Common Wait Types in SQL Server

Understanding the Top 10 Most Common Wait Types in SQL Server

When managing SQL Server Performance, understanding wait types is crucial for diagnosing and troubleshooting issues. Wait types provide insight into where SQL Server spends its time waiting, which often points to the underlying causes of performance bottlenecks. In this post, we’ll explore the 10 most common wait types you might encounter in SQL Server and what they typically indicate about your environment.

1. CXPACKET

The CXPACKET wait type is associated with parallel query execution. It indicates that a thread is waiting for other threads to complete their tasks in a parallel query. While CXPACKET waits are normal in parallel processing, high occurrences can sometimes point to issues with parallelism, such as uneven data distribution across threads. To address this, you might need to revisit your server’s MAXDOP (maximum degree of parallelism) settings or review the execution plans of queries causing high CXPACKET waits.

2. SOS_SCHEDULER_YIELD

This wait occurs when a SQL Server worker thread voluntarily yields the CPU, allowing other threads to execute. While some SOS_SCHEDULER_YIELD waits are expected, consistently high values may indicate CPU pressure. If your CPU is consistently under heavy load, consider optimizing your queries, indexing strategy, or even scaling up your hardware.

3. PAGEIOLATCH_SH / PAGEIOLATCH_EX

These wait types occur when SQL Server waits for pages to be read from disk into memory. PAGEIOLATCH_SH refers to a shared latch (read operation), while PAGEIOLATCH_EX refers to an exclusive latch (write operation). High PAGEIOLATCH waits often point to I/O bottlenecks, possibly due to slow storage subsystems or insufficient memory. Addressing these waits may involve upgrading your storage or increasing memory to reduce the frequency of disk reads.

4. LCK_M_X / LCK_M_S

Lock waits, particularly LCK_M_X (exclusive locks) and LCK_M_S (shared locks), are common in environments with high contention. These waits indicate that queries are waiting for locks on resources, leading to potential blocking issues. To mitigate high lock waits, review your transaction isolation levels, optimize your queries to lock fewer rows, or consider implementing a row-versioning-based isolation level like Snapshot Isolation.

5. ASYNC_NETWORK_IO

ASYNC_NETWORK_IO waits occur when SQL Server is waiting for the client to acknowledge that it has received data. This wait type can indicate slow client processing, network latency, or excessive data being sent to the client. If you see high ASYNC_NETWORK_IO waits, investigate network performance, or consider optimizing queries to return smaller result sets.

6. WRITELOG

This wait type is encountered when SQL Server is waiting for transaction log writes to complete. High WRITELOG waits often point to performance issues with the disk subsystem that hosts the transaction log files. To reduce WRITELOG waits, ensure your transaction log is on fast storage, like SSDs, and consider separating it from data files to avoid contention.

7. PAGEIOLATCH_UP

PAGEIOLATCH_UP is similar to PAGEIOLATCH_SH but occurs during update operations when SQL Server waits for a page to be read from disk into memory for modification. Frequent PAGEIOLATCH_UP waits suggest disk I/O bottlenecks. As with PAGEIOLATCH_SH, upgrading your storage or adding more memory to cache data pages can help alleviate these waits.

8. HADR_SYNC_COMMIT

In Always On Availability Groups, HADR_SYNC_COMMIT waits occur when a transaction is waiting for a synchronous replica to acknowledge that it has hardened the log. High HADR_SYNC_COMMIT waits can indicate network latency or performance issues on the secondary replica. To mitigate these waits, ensure low-latency, high-throughput network connections between your replicas and optimize the performance of your secondary nodes.

9. RESOURCE_SEMAPHORE

RESOURCE_SEMAPHORE waits happen when queries are waiting for memory to be available for execution. This wait type typically indicates memory pressure, especially for large queries requiring significant memory grants. Addressing RESOURCE_SEMAPHORE waits might involve adding more memory, optimizing large queries, or adjusting your server’s memory grant settings.

10. LOGBUFFER

LOGBUFFER waits occur when SQL Server is waiting for space in the log buffer to write new log records. These waits are often related to transaction log performance issues. If you encounter high LOGBUFFER waits, review your log buffer settings, and ensure your transaction log files are on high-performance storage.

Conclusion

Understanding and analyzing these common wait types can provide invaluable insights into SQL Server’s Performance issues. By monitoring these waits, you can identify the root causes of performance bottlenecks and take the necessary steps to optimize your environment. Whether it’s adjusting parallelism settings, optimizing disk I/O, or improving network performance, addressing these common wait types can lead to significant improvements in your SQL Server’s responsiveness and overall efficiency.

Recommendation: For ongoing wait stat monitoring, consider using Database Health Monitor. It’s a comprehensive tool designed to help DBAs monitor SQL Server performance, including wait statistics, and offers actionable insights to keep your environment running smoothly.

Try it today!

If you’re facing persistent wait types in your environment and need expert advice, consider reaching out for a comprehensive performance assessment tailored to your SQL Server setup.

10 Most Common Wait Types