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v$iostat_function tips

Oracle Database Tips by Donald BurlesonJanuary 8, 2015

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

  • 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).  Also see v$iostat_function_detail.

Here are the writer processes within Oracle that are collected by v$iostat_function:

select
   function_id,
   function_name

from
   v$iostat_function

order by
   function_id;


FUNCTION_ID FUNCTION_NAME
----------- ------------------
0           RMAN
1           DBWR
2           LGWR
3           ARCH
4           XDB
5           Streams AQ
6           Data Pump
7           Recovery
8           Buffer Cache Reads
9           Direct Reads
10          Direct Writes
11          Others


Quest expert Guy Harrison has this great SQL script to display The wait time for all background processes that perform system I/O (DBWR, ARCH, etc.)
 
col function_name    format a25         heading "File Type"
col reads            format 99,999,999  heading "Reads"
col writes           format 99,999,999  heading "Writes"
col number_of_waits  format 99,999,999  heading "Waits"
col wait_time_sec    format 999,999,999 heading "Wait Time|Sec"
col avg_wait_ms      format 999.99      heading "Avg|Wait ms"
 
set lines 80
set pages 10000
set echo on
 
select
   function_name,
   small_read_reqs + large_read_reqs reads,
   small_write_reqs + large_write_reqs writes,
   wait_time/1000 wait_time_sec,
   case when number_of_waits > 0 then
          round(wait_time / number_of_waits, 2)
       end avg_wait_ms
from
   v$iostat_function
order by
    wait_time desc;

 
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