Wait Type - SQL Server

WAIT TYPES refer to the specific events or conditions that cause a query or transaction to wait for a resource to become available. Understanding wait types is essential for troubleshooting and optimizing SQL Server performance.

SQL Server wait types can be broadly categorized into two groups:

External waits: These waits occur when SQL Server is waiting for an external resource, such as I/O operations or network requests. Examples of external wait types include PAGEIOLATCH_EX, NETWORK_IO, and ASYNC_NETWORK_IO.

DMVs related to External Waits:

sys.dm_io_virtual_file_stats: This DMV provides information on I/O statistics at the file level. It can be used to identify which database files are experiencing high I/O activity.

sys.dm_exec_requests: This DMV provides information on currently executing queries, including the wait type, wait time, and resource utilization.

Internal waits: These waits occur when SQL Server is waiting for an internal resource, such as a latch or a lock. Examples of internal wait types include PAGELATCH_SH, LCK_M_S, and CXPACKET.

DMVs related to Internal Waits:

sys.dm_os_waiting_tasks: This DMV provides information on queries that are currently waiting for a resource, including the wait type, wait duration, and resource utilization. It can be used to identify queries that are experiencing long wait times.

sys.dm_tran_locks: This DMV provides information on locks held by currently executing queries. It can be used to identify queries that are holding locks for a long time and causing contention.

Here are some of the common wait types in SQL Server, along with a brief description and examples:

PAGEIOLATCH_EX: This wait type occurs when SQL Server is waiting for data pages to be read from disk. It is a common wait type for queries that scan large tables or indexes.

CXPACKET: This wait type occurs when SQL Server is waiting for parallel query threads to synchronize. It is a common wait type for queries that use parallelism.

PAGELATCH_SH: This wait type occurs when SQL Server is waiting for a shared latch on a data page. It is a common wait type for queries that access the same data pages simultaneously.

LCK_M_S: This wait type occurs when SQL Server is waiting for a shared lock on a resource. It is a common wait type for queries that read or modify data.

WRITELOG: This wait type occurs when SQL Server is waiting for a log flush to complete. It is a common wait type for queries that modify data.

SOS_SCHEDULER_YIELD: This wait type occurs when a query voluntarily yields the CPU to other processes. It is a common wait type for queries that run for a long time or use a lot of CPU resources.

ASYNC_NETWORK_IO: This wait type occurs when SQL Server is waiting for asynchronous network I/O operations to complete. It is a common wait type for queries that transfer large amounts of data over the network.

Note - Wait Types alone do not necessarily indicate a problem. They simply provide insight into what SQL Server is waiting for. To troubleshoot performance issues, you need to identify the root cause of the wait, which may involve analyzing other performance metrics, such as CPU usage, memory usage, and disk I/O.

Comments

  1. zahidJuly 26, 2023 at 5:01 AM

    the main two dmvs to find the wait type, I heard,sys.dm_os_wait_stats, sys.dm_os_wait_task please explain these

    ReplyDelete

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