Detecting and resolving performance problems in SQL Server is crucial for database developers and administrators. Utilizing dynamic management views (DMVs) can provide insights into a server instance’s current state and aid in troubleshooting performance issues.
SQL Copy code
USE
master
GO
SELECT * FROM sys.dm_os_wait_stats
GO
The sys.dm_os_wait_stats view offers information on current waits within the system, including wait types, waiting task counts, wait times in milliseconds, and signal wait times (waiting for CPU resources).
Actionable Waits:
- ASYNC_NETWORK_IO:
- It occurs during network writes when a task is blocked behind the network.
- Potential causes include a slow network, an underpowered client machine, or excessive row-by-row processing on the application server.
- Collaboration with network administrators may be required.
- CXPACKET:
- Indicates improper configuration, often related to high CPU utilization.
- Adjusting the maximum degree of Parallelism (MaxDOP) and Cost Threshold for Parallelism (CTfP) settings can mitigate this wait type.
- Alternatively, it may suggest high CPU utilization, resolved through index tuning.
- SOS_SCHEDULER_YEILD:
- Indicates high CPU utilization, correlated with heavy workloads, large scans, or missing indexes.
- Collaboration with virtual machine administrators may help resolve associated issues.
- PAGEIOLATCH_XX (e.g., PAGEIOLATCH_SH):*
- Indicates SQL Server waiting to read data pages from storage.
- May point to disk subsystem issues, addressable through proper indexing before involving SAN administrators.
- LCK (e.g., LCK_M_S):*
- Locks resources during transactions, minimizing lock time can boost performance.
- Efficient code and proper indexing contribute to reducing lock time.
- WRITE_LOG:
- Long WRITE_LOG waits may indicate transaction log storage lagging behind workload.
- Addressed by ensuring transaction log storage keeps up with the workload.
- OLEDB:
- Indicates a wait for data from an OLE DB provider.
- Minimize linked server usage and investigate network performance for resolution.
- RESOURCE SEMAPHORE:
- Indicates queries waiting for memory availability, potentially due to excessive memory grants.
- Long runtimes or timeouts may be observed.
- Issues can stem from outdated statistics, missing indexes, or high query concurrency.
Benign Waits:
- CXCONSUMER:
- Normal wait type in parallel query execution.
- Separated from CXPACKET to avoid knee-jerk reactions to high CXPACKET waits.
- CHECKPOINT QUEUE:
- Occurs while the checkpoint task awaits the next checkpoint request.
- An idle process, safe to ignore.
- WAIT FOR:
- Results from a WAITFOR Transact-SQL statement.
- User-initiated wait for a specified duration.
- PARALLEL REDO LOG CACHE:
- Applies to SQL Server 2016 and later.
- Related to read threads on an Availability group replica, part of the parallel redo process.
- LAZYWRITER_SLEEP:
- Indicates the sleep time of a Lazy Writer during memory pressure.
- High wait times are generally positive as it means the server is not under memory pressure. Top of Form
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