Powerful DBCC dropcleanbuffers: 5 Usage

Discover the power of DBCC DROPCLEANBUFFERS in SQL Server.

The pursuit of improved performance is ongoing in the complex world of SQL Server optimization. Let’s introduce DBCC DROPCLEANBUFFERS, a command with considerable power that can potentially affect the speed and effectiveness of your database operations. In this guide, we’ll embark on a journey to demystify DBCC DROPCLEANBUFFERS, delving into its historical significance, advantages, and limitations. We’ll navigate through real-world scenarios, equipping you with the knowledge needed to harness its capabilities and elevate your SQL Server’s performance.

Introduction

Every tool and method in the field of SQL Server performance optimisation is essential to ensure that your database performs at its very best. DBCC DROPCLEANBUFFERS is one of these tools, and it has the ability to reveal important information about query performance and resource use. Although it may appear simple, this command is the key to understanding your SQL Server’s behaviour on a deeper level.

DBCC DROPCLEANBUFFERS

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A Historical Overview of this DBCC Development

Since earlier versions of SQL Server, this DBCC command has been a component of the SQL Server toolbox. It started off as a command intended to clean out data cache buffers, giving query execution a fresh start. Understanding the intricacies of its use, which has changed throughout time, is essential for enhancing the performance of contemporary databases.

Advantages of DROPCLEANBUFFERS

Advantages of DBCC DROPCLEANBUFFERS are given below:

1. Accurate Query Performance Assessment:

A precise evaluation of query performance is possible thanks to DBCC DROPCLEANBUFFERS, which clears the data cache buffers and removes the impact of cached data. By doing so, you may assess the actual effectiveness of your queries and spot any possible bottlenecks.

2. Resource Contention Testing:

Using DBCC DROPCLEANBUFFERS to clear the cache, you may mimic situations in which resources are under demand. This can be quite helpful in identifying performance problems under various load circumstances.

3. Effective Query Tuning:

The command provides a blank slate for evaluation when analyzing query execution strategies and performance. By giving a more realistic depiction of how queries execute against the actual data, it makes precise tuning decisions easier.

4. Testing Cache Effectiveness

DBCC DROPCLEANBUFFERS can be used to assess the efficiency of the SQL Server buffer pool. By comparing query performance with and without cached data, you may determine the cache’s impact on query execution times.

Example

Running the identical query before and after deleting the cache allows you to quantify the performance difference and demonstrate the advantages of having data cached in memory.

5. Analyzing Query Optimization

Clearing the data cache assists in determining the efficacy of query optimization tactics. When you improve a query, running it in a cold cache environment assures that any speed gains are due to the optimization itself rather than the cached data.

Example

After improving a difficult query, emptying the cache and rerunning the query helps demonstrate that the performance gains are due to the optimization rather than caching.

6. Database Management and Troubleshooting

During maintenance of the database and fixing problems, DBCC DROPCLEANBUFFERS can be used to reset the data cache status. This can aid in the diagnosis of data caching issues and ensure that cached data does not interfere with maintenance procedures.

Example

When diagnosing performance difficulties, deleting the cache might provide a fresh start, allowing you to isolate problems related to data caching from other potential concerns.

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Disadvantages of DROPCLEANBUFFERS

Disadvantages of DBCC DROPCLEANBUFFERS are given below:

1. Temporary Performance Impact

Clearing cache buffers has a short-term performance impact since the next time a query is executed, SQL Server must reload data into memory. The user experience may suffer briefly as a result of this first data retrieve.

2. User Experience Disruption

Running this DBCC command in a live environment may cause users to experience slower response times while the cache is being repopulated. It is important to give considerable thought to this possible downtime.

3. Increased disk I/O.

With the data cache empty, SQL Server must retrieve data from disk rather than memory. This increases disk I/O, which strains storage subsystems and reduces overall system performance.

Example

In high-transaction scenarios, excessive disk activity can cause bottlenecks, slowing the system and harming the user experience.

