SQL Server Wait Types - Essential Insights for Troubleshooting

Appendix: SQL Server Wait Types - Essential Insights for Troubleshooting

This appendix provides a list of the most common wait types you'll encounter during SQL Server troubleshooting. It outlines the conditions under which these waits may occur and offers tips on high-level troubleshooting strategies. This appendix should serve as a handy reference, complementing Microsoft's documentation and the SQLSkills Wait Types Library.

ASYNC_IO_COMPLETION

- Occurs during asynchronous I/O operations not related to the buffer pool, such as regular checkpoints, internal checkpoints during database backups, and reading data pages.

• Troubleshooting : A noticeable percentage of these waits may indicate an overloaded I/O subsystem. Perform I/O system troubleshooting (see PAGEIOLATCH waits for additional details).

ASYNC_NETWORK_IO

- Occurs when SQL Server waits for a client to consume data. Triggered by slow networks, under-provisioned VMs, or incorrect application design.

- Troubleshooting : Check network performance and client application design. Use the `sys.dm_os_waiting_tasks`, `sys.dm_exec_requests`, `sys.dm_exec_sessions`, and `sys.dm_exec_connections` views to detect problematic client applications.

BACKUPIO and BACKUPBUFFER

- Indicate insufficient backup/restore throughput due to slow or overloaded network/disk subsystems.

- Troubleshooting : Analyze and optimize backup process performance. Consider backup compression, stripe backups, and tuning backup options.

BTREE_INSERT_FLOW_CONTROL

- Indicates the existence of indexes with ever-increasing keys causing hotspots.

- **Troubleshooting**: Address hotspots by checking PAGELATCH waits and enabling the `OPTIMIZE_FOR_SEQUENTIAL_KEY` index option.

CXPACKET, CXCONSUMER, and EXCHANGE

- Occur during the execution of parallel execution plans. These waits are normal in data warehouse/reporting workloads but may indicate expensive queries in OLTP systems.

- Troubleshooting Optimize queries and tune parallelism settings. Do not set MAXDOP=1 as it hides the problem.

DIRTY_PAGE_TABLE_LOCK, DPT_ENTRY_LOCK, PARALLEL_REDO_FLOW_CONTROL, and PARALLEL_REDO_TRAN_TURN

- Occur on readable secondary nodes in AlwaysOn Availability Groups, indicating issues with the parallel redo process.

- Troubleshooting : Monitor for elevated CPU load and redo queue growth. Mitigate issues by patching SQL Server or disabling parallel redo with the T3459 trace flag.

HADR_GROUP_COMMIT

- Description : Occurs when the primary AlwaysOn Availability Group node tries to optimize replication performance by grouping multiple commit log records together before sending them to secondary nodes.

- Troubleshooting : This wait type is rarely problematic. If it becomes significant and impacts transaction throughput, troubleshoot AlwaysOn replication performance (refer to the HADR_SYNC_COMMIT wait). You can disable group commit with the T9546 trace flag for further mitigation.

HADR_SYNC_COMMIT

- Description : Occurs when the primary AlwaysOn Availability Group node waits for synchronous secondaries to harden transaction log records. This wait type is always present in Availability Groups using synchronous replication.

- Troubleshooting : Analyze the average wait time, which should be minimal and not exceed a few milliseconds when synchronous replicas are in the same datacenter as the primary node. If wait times are larger, investigate replication performance by examining network latency, throughput, disk performance, and the load on synchronous replicas. High HADR_SYNC_COMMIT wait times may contribute to system blocking, as transactions remain active with all locks held until the primary node receives confirmation that the COMMIT t-log record is hardened on synchronous secondaries.

HTBUILD, HTDELETE, HTMEMO, HTREINIT, and HTREPARTITION

- Description : These waits occur during the management of internal hash tables in batch-mode execution, particularly in environments with columnstore indexes. They may indicate poorly maintained columnstore indexes with large delta stores or unevenly sized rowgroups.

- Troubleshooting : Analyze rowgroup states with the `sys.column_store_row_group` view and rebuild indexes as needed.

O_COMPLETION

- Description : This wait type occurs during synchronous reads and writes to data files, as well as certain read operations in the transaction log. Examples include reading allocation map pages, reading the transaction log during recovery, and writing to tempdb during sort spills.

- Troubleshooting : High IO_COMPLETION waits may indicate an overloaded I/O subsystem. Perform I/O system troubleshooting, paying close attention to tempdb latency and throughput.

