Buffer cache hit ratio

The buffer cache in SQL Server is a critical component of the database engine's memory management system. It is a portion of the server's memory (RAM) that is dedicated to caching data pages and index pages from the database files. When SQL Server reads data from disk or writes data back to disk, it uses the buffer cache to store these pages in memory temporarily, reducing the need for frequent disk I/O operations and improving overall database performance.

The buffer cache typically holds frequently accessed data and index pages, allowing SQL Server to quickly retrieve and manipulate this data without the latency of disk access. The buffer cache is managed by the Buffer Pool Manager, which handles operations such as page allocation, eviction, and replacement to optimize memory usage and minimize contention.

It serves as a temporary storage area in memory where data pages from the database are loaded and kept for efficient access and manipulation. The buffer cache plays a crucial role in optimizing database performance by reducing disk I/O and improving data retrieval speeds.

 

Here's what you can find in the buffer cache:

1.      Data Pages: The primary content of the buffer cache is data pages retrieved from tables, indexes, and other database objects. These data pages contain actual data rows, index entries, and other information stored in the database.

2.      Index Pages: In addition to data pages, index pages are also cached in the buffer cache. Index pages store the structure of indexes, including keys and pointers to data pages, facilitating faster data retrieval through index seeks and scans.

3.      Stored Procedures and Query Plans: SQL Server caches compiled execution plans for stored procedures, queries, and ad-hoc SQL statements in the buffer cache. This helps in avoiding the overhead of recompiling query plans for frequently executed queries, leading to improved query performance.

4.      System Metadata: Certain system metadata, such as system tables and views, are also cached in the buffer cache to optimize system queries and operations.

5.      Temporary Objects: Temporary objects, such as temporary tables and table variables, can also be cached in the buffer cache during their lifespan, improving performance for queries involving these objects.

6.      Work Tables: When performing sorting or hashing operations, SQL Server may use work tables that are temporarily cached in the buffer cache to store intermediate results.

7.      Query Results: In some cases, the results of queries or subqueries may be cached in the buffer cache, especially for queries with caching hints or when caching is enabled at the application level.

It's important to note that the buffer cache is managed dynamically by SQL Server's memory management system, using algorithms to prioritize frequently accessed data pages and adapt to changing workload patterns. Proper configuration and monitoring of buffer cache usage are essential for optimizing database performance and ensuring efficient memory utilization.

The buffer hit ratio, also known as the buffer cache hit ratio or buffer cache hit rate, is a performance metric that indicates the efficiency of the buffer cache in serving data requests from memory rather than disk. It represents the percentage of data page requests that are satisfied from the buffer cache without requiring disk I/O operations.

The formula to calculate the buffer hit ratio is:

Buffer Hit Ratio=Buffer Cache Hits/(Buffer Cache Hits+Buffer Cache Misses) ​×100%

• Buffer Cache Hits: The number of times SQL Server finds the requested data page in the buffer cache.
• Buffer Cache Misses: The number of times SQL Server needs to read data pages from disk because they are not available in the buffer cache.

A high buffer hit ratio indicates that a significant portion of data requests are being served from memory, which is desirable for optimal performance. On the other hand, a low buffer hit ratio suggests that SQL Server is frequently fetching data from disk, which can impact performance due to increased I/O latency.

The recommended value for the buffer hit ratio can vary depending on the specific workload and environment. However, a buffer hit ratio of 90% or higher is often considered good, indicating that most data requests are served from memory. It's important to monitor the buffer hit ratio regularly and adjust memory settings, caching strategies, and query optimization as needed to maintain optimal performance.

SQL Server Buffer Manager counters:

