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Checking out SQL Server via empirical data points
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Okay, it’s a bit of a hyperbole. But let me elaborate. I regularly bump into SQL Server discussions, online or elsewhere, in which I hear people speaking with confidence that an operation is so and so because it is doing sequential disk I/Os or because Read More...
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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. Read More...
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All the previously posted results ( May 25 th and May 29 th ) 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 Read More...
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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 Read More...
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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 Read More...
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Logical scan fragmentation as reported by DBCC SHOWCONTIG is bad for queries that perform large scans such as table/index scans. Often, we hear that the higher the logical scan fragmentation percentage is, the worse it is for the scans. So the question Read More...
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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 Read More...
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A script to fragment a test file Well, this post won't cover any actual test results and is not specific to SAN. But in the spirit of full disclosure, I've attached the script I used to control the fragmentation level of the test files mentioned in this Read More...
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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, Read More...
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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 Read More...
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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 Read More...
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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 Read More...
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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 Read More...
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