LATCH_

- Description : These waits are triggered by latches not related to the buffer pool. SQL Server generates different LATCH waits depending on the latch type (shared, exclusive, etc.).

- Troubleshooting : Use the `sys.dm_os_latch_stats` view to get latch statistics and analyze potential bottlenecks.

LCK_M_

- Description : Waits starting with LCK_M_ occur during blocking. Each lock type in SQL Server has a corresponding wait type.

- Troubleshooting Analyze the specific lock type and address the root cause of the blocking. This appendix provides additional details for specific LCK_M_ wait types.

LCK_M_I

- Description : Intent locks (LCK_M_I*) are acquired at the object (table) and page levels, usually due to incompatibility with schema modification locks or full incompatible table-level locks held by other sessions.

- Troubleshooting : Address lock escalations and optimize queries. Use tools like the Blocking Monitoring Framework and `sys.dm_index_operational_stats` view.

LCK_M_R

- Description : Indicates waits for range locks, typically acquired in the SERIALIZABLE isolation level or nonclustered indexes with the IGNORE_DUP_KEY=ON option.

- **Troubleshooting**: Avoid using IGNORE_DUP_KEY=ON and the SERIALIZABLE isolation level unless necessary.

LCK_M_S

- Description : Indicates waits for shared (S) locks, acquired by SELECT queries in various isolation levels.

- Troubleshooting : Focus on query optimization and consider enabling READ_COMMITTED_SNAPSHOT to eliminate reader/writer blocking.

LCK_M_SCH_M

- Description : Indicates waits for schema modification (Sch-M) locks when sessions cannot obtain exclusive locks.

- Troubleshooting : Evaluate deployment strategies for database changes, index, and partition maintenance. Consider using low-priority locks for index rebuilds and partition switches.

LCK_M_SCH_S

- Description : Indicates waits for schema stability (Sch-S) locks during schema modifications.

- Troubleshooting : Follow strategies outlined for LCK_M_SCH_M waits.

LCK_M_U

- Description : Indicates a wait for update (U) locks, typically acquired during update scans caused by nonoptimized writer queries (UPDATE, DELETE, MERGE).

- **Troubleshooting**: Focus on query optimization to reduce these waits, as they often appear alongside PAGEIOLATCH* and CXPACKET waits. See Chapter 5 and Chapter 8 for detailed strategies.

LCK_M_X

- Description : Indicates waits for exclusive (X) locks, commonly due to artificial serialization points (e.g., counters table), excessive usage of REPEATABLE READ and SERIALIZABLE isolation levels, inefficient transaction management, long-running transactions, and table-level locking hints like (TABLOCKX).

- **Troubleshooting**: Analyze individual blocking cases to identify and address root causes. Refer to Chapter 8 for more details.

LOGBUFFER

- Description : Occurs when SQL Server waits for an available log buffer to write log records, often accompanied by WRITELOG waits, indicating insufficient transaction log throughput.

- Troubleshooting : Follow WRITELOG troubleshooting steps to improve transaction log throughput. Refer to Chapters 3 and 11 for additional information.

OLEDB

- Description : Occurs when SQL Server waits for data from an OLE DB provider, commonly during calls to linked servers, execution of SSIS packages, operations during DBCC CHECKDB, and queries against DMVs.

- Troubleshooting : Identify what triggered these waits and evaluate their impact. See Chapter 13 for detailed troubleshooting.

PAGEIOLATCH\

- Description : Indicates waits when SQL Server reads data pages from disk, common in most systems and associated with various operations with buffer pool pages.

- Troubleshooting : Review disk subsystem performance and latency with `sys.dm_io_virtual_file_stats`, analyze SQL Server and OS performance counters, tune the checkpoint process, and optimize I/O-intensive queries. Examine index metrics to identify the largest buffer pool consumers. Refer to Chapters 3, 4, 5, 14, and 15 for comprehensive troubleshooting.

PAGELATCH

- Description : Indicates latches related to the buffer pool, occurring when threads need to simultaneously access or modify data and allocation map pages. Common causes include contention in tempdb system pages and hotspots in ever-increasing indexes.

