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:

  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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.
  6. 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.
  7. OLEDB:
    • Indicates a wait for data from an OLE DB provider.
    • Minimize linked server usage and investigate network performance for resolution.
  8. 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    • Issues can stem from outdated statistics, missing indexes, or high query concurrency.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:

  1. CXCONSUMER:
    • Normal wait type in parallel query execution.
    • Separated from CXPACKET to avoid knee-jerk reactions to high CXPACKET waits.
  2. CHECKPOINT QUEUE:
    • Occurs while the checkpoint task awaits the next checkpoint request.
    • An idle process, safe to ignore.
  3. WAIT FOR:
    • Results from a WAITFOR Transact-SQL statement.
    • User-initiated wait for a specified duration.
  4. 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.
  5. 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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