This is in part a response to a comment by Paul White (@SQL_Kiwi) to my previous post on the performance impact of enabling hyperthreading (HT) on OLTP queries, and in part due to my desire to capture a more complete set of test data for future investigation on this very topic. I’m posting below the results of re-running the same exact test as ...

My previous post focuses on the performance impact of enabling hyperthreading (HT) on a machine with four Intel Westmere-EX processors on reporting queries. Let’s turn our attention to OLTP queries. To oversimplify it, reporting queries are generally processed by scanning a large number of pages, whereas quick index seeks are the hallmark of OLTP ...

A couple of years ago, John Sansom wrote a blog post comparing the performance of two different ways to get the maximum value from a column: MAX() and TOP (1). http://www.johnsansom.com/performance-comparison-of-select-top-1-verses-max/ In the conclusion, he states:  When a clustered index is present on the table & ...

The SQL Server support team just announced the release of a backup simulator. Many 3rd party utilities that do backup like IBM Tivoli, Symantec BackupExec, Quest Litespeed, Redgate SQL Backup etc. use the sqlvdi.dll to communicate to SQL Server and this allows you to simulate that activity. So if you want to test how your system reacts or more ...

One of the key performance metrics on any computer system is its memory speed (or memory bandwidth).   Memory speed can be measured in many different ways at different levels. At the OS level, one way to measure memory speed is to clock how fast (in terms of megabytes/gigabytes per second) it can read a piece of data from a memory location ...

This is another follow-up on the T-SQL exercise.   So the test query below is rather simple:   DBCC DROPCLEANBUFFERS go SELECT COUNT(*) FROM dbo.test;   But beneath its simple appearance, many factors are at play and interact in a complex way to influence the query performance. In other words, trying to predict its ...

SQL Server can run in one of two modes: thread mode or fiber mode. By default, SQL Server runs in thread mode in which a SQL Server worker is associated with a Windows thread throughout all phases of its execution. This can be changed with the sp_configure option ‘Lightweight Pooling’. When Lightweight Pooling is turned on, SQL Server runs in ...

In my previous post on the performance impact of having a large number of virtual log files (VLFs) in a transaction log, I showed that a large number of VLFs could be very bad for SQL Server 2008 performance. The test workloads were large batch delete, update, and insert. In other words, they were single monolithic transactions that ...

It is generally known that having a large number of virtual log files (VLFs) in a database transaction log is undesirable. A blog post by the Microsoft support team in Stockholm showed that a large number of virtual log files could seriously lengthen the database recovery time. Tony Rogerson also reported that lots of virtual log files were bad ...

Lies, damned lies, and statistics!   If you have read my three previous posts (1, 2, 3), you may walk away with an impression that on a drive presented from a high-end enterprise class disk array, Windows file fragmentation does not have a significant performance impact. And I’ve given you empirical data—oh yeah, statistics—to support that ...