Avg. Disk Bytes/Read Disk Read Bytes/sec in Visual C#

Generator Code 128 Code Set A in Visual C# Avg. Disk Bytes/Read Disk Read Bytes/sec

Avg. Disk Bytes/Read Disk Read Bytes/sec
Create Code-128 In Visual C#
Using Barcode creation for .NET Control to generate, create Code 128 image in VS .NET applications.
www.OnBarcode.com
Code 128A Reader In Visual C#
Using Barcode scanner for .NET framework Control to read, scan read, scan image in .NET framework applications.
www.OnBarcode.com
Query performance and disk I/O counters
Making Barcode In C#
Using Barcode creator for VS .NET Control to generate, create Barcode image in .NET framework applications.
www.OnBarcode.com
Paint UPC Code In Visual C#
Using Barcode generator for .NET Control to generate, create UPC-A image in VS .NET applications.
www.OnBarcode.com
Then, run the two test scripts in table 1, one at a time.
Code 128 Code Set A Generator In C#
Using Barcode creation for Visual Studio .NET Control to generate, create Code 128B image in VS .NET applications.
www.OnBarcode.com
Creating Barcode In Visual C#.NET
Using Barcode generation for VS .NET Control to generate, create Barcode image in .NET applications.
www.OnBarcode.com
Table 1 Test Test 1 Test scripts to see the impact of small random I/Os Script
Paint Matrix 2D Barcode In C#
Using Barcode generation for Visual Studio .NET Control to generate, create Matrix Barcode image in Visual Studio .NET applications.
www.OnBarcode.com
2 Of 5 Standard Drawer In C#.NET
Using Barcode printer for .NET framework Control to generate, create Industrial 2 of 5 image in .NET framework applications.
www.OnBarcode.com
-- table scan and large sequential I/Os DBCC DROPCLEANBUFFERS; go SELECT MAX(dt) FROM test; -- bookmark lookups and small random I/Os DBCC DROPCLEANBUFFERS; go SELECT MAX(dt) FROM test WITH (INDEX=2);
Code128 Generator In Java
Using Barcode encoder for Java Control to generate, create Code 128 Code Set B image in Java applications.
www.OnBarcode.com
Making Code 128A In Java
Using Barcode printer for BIRT Control to generate, create ANSI/AIM Code 128 image in BIRT applications.
www.OnBarcode.com
Test 2
Create Code 128 Code Set A In None
Using Barcode generation for Excel Control to generate, create Code 128 Code Set B image in Excel applications.
www.OnBarcode.com
Printing Data Matrix ECC200 In None
Using Barcode generation for Font Control to generate, create DataMatrix image in Font applications.
www.OnBarcode.com
In test 1, you will see that SQL Server chooses to process the SELECT query with a table scan. In test 2, SQL Server is forced to use the nonclustered index to retrieve the data pages through bookmark lookups.
Read European Article Number 13 In .NET Framework
Using Barcode reader for Visual Studio .NET Control to read, scan read, scan image in .NET applications.
www.OnBarcode.com
Barcode Generation In None
Using Barcode creation for Online Control to generate, create Barcode image in Online applications.
www.OnBarcode.com
NOTE
Universal Product Code Version A Drawer In Java
Using Barcode generation for Java Control to generate, create UPC Symbol image in Java applications.
www.OnBarcode.com
Barcode Reader In Visual C#.NET
Using Barcode reader for Visual Studio .NET Control to read, scan read, scan image in .NET framework applications.
www.OnBarcode.com
In each test script, DBCC DROPCLEANBUFFERS is run first to remove all the clean pages from the SQL Server buffer pool. This ensures that all pages are retrieved from disk by the subsequent SELECT query. Do not run this statement on a production server.
Generate Barcode In Java
Using Barcode generator for Java Control to generate, create Barcode image in Java applications.
www.OnBarcode.com
Print Barcode In Java
Using Barcode creator for BIRT Control to generate, create Barcode image in BIRT reports applications.
www.OnBarcode.com
Table 2 summarizes the performance statistics of the two tests in terms of query elapsed time, I/O size, and I/O throughput.
Data Matrix ECC200 Generation In Objective-C
Using Barcode generator for iPad Control to generate, create Data Matrix 2d barcode image in iPad applications.
www.OnBarcode.com
Making Barcode In None
Using Barcode generation for Office Excel Control to generate, create Barcode image in Microsoft Excel applications.
www.OnBarcode.com
Table 2 Performance impact of small random I/Os Query elapsed time 12 44 I/O size ~ 256 KB ~ 8 KB I/O throughput ~ 340 MB/sec ~ 80 MB/sec
Test Test 1 Test 2
Although the numbers may vary when you execute the tests in different environments, the following pattern should hold:
SQL Server issues large I/Os (~256 KB per read) in test 1, and small I/Os (8 KB ~ 64 KB per read) in test 2.
Depending on the storage capability, SQL Server should achieve much higher I/O throughput in test 1 than in test 2: a direct consequence of using large I/Os. The query performs significantly better in terms of elapsed time in test 1 than in test 2 because of the higher disk I/O throughput in test 1. In this experiment, the same query ran almost four times as fast in test 1 as it did in test 2. On a lower end disk subsystem that has lower throughput capacity, the difference will not be so pronounced. For instance, on a disk subsystem whose maximum
Performance scenarios
throughput capacity is 120 MB/sec, the 256 KB sequential scan was seen to achieve 100 MB/sec and the 8 KB random lookups reached 60 MB/sec. Although the gain was less than 100 percent, the difference was still significant.
Performance scenarios
These are the key messages in this chapter so far: Random I/Os are expensive. SQL Server is programmed to take advantage of sequential I/Os. You cannot directly observe whether I/Os are sequential or random. That being said, because sequential I/Os are associated with large I/Os and random I/Os with small I/Os, you can instead observe I/O sizes with the disk performance counters. Processing a large query with small I/Os is not efficient. This is interesting information in itself. But how does it help you in practice Let s explore four query performance scenarios where you can put this information to good use.
Scenario 1: constant checkpoints
This was a real-world case encountered by a database administrator (DBA). When reviewing the logged SQL Server performance counter values, the DBA discovered that checkpoints were taking place almost continuously on one of their busiest databases. At the same time, it was found that lazy writes were kept constantly active as well. Both checkpoints and lazy writes were working hard to flush out dirty pages and make room for free pages that are needed in the buffer pool, but they could barely keep up with the changes. Is there anything we could do to improve the situation The DBA was asked to provide an analysis and recommend a course of action for better performance. Fortunately, the disk performance counters were also logged along with the SQL Server counters. Correlating the disk performance counters with the SQL Server buffer manager counters led to the observation that SQL Server was doing small I/Os during checkpoints (Avg. Disk Bytes/Write <= ~16 KB). The performance capacity of the disk storage subsystem was not being fully utilized by the checkpoint I/O requests. The DBA realized that if the checkpoints could be done quicker, more free pages would be readily available, with the result of alleviating pressure on the lazy writer, keeping the pages in the buffer pool longer, and therefore improving the overall performance of the server. The solution in this case was to re-organize several key tables on different clustered indexes so that updates took place on pages that were more contiguous. This solution was evaluated against the potential increase in the lock contention, and in this case the I/O performance gain outweighed the downside risk of increased contention.
Copyright © OnBarcode.com . All rights reserved.