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When does “physical reads” include read-ahead reads in SQL Server?

pre-fetching mechanism

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!

I took my simple test query from this prior post out for a drive, and measured its physical reads and read-ahead reads in a few different ways. Here’s what I found.

STATISTICS IO output separates physical and read-ahead reads

SQL Server will return information about how many reads you did to your session’s Messages tab when you run “SET STATISTICS IO ON“.

Here’s what it says for our query:

(2 row(s) affected)
Table 'Worktable'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Workfile'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'FirstNameByBirthDate_1966_2015'. Scan count 1, logical reads 7758553, physical reads 5625, read-ahead reads 438117, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'FirstName'. Scan count 1, logical reads 2, physical reads 2, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

In this case, physical reads = 5,625 and read-ahead reads = 438,117 against our largest table. There are 2 other reads against the small FirstName table for a total of 443,384 physical pages read.

The sys.dm_exec_query_stats DMV combines all physical reads

When I query the sys.dm_exec_query_stats DMV and pull back execution statistics just for my query (with a diagnostic query like this), it doesn’t have a separate column for read-ahead reads. Instead, it reports all reads from disk under physical reads:

Notably, this DMV shows that more physical reads were done than were reported by STATISTICS IO! It saw 483,128 physical reads.

Extended Events trace on sql_statement_completed also combines physical reads

Well, how about a trace? I ran a quick trace on my session and collected sql_statement_completed. Like sys.dm_exec_query_stats, it doesn’t separate out read-ahead reads: it reports everything as physical reads.

I was really happy to see that the trace agreed with the DMV, and that they both saw 483,128 physical reads. It’s nice to have a little consistency!

What about tracing sqlserver.file_read_completed for read-ahead reads?

As Tim Chapman wrote over on the SQL PFE blog, the sqlserver.file_read_completed extended event lets you see the number of reads from disk and the size of physical reads done.

So I set up my Extended Events trace, cleared out my buffer pool, and ran my query. Here’s the physical read count and sizes, I saw, with some columns added for analysis:

The “read-ahead reads” column here has been converted to the page count read. I didn’t include single page reads  or the reads of a single extent in that column. (I also was a little lazy and didn’t bother to filter out “cache warming” pages read, which is probably around 9 pages, but don’t tell anyone, OK?)

This trace saw fewer read-ahead reads than STATISTICS IO reported. It also saw fewer physical reads than sys.dm_exec_query_stats or sql_statement_completed saw. But we’re in the ballpark, and these are different methods of measurement.

My view: STATISTICS IO isn’t super-precise, but it is cool that it separates out read-ahead reads

It’s actually not easy to see how many read-ahead reads your query is doing! STATISTICS IO is one of the few places that breaks it out in a way that’s simple to see. I see these numbers as a ballpark estimate, and I wouldn’t bet on them being “exactly right”. They’re good enough to be really useful.

When you’re looking at sys.dm_exec_query_stats, or you’re tracing statements completed, in those places “physical reads” includes reads of all sizes– including read-ahead reads.

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