Buffer cache hit ratio	Indicates the percentage of pages found in the buffer cache without having to read from disk. The ratio is the total number of cache hits divided by the total number of cache lookups over the last few thousand page accesses. After a long period of time, the ratio moves very little. Because reading from the cache is much less expensive than reading from disk, you want this ratio to be high. Generally, you can increase the buffer cache hit ratio by increasing the amount of memory available to SQL Server or by using the buffer pool extension feature.
Lazy writes/sec	Indicates the number of buffers written per second by the buffer manager's lazy writer. The lazy writer is a system process that flushes out batches of dirty, aged buffers (buffers that contain changes that must be written back to disk before the buffer can be reused for a different page) and makes them available to user processes. The lazy writer eliminates the need to perform frequent checkpoints in order to create available buffers.
Page life expectancy	Indicates the number of seconds a page will stay in the buffer pool without references.
Page lookups/sec	Indicates the number of requests per second to find a page in the buffer pool.
Page reads/sec	Indicates the number of physical database page reads that are issued per second. This statistic displays the total number of physical page reads across all databases. Because physical I/O is expensive, you might be able to minimize the cost, either by using a larger data cache, intelligent indexes, and more efficient queries, or by changing the database design.
Page writes/sec	Indicates the number of physical database page writes that are issued per second.
Checkpoint pages/sec	Indicates the number of pages flushed to disk per second by a checkpoint or other operation that require all dirty pages to be flushed.

SELECT object_name, counter_name, cntr_value AS [Buffer Cache Hit Ratio]

FROM sys.dm_os_performance_counters

WHERE object_name LIKE '%Buffer Manager%'

    AND counter_name = 'Buffer cache hit ratio'; 

To check what is currently in the Buffer Pool and monitor its usage, you can use various methods in SQL Server:

1. Dynamic Management Views (DMVs): SQL Server provides DMVs that allow you to query information about the Buffer Pool. For example, you can use the following query to get information about the Buffer Pool usage:

SELECT COUNT(*) AS TotalPages, COUNT(CASE WHEN is_modified = 1 THEN 1 ELSE NULL END) AS DirtyPages, COUNT(CASE WHEN is_modified = 0 THEN 1 ELSE NULL END) AS CleanPages FROM sys.dm_os_buffer_descriptors;

This query retrieves the total number of pages in the Buffer Pool, the number of dirty pages (modified but not written to disk), and the number of clean pages (not modified).

2. Performance Monitor (PerfMon): You can use PerfMon to monitor SQL Server Buffer Manager counters, such as "Buffer Cache Hit Ratio" and "Page Life Expectancy." These counters give insights into how efficiently the Buffer Pool is being utilized.
3. SQL Server Management Studio (SSMS): You can also use SSMS to view Buffer Pool-related performance counters and activity. Navigate to "Management" -> "Activity Monitor" -> "Memory" to see Buffer Cache Hit Ratio, Page Life Expectancy, and other relevant metrics.

Fixing Buffer Pool Performance Issues:

Increase Buffer Pool Size: If the Buffer Cache Hit Ratio is consistently low, consider increasing the max server memory configuration to allocate more memory to the Buffer Pool.

Optimize Queries and Indexes: Poorly performing queries and missing or inefficient indexes can lead to excessive buffer pool usage. Optimize queries and ensure indexes are properly designed and maintained.

Monitor Page Life Expectancy (PLE): PLE indicates how long data pages stay in the buffer pool. A low PLE value may indicate memory pressure and the need to adjust memory settings.

Consider Buffer Pool Extensions: If your database is large and experiences frequent disk I/O due to memory constraints, consider using Buffer Pool Extensions (available in SQL Server Enterprise edition) to extend the buffer pool onto SSDs.

Review Hardware Configuration: Ensure that your server hardware (CPU, RAM, storage) meets the demands of your workload. Upgrading hardware can sometimes resolve performance issues related to memory and I/O.

Increase Memory: Allocate more memory to SQL Server if possible, especially if you notice frequent page reads from disk or a low page life expectancy. More memory can reduce disk I/O and improve overall performance.

Optimize Queries: Poorly performing queries can put unnecessary pressure on the buffer pool. Review and optimize queries by using appropriate indexes, avoiding excessive joins or subqueries, and optimizing where clauses.

Monitor and Tune: Continuously monitor buffer pool metrics using DMVs and PerfMon counters. Tune SQL Server configuration settings such as max server memory, min server memory, and buffer pool extensions (if using SQL Server Enterprise Edition).

Review Disk Subsystem: Slow disk I/O can impact buffer pool performance. Ensure that your disk subsystem (RAID configurations, disk types, etc.) can handle the I/O load efficiently.

Consider Buffer Pool Extensions (BPE): In SQL Server Enterprise Edition, you can use BPE to extend the buffer pool into SSD storage, improving performance for read-heavy workloads.

