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  • Performance impact: hyperthreading for OLTP queries -- II

    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 ...
    Posted to Linchi Shea (Weblog) by Linchi Shea on January 11, 2012
  • Performance impact: hyperthreading for OLTP queries

    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 ...
    Posted to Linchi Shea (Weblog) by Linchi Shea on January 5, 2012
  • Why generalizations are dangerous

    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). In the conclusion, he states:  When a clustered index is present on the table & ...
    Posted to Aaron Bertrand (Weblog) by AaronBertrand on September 15, 2011
  • SQL Server Backup Simulator

    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 ...
    Posted to Andrew Kelly (Weblog) by Andrew Kelly on October 28, 2010
  • Performance impact: memory effectiveness of virtualization

    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 ...
    Posted to Linchi Shea (Weblog) by Linchi Shea on July 29, 2009
  • T-SQL Exercise: Simplest data-loading script to produce worst query performance – May 29th Update

    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 ...
    Posted to Linchi Shea (Weblog) by Linchi Shea on May 29, 2009
  • Performance impact: thread mode vs. fiber mode

    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 ...
    Posted to Linchi Shea (Weblog) by Linchi Shea on May 4, 2009
  • Performance impact: a large number of virtual log files – Part II

    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 ...
    Posted to Linchi Shea (Weblog) by Linchi Shea on February 12, 2009
  • Performance impact: a large number of virtual log files – Part I

    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 ...
    Posted to Linchi Shea (Weblog) by Linchi Shea on February 9, 2009
  • Performance impact: file fragmentation and SAN – Part IV

    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 ...
    Posted to Linchi Shea (Weblog) by Linchi Shea on December 22, 2008
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