Wait Type HADR_COMPRESSED_CACHE_SYNC

Wait statistics, in the context of SQL Server, refer to the amount of time that a query spends waiting to access data in the database. When a client application requests data from the database, the request is placed in a queue and the client application must wait for its turn to access the data. The time that the query spends waiting is called a "wait" and is tracked by SQL Server. This information can be used to identify potential performance bottlenecks and optimize the performance of the database. Wait statistics are commonly used by database administrators to diagnose and troubleshoot performance issues in SQL Server.

SQL Server Always On Availability Groups provide powerful high availability and disaster recovery capabilities, but they also introduce unique wait types that administrators should understand. One such wait type is HADR_COMPRESSED_CACHE_SYNC. Knowing what it represents and how to address it can help maintain a reliable and high-performing environment.

What is HADR_COMPRESSED_CACHE_SYNC?

The HADR_COMPRESSED_CACHE_SYNC wait type occurs in Always On Availability Groups when SQL Server is synchronizing compressed log records between the primary and secondary replicas. This wait type specifically pertains to the cache of compressed data, which is used to optimize the transfer of transaction logs across replicas.

In simpler terms, this wait type shows that SQL Server is busy ensuring compressed log records are synchronized to keep your replicas up-to-date. While some wait time is normal, excessive HADR_COMPRESSED_CACHE_SYNC waits could indicate bottlenecks in the compression or synchronization processes.

Why Does HADR_COMPRESSED_CACHE_SYNC Happen?

There are several reasons why HADR_COMPRESSED_CACHE_SYNC waits might occur, including:

• High transaction log generation on the primary replica, increasing the volume of compressed data.
• Resource bottlenecks on the primary or secondary replicas, such as CPU or memory constraints.
• Slow or unreliable network connections between replicas, causing delays in synchronization.
• Insufficient disk I/O performance on the secondary replicas, impacting the ability to apply log records.
• Suboptimal Always On Availability Group configurations.

Identifying the root cause of these waits can help improve the performance and reliability of your Always On setup.

How to Monitor HADR_COMPRESSED_CACHE_SYNC Waits

To track and analyze HADR_COMPRESSED_CACHE_SYNC waits, monitoring tools like the Database Health Monitor are invaluable. Its Historic Waits Monitoring feature provides detailed insights into SQL Server wait types, including this one, helping you identify patterns and correlations with system activity.

Using Database Health Monitor, you can determine if these waits are occurring during specific workloads or at times of high resource usage. This data enables you to make informed decisions to resolve issues effectively.

What Can You Do About HADR_COMPRESSED_CACHE_SYNC Waits?

If you notice frequent or prolonged HADR_COMPRESSED_CACHE_SYNC waits, here are steps you can take:

• Review and optimize queries and indexes to reduce transaction log generation on the primary replica.
• Ensure the network between the primary and secondary replicas has low latency and sufficient bandwidth for log synchronization.
• Upgrade or optimize CPU, memory, and disk I/O resources on both primary and secondary replicas.
• Evaluate your Always On Availability Group configuration to ensure it matches your workload and performance requirements.
• Distribute workloads across secondary replicas to balance resource usage and avoid contention.

These actions can help reduce HADR_COMPRESSED_CACHE_SYNC waits, improving the overall efficiency of your Always On environment.

How Stedman Solutions Can Help

At Stedman Solutions, we specialize in SQL Server performance tuning and troubleshooting, including resolving issues with wait types like HADR_COMPRESSED_CACHE_SYNC. Our managed services provide expert support to ensure your Always On Availability Groups are optimized and reliable. Additionally, our Database Health Monitor offers robust tools to track and analyze SQL Server waits, giving you the insights needed to maintain peak performance.

If your Always On environment is experiencing performance challenges or you want to optimize your SQL Server setup, contact us today. Let us help you achieve a fast, reliable, and efficient database environment!

Find out more about our SQL Server Managed Services

Applies to

Related Waits

See Also

• More About HADR_COMPRESSED_CACHE_SYNC