4. Impact on Performance Testing.

While the command is beneficial for performance testing, repeated use can produce inconsistent results. It may not correctly reflect real-world settings in which data is frequently accessed from caches.

Example

Using this command to simulate a cold cache environment intentionally may result in deceptive performance test results.

5. High resource consumption.

Repopulating the cache necessitates significant CPU and memory resources. This can have an influence on other processes on the server, potentially resulting in resource congestion and decreased performance.

Example

A server under excessive stress may struggle to rebuild the cache, resulting in decreased response times for all apps that use the database.

Why do we need DBCC DropCleanBuffers?

DBCC DROPCLEANBUFFERS is a command that clears the data cache, requiring SQL Server to read data from disk rather than memory for subsequent queries. While it can substantially influence performance when used incorrectly, it is a useful tool in certain situations, particularly for performance testing, benchmarking, and troubleshooting. Here are the main reasons why you may need DBCC DROPCLEANBUFFERS:

Performance testing and benchmarking

One of the primary motivations for using DBCC DROPCLEANBUFFERS is to replicate a cold cache environment during performance testing. This aids in understanding how queries run when data is not pre-loaded into memory, which is critical for proper benchmarking and comparison of various database settings or query improvements.

Detecting Disk I/O bottlenecks

Clearing the cache allows you to uncover performance issues associated with disk I/O. Running queries after clearing the cache causes the SQL Server to fetch data from the disk, demonstrating how efficiently the storage subsystem handles the load.

Evaluate Cache Effectiveness

Clearing the data cache allows you to evaluate the efficacy of SQL Server’s buffer pool. By comparing query performance with a cold cache and a warm cache, you can see how caching affects query execution times.

Analyzing query optimization

Clearing the data cache aids in the evaluation of query optimization options. When you optimize a query, run it in a cold cache environment to confirm that any speed advantages are due to the optimization rather than the existence of cached data.

Database Management and Troubleshooting

During database maintenance and troubleshooting, DBCC DROPCLEANBUFFERS can be used to reset the data cache status. This can aid in diagnosing data caching issues and ensure that cached data does not interfere with maintenance procedures.

Syntax of DBCC DROPCLEANBUFFERS

Syntax of this DBCC command in SQL Server and Azure SQL Database:

DBCC DROPCLEANBUFFERS [ WITH NO_INFOMSGS ]

and in Azure Synapse Analytics and Analytics Platform System:

DBCC DROPCLEANBUFFERS ( COMPUTE | ALL ) [ WITH NO_INFOMSGS ]
DBCC DROPCLEANBUFFERS in SQL Server

Parameters

WITH NO_INFOMSGS

All informational messages are suppressed in arguments WITH NO_INFOMSGS. On the Analytics Platform System (PDW) and Azure Synapse Analytics, informational notifications are always hidden.

ALL

Clean out the RAM data cache on the Control node and each Compute node. If you don’t give a value, this setting will be used by default.

COMPUTE

each Compute node’s memory data cache should be cleared.

Note:

Use CHECKPOINT to create a cold buffer cache before dropping clean buffers from the buffer pool and columnstore objects from the columnstore object pool. CHECKPOINT clears the buffers and forces the writing to disc of all dirty pages for the current database. You may use the DBCC DROPCLEANBUFFERS command to purge the buffer pool of all buffers following a database checkpoint.

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Permission required to execute this DBCC command

A user must be a member of the administrator fixed server role or the db_owner fixed database role to run this DBCC command. Only individuals who are authorized to do so can carry out the order thanks to these rights.

In SQL Server, Analytics Platform System (PDW):
Requires sysadmin permission on the server.

Starting from SQL Server 2022 or later:
needs ALTER SERVER STATE permission.

The user should be a member of server role ##MS_ServerStateManager## in Azure SQL Database.

Example of DBCC DROPCLEANBUFFERS

1. Query Performance Baseline

To assess the impact of this DBCC command, compare the baseline performance of a query before and after using it.