- Troubleshooting : Identify triggers by analyzing the `wait_resource` column in the `sys.dm_os_waiting_tasks` view or capturing the `sqlserver.latch_suspend_end` xEvent. Ensure tempdb is configured correctly, reduce tempdb load, and consider enabling memory-optimized tempdb metadata. Use the `sys.dm_db_index_operational_stats` function to identify indexes with hotspots. Enable the `OPTIMIZE_FOR_SEQUENTIAL_KEY` index setting in SQL Server 2019 and later.

PARALLEL_REDO_FLOW_CONTROL and PARALLEL_REDO_TRAN_TURN

- Description : Related to issues with the parallel redo process in AlwaysOn Availability Groups.

- Troubleshooting : See the DIRTY_PAGE_TABLE_LOCK wait type for details.

PREEMPTIVE_OS_ACCEPTSECURITYCONTEXT and PREEMPTIVE_OS_AUTH\

- Description : These waits occur during user authentication, often due to performance issues with Active Directory controllers or the use of EXECUTE AS context.

- Troubleshooting : Ensure SQL Server does not authenticate against remote Active Directory controllers. Check if EXECUTE AS OWNER or EXECUTE AS USER context is being used.

PREEMPTIVE_OS_LOOKUPACCOUNTSID

- Description : Similar to PREEMPTIVE_OS_AUTH\* waits, occurring during user authentication.

- Troubleshooting : See strategies for PREEMPTIVE_OS_AUTH\* waits.

PREEMPTIVE_OS_WRITEFILE

- Description : Indicates bottlenecks during synchronous writes to files, potentially due to multiple SQL Traces or SQL audits, or the existence of database snapshots.

- **Troubleshooting**: Check for these conditions and address them accordingly.

PREEMPTIVE_OS_WRITEFILEGATHER

- Description : Occurs during the zero-initializing process.

- Troubleshooting : Enable instant file initialization by granting the `Perform volume management tasks` (SE_MANAGE_VOLUME_NAME) permission to the SQL Server startup account. Review transaction log auto-growth parameters and ensure no processes regularly shrink the transaction log.

QDS\

- Description : Query Store related waits, indicating overhead from Query Store data collection.

- Troubleshooting : Check Query Store settings, avoid using QUERY_CAPTURE_MODE=ALL, reduce Query Store size, and enable trace flags T7745 and T7752.

RESOURCE_SEMAPHORE

- Description : Occurs when queries wait for memory grants to execute, indicating potential memory pressure.

- Troubleshooting : Analyze memory usage, check memory clerks, and ensure proper server provisioning and SQL Server configuration. Use the `sys.dm_exec_query_memory_grants` view to optimize specific queries.

RESOURCE_SEMAPHORE_QUERY_COMPILE

- Description : Indicates insufficient memory to compile queries, requiring investigation of excessive compilations and general memory usage.

- **Troubleshooting**: Reduce query compilations by parameterizing queries and perform memory usage troubleshooting as described for RESOURCE_SEMAPHORE waits.

THREADPOOL

- Description : Occurs when SQL Server does not have available workers to handle user requests. This is a critical wait that needs immediate investigation.

- Common Causes : Incorrect max worker threads configuration, insufficient SQL Server memory, long blocking chains, excessive memory pressure, a large number of connected clients, and excessive parallel queries.

- Troubleshooting : Review and adjust the max worker threads configuration, ensure adequate SQL Server memory, address blocking chains, manage memory pressure, and optimize query workloads. In cloud environments, this may indicate an under-provisioned cloud service.

WRITE_COMPLETION

- Description : Occurs during synchronous write operations to data and log files, most commonly with database snapshots.

• Troubleshooting : Check for the presence of database snapshots, including those created by internal processes like DBCC CHECKDB. If IO_COMPLETION waits are also present, perform I/O system troubleshooting.

WRITELOG

- Description : Occurs when SQL Server writes log records to the transaction log. While normal, a high percentage or long average wait time may indicate a transaction log bottleneck.

- Troubleshooting : Analyze the average wait time and transaction log write latency using the `sys.dm_io_virtual_file_stats` view. Address transaction log performance issues and perform I/O system troubleshooting if necessary.

By understanding and effectively troubleshooting these wait types, you can enhance the performance and reliability of your SQL Server environment. This appendix serves as a quick reference for identifying and addressing common SQL Server wait types during your troubleshooting efforts. For more comprehensive information, refer to the specific chapters mentioned and complement this guide with official Microsoft documentation and the SQLSkills Wait Types Library.

Happy troubleshooting!