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Don Burleson Blog 







v$metricgroup and MMON

Oracle Tips by Burleson Consulting

The v$metricgroup and MMON

The MMON is the new background process that is responsible for gathering database metrics. MMON computes short duration metrics every 15 seconds and long duration metrics every 60 seconds. Database metrics are grouped by their meaning and duration. The available metric groups are available in the v$metricgroup view:









The interval_size column specifies the sampling frequency in hundredths of a second for every metric group.


Some of these metric values are more important than others, so the details on the most important metrics are presented in groups as follows:

§       User Metrics are useful for getting details on total user waits and PGA usage.


METRIC_NAME                              METRIC_UNIT

---------------------------------------- -----------------

Average Users Waiting Counts             Users

Blocked User Session Count               Sessions

PGA Memory (Session)                     Bytes


§       Wait Metrics are useful for getting details on total waits and total session waits.  These metrics are also very valuable in the AWR trend identification process as they can serve as a baseline value to compare with other AWR data.  For example, a query could be written to determine the long-term relationship between Database Time Spent Waiting and suspected bottleneck events such as physical reads, CPU, Network waits, etc.


METRIC_NAME                              METRIC_UNIT

---------------------------------------- -----------------

Database Time Spent Waiting (%)          % (TimeWaited /

Total Time Waited                        CentiSeconds

Total Wait Counts                        Waits

Number of Sessions Waiting (Event)       Sessions

Total Time Waited                        CentiSeconds

Total Wait Counts                        Waits


§       I/O Metrics are valuable for any I/O-bound Oracle database, and they show the overall performance of the disk I/O sub-system.  Tracking these events is especially useful when determining where to replace platter disks with solid-state disks.


METRIC_NAME                              METRIC_UNIT

---------------------------------------- -----------------

Average File Read Time (Files-Long)      CentiSeconds Per Read

Average File Write Time (Files-Long)     CentiSeconds Per Write

Physical Block Reads (Files-Long)        Blocks

Physical Block Writes (Files-Long)       Blocks

Physical Reads (Files-Long)              Reads

Physical Writes (Files-Long)             Writes

Physical Reads (Session)                 Reads

Physical Reads Ratio (Sess/Sys) %        %

Logical Reads Ratio (Sess/Sys) %         %


§       CPU Metrics are especially valuable since Oracle’s SQL costing algorithm has shifted from I/O costing to CPU costing.  Oracle has recognized that large data buffer caches such as db_cache_size and db_keep_cache_size have reduced overall disk I/O and driven up CPU consumption on the typical Oracle 10g database.   These CPU metrics are also valuable for trend analysis in which overall “CPU Time (Session)” might be compared with consistent gets from the RAM data buffers.


METRIC_NAME                              METRIC_UNIT

---------------------------------------- ----------------------

CPU Time Per User Call                   Microseconds Per Call

Elapsed Time Per User Call               Microseconds Per Call

CPU Time (Session)                       CentiSeconds

CPU Usage Per Sec                        CentiSeconds Per

CPU Usage Per Txn                        CentiSeconds Per Txn


The AWR stores metrics history for those times when MMON takes the corresponding snapshot, but the AWR does not retain all of the database metric history as it occurred over the course of time.



This is an excerpt from my latest book "Oracle Tuning: The Definitive Reference". 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts:




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