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Don Burleson Blog 







v$iostat_consumer_group tips

Oracle Database Tips by Donald BurlesonJanuary 8, 2015

Question:  I see that Oracle has a v$iostat_consumer_group  view that allows me to see I/O timings.  Are these the same number is an AWR report?  When is it appropriate to use v$iostat_consumer_group  ?
Answer:  Oracle does not run in a vacuum, and Oracle has introduced many tools like v$iostat_consumer_group  to monitor the external environment. 

An Oracle consumer group is a named entities, which are groups of users or sessions combined together, based on their processing and resource needs.

NOTE:  We do not recommend using consumer groups except in special cases.  Consumer groups will limit the available resources of end-users, and when consumer group limits are reached, the end-user session is terminated.  It is never a good idea to terminate an end user.

Remember, no amount of tuning is going to help a database that has an external bottleneck, such as I/O enqueues.

  • The dbms_stats.gather_system_stats to sample the external environment for CPU and disk latency.  See using dbms_stats.gather_system_stats for external monitoring.
  • Oracle has internal instrumentation mechanisms capture disk I/O metrics automatically for all tablespaces and files and stored them in the v$ structures, the dba_hist tables and the ASH tables.

Internally Oracle knows the individual I/O latency for all tablespaces and datafiles. As we see these iostat details in all AWR and STATSPACK reports.
Oracle has introduced three iostat views to display I/O details:

  • v$iostat_consumer_group:  If Oracle Database Resource Manager is enabled this view captures I/O statistics for consumer groups.
  • v$iostat_file:  This v$iostat_file view displays I/O statistics of database files that are or have been accessed. The small_sync_read_latency column displays the latency for single block synchronous reads (in milliseconds), which translates directly to the amount of time that clients need to wait before moving onto the next operation.  The normal range of I/O speeds range from .002 ms for solid state disks (RAM disk) to 5 ms for a disk array, up to 15 ms for a standard platter disk without any RAM caching.  See Monitoring I/O with v$iostat_file.
  • v$iostat_function:  This v$iostat_function view captures I/O statistics for database background processes.  Oracle has named the sum of the background processes as functions (ARCH, LGWR, DBWR, &c).  See Monitoring I/O for Oracle background processes with v$iostat_function. 

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