Category Archives: Performance Tuning

Reads and Writes per DB, Using DMV’s

So years ago when I was working with a SQL 2000 database I had a need to see how many reads and writes were happening on each DB so I could properly partition a new SAN we had purchased. The SAN guys wanted Reads/Sec and Writes/Sec and IO’s/Sec total. Perfmon couldn’t give this for each DB so I had to use fn_virtualFileStats. I wrote a procedure that would tell me per DB what was going on and then store it down into a table for later comparison.

I’ve found a need for this again since I’m running some load tests and want to know what my data files are doing. This is easier now thanks to sys.dm_io_virtual_file_stats. It’s even better now that so many people in the community provide great content! Glen Alan Berry (Blog | Twitter) wrote and excellent DMV a day blog post series and in one of them he gives a great query on Virtual File stats. This gives you an excellent general point in time look at your server but was not exactly what I needed. I wanted to know for a specific period what were my reads and writes. This works better for my load testing needs to know just what is going on during a specific time.

You might want to use this during heavy business hours as well. The DMV is keeping track since the last Server restart so if you look at just the DMV you’re going to see everything that has occurred. Maintenance items (checkdb, Indexes, Backups) will all show up inside there so one DB might be very large in size and at night when the backup kicks off it might be using lots of Reads making it’s percent much higher but during the day no one really uses the DB so the DMV might show you that the DB is very busy when in reality your other DB’s are the busy one’s during the day. So If I’m planning for a new SAN or moving around files I would run this query at specific times so I could compare data points and find out what my databases are actually doing during critical times.

This script takes a Baseline record on the DMV. It then waits the amount of time you specify and takes a comparison line. It then compares the two and returns the percent’s. Lots more could be done with the comparison and certain things I’ve left in for later changes like Size. Right now I don’t do anything with Size but I plan to show the growth from the data points. I chose not to create the script as a stored proc just so it’s easy for you to put where you want. It only deals with 2 data points to keep it simple for now I have considered changing this in the future. The only setting you need to change is the @DelaySeconds parameter. Just set that to the number of seconds you want it to wait. 60 seconds is usually a good default to get a quick snapshot on the server. My suggestion to discover what your databases are doing during busy times I would probably run it for about 300 seconds (5 minutes) and then do that 2-3 times in an hour to see what the data looks like.

Would love to hear any comments on if this works for you!  Thanks.

File Stats Per DB
Date Created: 07-22-2010
Written by Pat Wright(@SqlAsylum)
This Scrpit is Free to download for Personal,Educational, and Internal Corporate Purposes,
provided that this main header is kept along with the script.  Sale of this script is
Prohibited in whole or in part is prohibited without the author’s consent.

[databaseName] [NVARCHAR](128) NULL,
[FileType] [NVARCHAR](60) NULL,
[physical_name] [NVARCHAR](260) NOT NULL,
[DriveLetter] VARCHAR(5) NULL,
[BytesRead] [BIGINT] NOT NULL,
[BytesWritten] [BIGINT] NOT NULL,

DECLARE @DelaySeconds INT

–Set Parameters
The counter is just to initialize the number to 1
The Delayseconds Tells SQL Server how long to wait before it runs the second data point.
How long you want this depends on what your needs are.  If I have a load test running for
5 minutes and I want to know what the read and Write percents were during those 5 minutes
I set it to 300. If I just want a quick look at the system I’ll usually set it to 60 seconds,
To give me a one minute view.  This depends on if it’s a busy time and what’s going on during that time.
SET @Counter = 1
SET @DelaySeconds = 60
SET @TestTime = DATEADD(SS,@delayseconds,GETDATE())

WHILE @Counter <=2
#FileStatsPerDb (DatabaseName,FileType,Physical_Name,DriveLetter,READS,BytesRead,Writes,BytesWritten,SIZE)
DB_NAME(mf.database_id) AS DatabaseName
,Mf.Type_desc AS FileType
,Mf.Physical_name AS Physical_Name
,LEFT(Mf.Physical_name,1) AS Driveletter
,num_of_reads AS READS
,num_of_bytes_read AS BytesRead
,num_of_writes AS Writes
,num_of_bytes_written AS BytesWritten
,size_on_disk_bytes AS SIZE
FROM sys.dm_io_virtual_file_stats(NULL, NULL) AS fs
JOIN sys.master_files AS mf ON mf.database_id = fs.database_id
IF @Counter = 1
TIME @TestTime
@Counter = @Counter + 1

