Buffer pool scans causing slow operations on large-memory computers have long been a concern for SQL Server users. However, with the release of SQL Server 2022, this problem has been effectively addressed. In this article, we will explore the previous challenges associated with buffer pool scans and highlight the improvements introduced in SQL Server 2022 that mitigate these issues.
Understanding Buffer Pool Scans
The buffer pool is a critical component of SQL Server that caches database pages in memory. Certain operations, such as database startup, shutdown, backups, and other maintenance tasks, can trigger buffer pool scans. On systems with a substantial amount of RAM, specifically 1 TB or more, scanning the buffer pool could be time-consuming, resulting in performance degradation.
Challenges and Symptoms
In earlier versions of SQL Server, when a buffer pool scan occurred on a large-memory computer, it could significantly slow down the operation that triggered it. This issue was particularly noticeable during tasks like database startup, shutdown, backup, restoration, and DBCC operations. The SQL Server Error log would record instances where a buffer pool scan took an extended period, typically exceeding 10 seconds.
To aid administrators in diagnosing long buffer pool scans, SQL Server 2016 SP3, SQL Server 2017 CU23, and SQL Server 2019 CU9 introduced the buffer_pool_scan_complete Extended Event. This event provides detailed information about elapsed time, the command or operation triggering the scan, and the number of scanned and iterated buffers. These enhancements were invaluable in identifying and troubleshooting buffer pool scan-related performance issues.
Previous Workaround Limitations
Prior to SQL Server 2022, there were limited options for mitigating the buffer pool scan problem. Although some administrators attempted to clear the buffer pool using DBCC DROPCLEANBUFFERS, this workaround often resulted in significant performance degradation. Removing database pages from memory forced subsequent query executions to retrieve data from disk, leading to slower query performance due to increased disk I/O.
Solution in SQL Server 2022
SQL Server 2022 introduces a breakthrough solution to address buffer pool scan performance on large-memory computers. The key improvement lies in the parallelization of buffer pool scans, leveraging multiple processor cores to expedite the scanning process. This parallelization enables SQL Server to utilize one task per 8 million buffers (64 GB), ensuring efficient and optimized scanning. In cases where the number of buffers is below 8 million, a serial scan is still employed. This enhancement significantly reduces the time taken for buffer pool scans and consequently improves the overall performance of affected operations.
During my popular Comprehensive Database Performance Health Check service, I encountered instances with some clients where they experienced challenges related to buffer pool.
With the release of SQL Server 2022, the longstanding problem of slow operations triggered by buffer pool scans on large-memory computers has been successfully addressed. The introduction of parallel scans in SQL Server 2022 optimizes the scanning process and enhances overall performance. Administrators can now benefit from improved efficiency and reliability when performing database startup, shutdown, backups, and maintenance operations.
I must also admit that I tried to create demonstration on this topic but I could not build a reliable and reproducable demo on this topic.
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Reference: Pinal Dave (http://blog.SQLAuthority.com)