Wait Type PREEMPTIVE_COM_SEQSTRMREAD

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.

In SQL Server, wait types help administrators identify where the database might be experiencing delays. One of these wait types, PREEMPTIVE_COM_SEQSTRMREAD, occurs when SQL Server is waiting on an external operation related to reading data using a COM (Component Object Model) sequential stream. This wait type is typically seen in environments where SQL Server interacts with external data sources or applications through COM objects. In this post, we’ll discuss what PREEMPTIVE_COM_SEQSTRMREAD means, when it appears, and how it could affect your SQL Server performance.

What Is the PREEMPTIVE_COM_SEQSTRMREAD Wait Type?

The PREEMPTIVE_COM_SEQSTRMREAD wait type appears when SQL Server is waiting for a sequential read operation via an external COM object. COM (Component Object Model) is a Microsoft technology that allows different applications and components to communicate with one another. SQL Server may use COM components to read or retrieve data from non-SQL data sources, such as linked servers, file systems, or external applications.

When SQL Server uses a COM object to read data in a "sequential stream" (a stream of data that is read in a fixed order), it may need to wait for the operating system or external application to complete the read operation. In these situations, SQL Server hands over control to the operating system, entering a "preemptive" mode, and waits until the read operation finishes. This wait is recorded as PREEMPTIVE_COM_SEQSTRMREAD.

When Does PREEMPTIVE_COM_SEQSTRMREAD Appear?

This wait type generally appears in SQL Server environments where external data sources or applications are accessed via COM objects. Typical scenarios that may lead to PREEMPTIVE_COM_SEQSTRMREAD waits include:

• Accessing Linked Servers – When SQL Server retrieves data from a linked server using OLE DB providers, it may encounter PREEMPTIVE_COM_SEQSTRMREAD waits as it waits for the data stream to be read.
• Reading Files or External Data Sources – SQL Server might use COM objects to interact with the file system or read data from other sources outside SQL Server, leading to waits if the read operation takes time.
• External Application Integration – If SQL Server is integrated with other applications or services that require COM to read sequential data, these reads may cause PREEMPTIVE_COM_SEQSTRMREAD waits.

While some PREEMPTIVE_COM_SEQSTRMREAD waits are normal when accessing external resources, high or prolonged waits may indicate a bottleneck in the external data source or network.

Why PREEMPTIVE_COM_SEQSTRMREAD Waits Matter

While PREEMPTIVE_COM_SEQSTRMREAD waits are common in environments that use external COM objects for sequential reads, frequent or lengthy waits can negatively impact SQL Server performance. High wait times in this category suggest that SQL Server is spending too much time waiting on external resources, which can lead to delays for other processes or queries.

Common causes of high PREEMPTIVE_COM_SEQSTRMREAD waits include:

• Slow Response from External Systems – If the external data source or linked server is slow to respond, it will increase wait times.
• Network Latency – When external data is accessed over a network, high latency can extend wait times for sequential reads.
• Resource Constraints on the External System – Limited resources on the external server, such as CPU or memory, may lead to delays in completing read operations.

How to Address PREEMPTIVE_COM_SEQSTRMREAD Waits

If you are seeing high PREEMPTIVE_COM_SEQSTRMREAD waits, here are some strategies to help reduce these wait times and improve performance:

• Optimize Linked Server Usage – Try to minimize the use of linked servers or optimize queries that access external data to reduce dependency on external reads.
• Monitor Network Performance – If data is accessed over a network, check network performance to ensure low latency and a stable connection.
• Increase Resources for External Systems – Ensure that the external systems or applications have adequate CPU, memory, and storage resources to handle SQL Server’s data requests efficiently.
• Limit COM Component Usage – Reduce reliance on COM objects for accessing external data if possible, as they introduce dependency on external processes.

Conclusion

The PREEMPTIVE_COM_SEQSTRMREAD wait type in SQL Server indicates that the system is waiting for an external COM object to complete a sequential read operation. Although some waits of this type are expected in environments with external dependencies, high or frequent waits could suggest issues with the external data source, network latency, or resource limitations. By optimizing linked server usage, monitoring network performance, and ensuring that external systems are adequately resourced, you can help reduce PREEMPTIVE_COM_SEQSTRMREAD waits and improve SQL Server performance.

For assistance with SQL Server performance tuning and optimization for external integrations, Stedman Solutions offers managed services to keep your SQL Server environment running efficiently and reliably.

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Applies to

• SQL Server 2016
• SQL Server 2017
• SQL Server 2019
• SQL Server 2022

Related Waits

See Also

• More About PREEMPTIVE_COM_SEQSTRMREAD