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  SQL Server Tips by Robin Schumacher

Extent Proximity

Like many other database engines, SQL Server has a prefetch mechanism called the Read Ahead manager that will pull data into the memory caches it believes will be needed for operations such as full table scans. Having the data already present in memory ensures that response times are the shortest possible, so the Read Ahead capabilities are indeed good to have. However, the Read Ahead managerís ability to perform as efficiently as possible is somewhat dependent on the organization of the data that it prefetches.

If the data is contiguous in nature, the Read Ahead manager can work very well and read large chunks of data at a time. If the data is scattered and mixed throughout extents that contain other objects, the Read Ahead manager cannot move in one fluid direction and instead must skip around the file(s) to obtain the data it believes will be needed.

As a result, it is desirable to have the object data organized as contiguously as possible. The question is: how can one tell if this is the case? Viewing the extent fragmentation scores for each object is one way, but another way is to view object fragment placements throughout the database. This is best viewed through a third party tool that can visually present the extents in a way that allows one to easily diagnose extent proximity issues, but nonetheless, the up_bn_storage_map procedure below is needed to get the raw diagnostic data:

  • up_bn_storage_map

    -- Script is available in the Online Code Depot

The above book excerpt is from:

High-Performance SQL Server DBA
Tuning & Optimization Secrets

ISBN: 0-9761573-6-5
Robin Schumacher  


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