Buffer Cache Hit Ratio shows how SQL Server utilizes buffer cache

It gives the ratio of the data pages found and read from the SQL Server buffer cache and all data page requests. The pages that are not found in the buffer cache are read from the disk, which is significantly slower and affects performance.

Ideally, SQL Server would read all pages from the buffer cache and there will be no need to read any from disk. In this case, the Buffer Cache Hit Ratio value would be 100. The recommended value for Buffer Cache Hit Ratio is over 90. When better performance is needed, the minimal acceptable value is 95. A lower value indicates a memory problem.

To find 'Buffer cache hit ratio' value use below code:

SELECT object_name, counter_name, cntr_value FROM sys.dm_os_performance_counters WHERE [object_name] LIKE '%Buffer Manager%'AND [counter_name] = 'Buffer cache hit ratio'

If Buffer cache hit ratio value exceeded above threshold.

Due to inert behavior of Buffer Cache Hit Ratio, the values it shows can be misleading and it’s recommended to check values of other SQL Server Buffer Manager counters, such as Page Life Expectancy, Free list stalls/sec, Page reads/sec, etc.

SQL Server has more chances to find the pages in the buffer pool if they stay there longer. If the page is not in the buffer pool, it will be read from disk, which affects performance. If there’s insufficient memory, data pages are flushed from buffer cache more frequently, to free up the space for the new pages When there’s sufficient memory on the server, pages have a high life expectancy.

The normal values are above 300 seconds (5 minutes) and the trend line should be stable. It’s recommended to monitor the values over time, as frequent quick drops indicate memory issues. Also, a value drop of more than 50% is a sign for deeper investigation As typical hardware configurations used currently provide more resources, the normal values are often high above 300. If there’s enough memory on the machine, the Page Life Expectancy value will be over 1,000. If the value is below 300, it’s a clear indication that something is wrong “For example, a server with 230GB RAM allocated to the SQL Server buffer pool and a Page Life Expectancy of 300 would equate roughly to 785MB/sec of I/O activity to maintain the page churn inside of the buffer pool. While it might be possible for the I/O subsystem to keep up with this demand, this represents a significant amount of page churn in the buffer pool” [3]

To get Page life expectancy value use below script:

SELECT object_name, counter_name, cntr_value FROM sys.dm_os_performance_counters WHERE [object_name] LIKE '%Buffer Manager%'AND [counter_name] = 'Page life expectancy'

The recommended value for Buffer Cache Hit Ratio is over 90

Page life expectancy is greater than 300

Pages read per second <=750

Note: The cause for Page Life Expectancy values below 300 can be poor index design, missing indexes, mismatched data types, insufficient memory, etc.

Cause    : The cause of the low readings was insufficient RAM.

Solution : After freeing some RAM or adding more memory to your server you will see the readings back to normal.

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Method-2:

Buffer Cache Hit Ratio  is the percentage of  sql server pages requested and retrieved from the buffer cache without reading from disk.

SQL Server  reads data pages into a pool of memory called the Buffer Cache. The Buffer Cache Hit Ratio is a good indicator of Buffer Cache performance.

The calculation is : total number of cache hits \ total number of cache lookups from the previous few thousand page requests.

Reading from disk is expensive , it is generally faster to read from cache. Maintaining a high Buffer Cache Hit Ratio  indicates a large amount of pages are accessed from cache – meaning data will return faster.

The Buffer Cache Hit Ratio is a good measure of memory pressure,along with the Page Life Expectancy measure . They both measure slightly different aspects of the buffer cache.

When using Buffer Cache Hit Ratio don’t use the absolute value , but measure over a period of time. Look for a trend. I prefer to monitor for at least 30 minutes over a typical user workload.

 The Microsoft recommendation is 95% and higher. If the percentage drops less for a period of time – one way of increasing the Buffer Cache Hit Ratio could be to increase physical memory.

 There are situations where it’s obvious increasing physical memory will improve the Buffer Cache Hit Ratio, but not always. In those situations look first to:

a) index optimization and  SQL Server tuning.
b) table design.
c) quality queries , that could mean rewriting queries.
d) SQL Server memory configuration.