only.
RECOVERY_MANAGER Internal use only.
SECURITY_OPERATION_RULE_TABLE Internal use only.
SECURITY_OBJPERM_CACHE Internal use only.
SECURITY_CRYPTO Internal use only.
SECURITY_KEY_RING Internal use only.
SECURITY_KEY_LIST Internal use only.
SERVICE_BROKER_CONNECTION_RECEIVE Internal use only.
SERVICE_BROKER_TRANSMISSION Internal use only.
SERVICE_BROKER_TRANSMISSION_UPDATE Internal use only.
SERVICE_BROKER_TRANSMISSION_STATE Internal use only.
SERVICE_BROKER_TRANSMISSION_ERRORS Internal use only.
SSBXmitWork Internal use only.
SERVICE_BROKER_MESSAGE_TRANSMISSION Internal use only.
SERVICE_BROKER_MAP_MANAGER Internal use only.
SERVICE_BROKER_HOST_NAME Internal use only.
SERVICE_BROKER_READ_CACHE Internal use only.
SERVICE_BROKER_WAITFOR_MANAGER Used to synchronize an instance level map of waiter queues. One queue exists per database ID, Database Version, and Queue ID tuple. Contention on latches of this class can occur when many connections are: In a WAITFOR(RECEIVE) wait state; calling WAITFOR(RECEIVE); exceeding the WAITFOR timeout; receiving a message; committing or rolling back the transaction that contains the WAITFOR(RECEIVE); You can reduce the contention by reducing the number of threads in a WAITFOR(RECEIVE) wait state.
SERVICE_BROKER_WAITFOR_TRANSACTION_DATA Internal use only.
SERVICE_BROKER_TRANSMISSION_TRANSACTION_DATA Internal use only.
SERVICE_BROKER_TRANSPORT Internal use only.
SERVICE_BROKER_MIRROR_ROUTE Internal use only.
TRACE_ID Internal use only.
TRACE_AUDIT_ID Internal use only.
TRACE Internal use only.
TRACE_CONTROLLER Internal use only.
TRACE_EVENT_QUEUE Internal use only.
TRANSACTION_DISTRIBUTED_MARK Internal use only.
TRANSACTION_OUTCOME Internal use only.
NESTING_TRANSACTION_READONLY Internal use only.
NESTING_TRANSACTION_FULL Internal use only.
MSQL_TRANSACTION_MANAGER Internal use only.
DATABASE_AUTONAME_MANAGER Internal use only.
UTILITY_DYNAMIC_VECTOR Internal use only.
UTILITY_SPARSE_BITMAP Internal use only.
UTILITY_DATABASE_DROP Internal use only.
UTILITY_DYNAMIC_MANAGER_VIEW Internal use only.
UTILITY_DEBUG_FILESTREAM Internal use only.
UTILITY_LOCK_INFORMATION Internal use only.
VERSIONING_TRANSACTION Internal use only.
VERSIONING_TRANSACTION_LIST Internal use only.
VERSIONING_TRANSACTION_CHAIN Internal use only.
VERSIONING_STATE Internal use only.
VERSIONING_STATE_CHANGE Internal use only.
KTM_VIRTUAL_CLOCK Internal use only.

See Also

DBCC SQLPERF (Transact-SQL)

SQL Server Operating System Related Dynamic Management Views (Transact-SQL)