SQL Server: Large Memory Can Cause Slow Buffer Pool Scans - Potential Performance Bottleneck

Table of Contents

This article explains how scanning the SQL Server buffer pool can be time-consuming on computers with substantial memory resources.

Symptoms¶

Certain operations within Microsoft SQL Server initiate a scan of the buffer pool. The buffer pool serves as a cache, storing database pages in memory for rapid access. On systems equipped with a large amount of RAM, specifically 1 TB or more, scanning this buffer pool can become a lengthy process. This extended scan duration subsequently slows down the operation that originally triggered it.

Operations That Cause a Buffer Pool Scan¶

Several operations within SQL Server can lead to a buffer pool scan. Understanding these operations is crucial for diagnosing potential performance bottlenecks. These operations include:

• Database startup
• Database shutdown or restart
• Availability Group (AG) failover
• Database removal (drop)
• File removal from a database
• Full or differential database backup
• Database restoration
• Transaction log restoration
• Online restoration
• DBCC CHECKDB or DBCC CHECKTABLE operations

Error Log Shows That a Scan Took a Long Time¶

Starting with specific updates for various SQL Server versions, a new error message has been introduced to the SQL Server Error log. This message is designed to highlight buffer pool scans that exceed a duration of 10 seconds. These versions include:

• SQL Server 2016 SP3
• SQL Server 2017 CU23
• SQL Server 2019 CU9

The error message provides details about the scan, including the database ID, the command that triggered it, the operation performed, the number of buffers scanned, the total buffers iterated, and the wait time. An example of this error message is shown below:

Buffer Pool scan took 14 seconds: database ID 7, command ‘BACKUP DATABASE’, operation ‘FlushCache’, scanned buffers 115, total iterated buffers 204640239, wait time 0 ms. See ‘https://go.microsoft.com/fwlink/?linkid=2132602’ for more information.

This error message serves as an indicator that a buffer pool scan is taking longer than expected and could be contributing to performance issues.

Extended Event to Diagnose a Long Scan¶

To further aid in diagnosing lengthy buffer pool scans, the buffer_pool_scan_complete Extended Event has been introduced in the same SQL Server updates mentioned previously (SQL Server 2016 SP3, SQL Server 2017 CU23, and SQL Server 2019 CU9).

This Extended Event is recorded when a buffer pool scan takes more than 1 second, offering a more granular view of scan durations. When enabled, the XEvent captures the following information:

Name	Description
name	Event name (buffer_pool_scan_complete)
database_id	ID of the database involved
elapsed_time_ms	Duration of the scan in milliseconds
command	Command that triggered the scan
operation	Specific operation performed during the scan
scanned_buffers	Number of buffers scanned
total_iterated_buffers	Total number of buffers iterated

Below is an example of the data captured by the buffer_pool_scan_complete Extended Event:

name                       | database_id | elapsed_time_ms | command         | operation   | scanned_buffers | total_iterated_buffers
----------------------------|-------------|-----------------|-----------------|-------------|-----------------|-----------------------
buffer_pool_scan_complete | 7           | 1308            | BACKUP DATABASE | FlushCache    | 243             | 19932814

The threshold for triggering the Extended Event is set lower than the error log threshold (1 second vs 10 seconds) to enable capturing more detailed information about buffer pool scan performance. This finer granularity allows for more proactive identification of potential issues.

Workaround¶

Prior to SQL Server 2022, there was no direct method to resolve the issue of slow buffer pool scans on large memory systems. It is generally discouraged to manually clear the buffer pool as actions like dropping clean buffers using DBCC DROPCLEANBUFFERS can lead to significant performance degradation. Removing database pages from memory forces subsequent queries to reread data from disk, resulting in slower query execution due to disk I/O operations.

SQL Server 2022 introduces a significant improvement to address this problem. Buffer pool scans are now parallelized, leveraging multiple CPU cores to accelerate the scanning process. The parallel scan mechanism employs one task for every 8 million buffers, which equates to approximately 64 GB of memory. For systems with buffer pools smaller than 8 million buffers, a serial scan is still utilized.

This parallel scanning capability in SQL Server 2022 substantially mitigates the performance bottleneck associated with buffer pool scans on large-memory servers. By distributing the workload across multiple cores, the overall scan time is significantly reduced, leading to faster operation completion and improved system responsiveness.

More Information¶

The introduction of parallel buffer pool scans in SQL Server 2022 represents a crucial enhancement for managing large memory environments. This optimization directly tackles the performance challenges posed by lengthy buffer pool scans, particularly in scenarios involving database backups, restores, and other metadata operations.

The use of parallel processing for buffer pool scans is a significant architectural improvement, ensuring that SQL Server can effectively manage and utilize large memory configurations without incurring performance penalties due to serial buffer pool scanning. Organizations leveraging SQL Server 2022 on systems with substantial RAM will benefit directly from this enhancement, experiencing improved performance and efficiency in database operations.

Furthermore, the diagnostic tools introduced in earlier versions of SQL Server (error log messages and Extended Events) remain valuable for monitoring and troubleshooting buffer pool scan performance. These tools, combined with the parallel scan capabilities of SQL Server 2022, provide a comprehensive approach to managing buffer pool performance in modern, memory-rich SQL Server environments.

If you have experienced slow buffer pool scans or have further questions about optimizing SQL Server performance on large memory systems, please feel free to leave a comment below. Your insights and experiences are valuable to the community.