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Home » SQL Server 2008 » SQL Server 2008 Administration » What is a read ?
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SQLSACT
Posted Saturday, October 06, 2012 3:19 PM
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Hi All

When using DMV's to assess your SQL instance, what is a read?

For example, when dealing with DMV's like:
sys.dm_exec_query_stats
sys.dm_exec_requests

There are columns like logical_reads, physical_reads etc...

My questions is, is a read when SQL reads one page?

Thanks
Post #1369469
Perry Whittle
Posted Saturday, October 06, 2012 3:39 PM


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a logical read is the pages that are read in memory, a physical read is when the page is brought in from disk

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Post #1369470
SQLSACT
Posted Saturday, October 06, 2012 8:09 PM
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Perry Whittle (10/6/2012)
a logical read is the pages that are read in memory, a physical read is when the page is brought in from disk


Thanks

Does 1 read mean 1 page?
Post #1369491
Perry Whittle
Posted Sunday, October 07, 2012 12:56 AM


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Yes a read is a page. A logical read may need to initiate a physical read first if the requested page is not in cache.

more info may be found at this link

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Post #1369501
CYates
Posted Monday, October 08, 2012 12:57 PM
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I've always gone by Microsoft's explanation:

“The I/O from an instance of SQL Server is divided into logical and physical I/O. A logical read occurs every time the database engine requests a page from the buffer cache. If the page is not currently in the buffer cache, a physical read is then performed to read the page into the buffer cache. If the page is currently in the cache, no physical read is generated; the buffer cache simply uses the page already in memory.”

Also, keep in mind that when you run a trace it is considered a logical read and not physical.

Physical reads for me have been a pain point when dealing with performance.
Post #1370004
Jeff Moden
Posted Monday, October 08, 2012 3:02 PM


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CYates (10/8/2012)
I've always gone by Microsoft's explanation:
...
{snip}
Also, keep in mind that when you run a trace it is considered a logical read and not physical.


Have you got an MS link that explains that as well?

--Jeff Moden
"RBAR is pronounced "ree-bar" and is a "Modenism" for "Row-By-Agonizing-Row".

First step towards the paradigm shift of writing Set Based code:
Stop thinking about what you want to do to a row... think, instead, of what you want to do to a column."

For better, quicker answers on T-SQL questions, click on the following...
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Post #1370067
Jeff Moden
Posted Monday, October 08, 2012 3:05 PM


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Never mind. I got it. BOL states:
Reads
The number of read operations on the logical disk that are performed by the server on behalf of the event. These read operations include all reads from tables and buffers during the statement's execution.


In this case, I don't believe that "logical" means just "logical reads". It would appear to include both physical and logic reads for the "logically named disk".

Of course, I could have misinterpreted that. Any one else have something more finite?

--Jeff Moden
"RBAR is pronounced "ree-bar" and is a "Modenism" for "Row-By-Agonizing-Row".

First step towards the paradigm shift of writing Set Based code:
Stop thinking about what you want to do to a row... think, instead, of what you want to do to a column."

For better, quicker answers on T-SQL questions, click on the following...
http://www.sqlservercentral.com/articles/Best+Practices/61537/

For better answers on performance questions, click on the following...
http://www.sqlservercentral.com/articles/SQLServerCentral/66909/
Post #1370071
Jeff Moden
Posted Monday, October 08, 2012 3:17 PM


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Ah... got it. If found (and immediately forgot to copy the URL for) a post by Microsoft Certified Master Gail Shaw where she explained that SQL Server actually only reads from the "buffer pool" which is all "logical". Reads from the physical disk are always read into the "Buffer Pool". It's the reads from that "Buffer Pool" that SQL Profiler counts.

It would appear that the only way to easily find out if physical reads were actually involved is to SET STATISTICS IO ON before you run the query.

--Jeff Moden
"RBAR is pronounced "ree-bar" and is a "Modenism" for "Row-By-Agonizing-Row".

First step towards the paradigm shift of writing Set Based code:
Stop thinking about what you want to do to a row... think, instead, of what you want to do to a column."

For better, quicker answers on T-SQL questions, click on the following...
http://www.sqlservercentral.com/articles/Best+Practices/61537/

For better answers on performance questions, click on the following...
http://www.sqlservercentral.com/articles/SQLServerCentral/66909/
Post #1370080
Perry Whittle
Posted Monday, October 08, 2012 3:28 PM


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Jeff Moden (10/8/2012)
CYates (10/8/2012)
I've always gone by Microsoft's explanation:
...
{snip}
Also, keep in mind that when you run a trace it is considered a logical read and not physical.


Have you got an MS link that explains that as well?

Yes, the link i posted above

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Post #1370086
GilaMonster
Posted Monday, October 08, 2012 4:03 PM


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Jeff Moden (10/8/2012)
Reads from the physical disk are always read into the "Buffer Pool". It's the reads from that "Buffer Pool" that SQL Profiler counts.


Easiest way to think about it is that all reads are logical. Some of the logical reads may additionally be physical reads (the page required wasn't in the buffer pool)

Gail Shaw
Microsoft Certified Master: SQL Server 2008, MVP
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

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