Performance Tuning with SQL Server Dynamic Management Views

Chapter 6: Physical Disk Statistics and Utilization
necessarily directly comparable to the previous count-based ratio, obtained from sys_
dm_io_virtual_file_stats. These numbers, however, provide a better representation
of the actual number of read and write operations, as they are counted at the object
level, whether or not the utilization is logical or physical. Note, too, that only objects that
have been used in a DML statement will be included, so after a reboot, you may get not
get any results for some objects.
The query shown in Listing 6.20 is based on one first suggested by Jamie Massie, in his
blog entry entitled Is 80/20 a 90's Estimate? For all objects in a given database, it sums
seeks, scans and lookups as read operations, and updates as changes to the data (the CASE
expressions prevent divide by zero errors when the table has never been used).
DECLARE @databaseName SYSNAME
SET @databaseName = 'BusyDatabase' --obviously not the real name
--'%' gives all databases
SELECT CASE
WHEN ( SUM(user_updates + user_seeks + user_scans + user_lookups) = 0 )
THEN NULL
ELSE ( CAST(SUM(user_seeks + user_scans + user_lookups)
AS DECIMAL)
/ CAST(SUM(user_updates + user_seeks + user_scans
+ user_lookups) AS DECIMAL) )
END AS RatioOfReads ,
CASE
WHEN ( SUM(user_updates + user_seeks + user_scans + user_lookups) = 0 )
THEN NULL
ELSE ( CAST(SUM(user_updates) AS DECIMAL)
/ CAST(SUM(user_updates + user_seeks + user_scans
+ user_lookups) AS DECIMAL) )
END AS RatioOfWrites ,
SUM(user_updates + user_seeks + user_scans + user_lookups)
AS TotalReadOperations ,
SUM(user_updates) AS TotalWriteOperations
FROM sys.dm_db_index_usage_stats AS ddius
WHERE DB_NAME(database_id) LIKE @databaseName
Listing 6.20: Read:write ratio for all objects in a given database.
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