WITH FileStatCTE (Databasename,Filetype,Driveletter,TotalReads,TotalWrites,TotalSize,TotalBytesRead,TotalBytesWritten)
(SELECT BL.Databasename,BL.FileType,Bl.DriveLetter,
NULLIF(SUM(cp.Readsbl.Reads),0) AS TotalReads,
NULLIF(SUM(cp.Writesbl.Writes),0) AS TotalWrites,
NULLIF(((SUM(cp.Sizebl.Size))/1024),0) AS TotalSize,
NULLIF(((SUM(cp.BytesReadbl.BytesRead))/1024),0) AS TotalKiloBytesRead,
NULLIF(((SUM(cp.BytesWrittenbl.BytesWritten))/1024),0) AS TotalKiloBytesWritten
( SELECT insertdate,Databasename,FileType,DriveLetter,READS,BytesRead,Writes,BytesWritten,SIZE
FROM #FileStatsPerDb
WHERE InsertDate IN (SELECT MIN(InsertDate) FROM #FileStatsPerDb) ) AS BL –Baseline
( SELECT insertdate,Databasename,FileType,DriveLetter,READS,BytesRead,Writes,BytesWritten,SIZE
FROM #FileStatsPerDb
WHERE InsertDate IN (SELECT MAX(InsertDate) FROM #FileStatsPerDb) ) AS CP — Comparison
ON BL.Databasename = cp.Databasename
AND bl.filetype = cp.filetype
AND bl.DriveLetter = cp.DriveLetter
GROUP BY BL.databasename,BL.filetype,Bl.driveletter)

Return the Read and write percent for Each DB and file.  Order by ReadPercent
SELECT databasename,filetype,driveletter,
100. * TotalReads / SUM(TotalReads) OVER() AS ReadPercent,
100. * TotalWrites / SUM(TotalWrites) OVER() AS WritePercent
ORDER BY ReadPercent DESC,WritePercent DESC

In my haste I forgot to add a sample of what you would get from this.  The names of the DB’s have been changed.


When a Second just isn’t fast enough

So I find myself performance tuning queries and statements all the time for SQL Server.  Many times I run into easy statements that are missing an index or calling a function 20 million times and causing problems.  These queries typically move from 20 seconds to 1-2 seconds very easily with some quick changes.  My problem this week was a query that took  1.5 seconds.  I needed to get this to run under 1 second consistently. I knew could get a little more out of the query by looking at the execution plan but I also knew I would have to consider the Architecture and the application itself to really get it the way I wanted it. 

I was surprised to find something in the execution plan that helped me out more than I expected.  The Key Lookup (In 2000 they are called Bookmark Lookups).  These show up in the execution plan and look like this. 


You will usually find these in your execution plans along with an Index seek/scan.  But what really do they mean?  SQL Server has found your data in the index but some of the values you have asked for in the query are not located in the index pages and it must go and lookup the data.  This will take a little more time as SQL Server goes and fetches your data. The funny thing about this query is that for the Execution plan it only put cost at 10% on this particular lookup.  Which seemed odd to me because when I looked at the IO STATS ( Set STATISTICS IO ON) it seemed like many of my reads were to pull back this data.  I decided I would ignore the Cost %  and place a covering index on the fields that are getting pulled back. I changed the index that it was using in the seek to add columns in my select statement as INCLUDE columns in my index.  This made a larger improvement than just the 10% that it had estimated.  It removed about 500ms of time from the query.  Which is a lot when your trying to go from 1.5 seconds to <1 second. 

Of course you do have a trade off here. By adding the columns to the index I’ve made my index larger and take up more space so don’t go around just adding all your fields into the INCLUDE of a index.  You need to evaluate and find out if the trade off is worth it.  In this case it is.  I’m also making changes to the application so that it doesn’t request columns it doesn’t need then I can remove them from the INCLUDE and save myself some space. 

If there is one thing I’ve learned from performance tuning and reading execution plans is never take them at face value.  They are a tool that was created to help you make better decisions.  They are not always right so make sure to do your homework and really test your procedures and break them down to find the real bottlenecks. 

Happy Performance Tuning