2. Determining the resource bottleneck

Use this DBCC command to examine how resource conflict impacts crucial processes during high use.

3. Performance Testing with DBCC DropCleanBuffers

Assume you’re a database administrator who has to evaluate the performance implications of a new indexing approach on a huge table. You wish to assess query performance in both warm caches (data stored in memory) and cold caches (data fetched from drive).

-- Script to create a new orders table in SQL Server with sample data

CREATE TABLE Orders (
    OrderID INT PRIMARY KEY,
    OrderDate DATETIME,
	ItemID INT,
	Quantity DECIMAL(10, 2),
	ItemBasePrice DECIMAL(10, 2),
	ItemSalePrice DECIMAL(10, 2),
    ClientID INT,
    OrderAmount DECIMAL(10, 2)
);

-- Script to insert a few sample data in Orders table in SQL Server 

INSERT INTO Orders (OrderID, OrderDate, ItemID, Quantity, ItemBasePrice, ItemSalePrice, ClientID, OrderAmount)
SELECT [TransactionID]
      ,(CASE 
		WHEN YEAR([ModifiedDate])=2013 THEN DATEAdd(YEAR,10,[ModifiedDate])
		WHEN YEAR([ModifiedDate])=2014 THEN DATEAdd(YEAR,10,[ModifiedDate])
		ELSE [ModifiedDate] END) AS OrderDate
      ,[ProductID]  ,[Quantity]
      ,CAST((CASE 
		WHEN [ActualCost]>0 and [ActualCost]<=100 THEN ([ActualCost] - [ActualCost]*0.25)
		WHEN [ActualCost]>100 and [ActualCost]<=200 THEN ([ActualCost] - [ActualCost]*0.20)
		WHEN [ActualCost]>200 and [ActualCost]<=300 THEN ([ActualCost] - [ActualCost]*0.15)
		WHEN [ActualCost]>300 and [ActualCost]<=400 THEN ([ActualCost] - [ActualCost]*0.10)
		ELSE ([ActualCost] - [ActualCost]*0.05)
		END) AS DECIMAL(10,2)) AS ItemBasePrice
	  ,CAST([ActualCost] AS DECIMAL(10,2)) AS ItemSalePrice
      ,[ReferenceOrderID],
	  CAST(Quantity * (CASE 
		WHEN [ActualCost]>0 and [ActualCost]<=100 THEN ([ActualCost] - [ActualCost]*0.25)
		WHEN [ActualCost]>100 and [ActualCost]<=200 THEN ([ActualCost] - [ActualCost]*0.20)
		WHEN [ActualCost]>200 and [ActualCost]<=300 THEN ([ActualCost] - [ActualCost]*0.15)
		WHEN [ActualCost]>300 and [ActualCost]<=400 THEN ([ActualCost] - [ActualCost]*0.10)
		ELSE ([ActualCost] - [ActualCost]*0.05)
		END) AS DECIMAL(10, 2))  AS OrderAmount
  FROM [AdventureWorks2016].[Production].[TransactionHistory]
  WHERE [ActualCost]>0 

-- Script to create an index on OrderDate on Orders table

CREATE INDEX NCI_OrderDate ON Orders(OrderDate);


-- To get the count of records for a particular time span in SQL Server & load the data into the memory

SELECT COUNT(*) AS [NoOfRecords]
FROM Orders
WHERE OrderDate BETWEEN '2023-08-01' AND '2023-08-31';


-- Script to clear the cache on the server
-- Issue checkpoint to transfer all dirty pages from memory to disk

CHECKPOINT;  
DBCC DROPCLEANBUFFERS;

-- To capture details of query performance, need to enable statistics

SET STATISTICS IO ON;
SET STATISTICS TIME ON;

-- Now execute the query once again to capture statistics

SELECT COUNT(*) AS [NoOfRecords]
FROM Orders
WHERE OrderDate BETWEEN '2023-08-01' AND '2023-08-31';

-- Finally disable the statistics

SET STATISTICS IO OFF;
SET STATISTICS TIME OFF;
Step-by-Step process to test:

Setup: Create a sample table named Orders and fill it with data.

