In this previous post I asked whether 100% logical scan fragmentation is always worse than 85% local scan fragmentation for table/index scans. (To be precise, I was talking about a B-tree table, i.e. a table with a clustered index).   The answer is no. 100% logical scan fragmentation is not always worse than 85% logical scan fragmentation ...

All the previously posted results (May 25th and May 29th) on this exercise were obtained with query parallelism disabled (i.e. the sp_configure ‘max degree of parallelism’ option was set to 1).   Since the following test query is sensitive to query parallelism, we need to see what impact query parallelism may have.   DBCC ...

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 ...

This is a quick update on the T-SQL exercise I posted a few days ago. The goal was to write a simple T-SQL script to generate and load 4,000,000 rows into a table so that the following query would produce the worst performance, i.e. take longest time to finish:   DBCC DROPCLEANBUFFERS go SELECT COUNT(*) FROM dbo.test;   The ...

Here is a T-SQL scripting exercise in case you have a few minutes to spare or are bored with whatever else you are doing.    Objective The task is to write a simple T-SQL script to generate and load 4,000,000 rows into a test table. The objective is to make the following simple test query to have the worst performance in terms of ...

SQL Server workloads   So far, the discussions in all the previous posts (1, 2, 3, and 4) on the performance impact of file fragmentation on a drive presented from a high-end enterprise-class disk array are related to disk I/O workloads. Ultimately, you want to know how file fragmentation may impact your SQL Server workloads.   In ...

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 ...

256KB Sequential Reads   In my two previous posts (1, 2), I highlighted the fact that while file fragmentation had a huge adverse performance impact on directly attached storage (DAS), it did not have much, if any, impact on the drive presented from a high end enterprise class disk array. That observation was derived from running disk I/O ...

1KB Sequential Writes on DAS   There were some questions about the use 1KB sequential writes in my previous post to test the performance impact of file fragmentation on a drive presented from a high end enterprise class disk array.   There were two reasons for testing 1KB sequential writes: ·      SQL ...

1KB Sequential Writes   It’s well known that disk I/O performance can be severely impacted by fragmentation at the file system level. In other words, when a file is allocated space from many small fragments, its performance can be much worse than when its space is allocated from a single contiguous chunk. The impact is most pronounced with ...