Physical Versus Read-Ahead Reads

Kendra Little explains which SQL Server diagnostic tools include read-ahead reads versus “regular” physical reads:

SQL Server has more than one way to pull pages in from disk for your queries. SQL Server can do a physical read of an 8KB page, or an extent of 8  of those 8KB pages.

SQL Server can also use the “read-ahead” mechanism to pull even larger chunks of data in from disk when you have a query that wants to read a lot of data — because just plucking one 8KB page or even 64KB of pages into disk isn’t super fast when you need lotsa pages.

But these terms get a little confusing when you’re changing between different diagnostic tools in SQL Server, because some of these tools include read-ahead reads in physical reads, and some don’t!

There is some good information here, so read the whole thing.

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