Initial Query Execution: Run a query to load data into the cache.

Query to Fetch October Data

Clean the Cache: To clean the data cache, run DBCC DROPCLEANBUFFERS in SQL Server.

DBCC DROPCLEANBUFFERS Result

Query Execution Following Cache Clear: Rerun the query to evaluate performance with a cold cache.

Comparison: Evaluate the performance results.

Execution Plan after using DBCC DROPCLEANBUFFERS
Explanation

Setup: The sample table Orders is created and loaded with approx 80000 rows of data. To facilitate the query, an index is established on the OrderDate column.

Initial Query Execution: The initial query execution loads data into the cache, allowing subsequent queries to access the data from memory.

Clear the Cache: DBCC DROPCLEANBUFFERS clears the data cache, imitating a cold cache environment.

Query Execution Post Cache Clear: The same query is run again, causing the SQL Server to read data from the disk and establishing a baseline for cold cache performance.

Comparison: Execution statistics are collected to compare query performance in warm and cold cache environments.

Conclusion

A powerful tool in the SQL Server toolbox, DBCC DROPCLEANBUFFERS provides a look into query performance and resource usage. You can successfully use its capabilities if you are aware of its benefits, drawbacks, and execution requirements. When using this DBCC command to optimize SQL Server performance, approach it with a strategic mindset and use it as a compass.

FAQs

Q: Can data loss be caused by DBCC DROPCLEANBUFFERS?
Ans:
No, the database integrity is preserved since this DBCC command just clears cached data.

Q: Is this DBCC command appropriate for production use?
Ans:
It is advised against using this DBCC command in production without careful thought; instead, utilize them in controlled situations for analysis.

Q: Do indexes become affected by this DBCC command?
Ans:
No, just cached data is removed; indexes are left untouched.

Q: How frequently should I run this DBCC command command?
Ans:
Don’t use it on a regular basis; just use it for specialized analyses.

Q: Do DBCC DROPCLEANBUFFERS have the ability to optimize sluggish queries?
Ans:
Although optimization necessitates query analysis and tweaking, it can help with accurate performance measurement.

Q: Does SQL Server’s DBCC DROPCLEANBUFFERS remove the whole database cache or only the data cache?
Ans:
Data cache, which stores data pages in memory, is explicitly cleared by this DBCC command. Other caches, such as the plan cache, are not cleared.

Q: Can stored procedures or query plans be affected by DBCC DROPCLEANBUFFERS?
Ans:
Stored procedures and query plans are unaffected by DBCC DROPCLEANBUFFERS. Execution plans are not cleared; only the data cache is.

Q: Can you reverse DBCC DROPCLEANBUFFERS? After emptying the cache, can I put it back?
Ans:
No, the cache cannot be restored after it has been emptied with this DBCC command. Executing queries is required to refresh the cache with new data.

Q: Is it possible to utilise DBCC DROPCLEANBUFFERS in a production setting?
Ans:
Unless specifically for analytical purposes, using this DBCC command in a production environment is not advised. Cache repopulation has the potential to affect user experience.

Q: How frequently should I use DBCC DROPCLEANBUFFERS to evaluate performance?
Ans:
Reserve it for particular evaluation circumstances only. Frequent usage can reduce SQL Server performance and result in needless cache cleaning.

Q: Does DBCC DROPCLEANBUFFERS have an impact on configuration options relating to memory?
Ans:
No, the memory configuration settings are not changed by this DBCC command. Only the data cache buffers are cleared.

Q: Can DBCC DROPCLEANBUFFERS be executed on a database-by-database basis?
Ans:
No, the entire SQL Server instance must run this DBCC command. All databases’ data cache buffers are cleared.

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