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Find disk constrained Oracle instance

Oracle Database Tips by Donald BurlesonJuly 4, 2013

Disk Constrained Databases

In a disk-bound database, the majority of the wait time is spent accessing data blocks. This can be db file sequential read waits, which is usually index access, and db file scattered read waits, which is usually full-table scans as evidenced by the following AWR report section:

Top 5 Timed Events
                                          % Total
    Event                    Waits        Ela Time
--------------------------- ------------ -----------
db file sequential read         2,598         48.54
db file scattered read         25,519         22.04
library cache load lock           673          9.26
CPU time                           44          7.83
log file parallel write        19,157          5.65


See my notes on disk I/O tuning for further details on Oracle disk constrained databases.

I/O bound databases can be tuned down by any number of holistic techniques including large data buffers with 64-bit Oracle, changing RAID levels on the disk and using high speed solid-state RAM-disk.

The dba_hist_sqlstat table is a great way to find disk bottlenecks. This view is very similar to the v$sql view, but it contains important SQL metrics for each snapshot. These include important delta (change) information on disk reads and buffer gets, as well as time-series delta information on application, I/O and concurrency wait times. The following is a script that queries this view for this performance information.

awr_sqlstat_deltas.sql

col c1 heading 'Begin|Interval|time'    format a8
col c2 heading 'SQL|ID'                 format a13
col c3 heading 'Exec|Delta'             format 9,999
col c4 heading 'Buffer|Gets|Delta'      format 9,999
col c5 heading 'Disk|Reads|Delta'       format 9,999
col c6 heading 'IO Wait|Delta'          format 9,999
col c7 heading 'Application|Wait|Delta' format 9,999
col c8 heading 'Concurrency|Wait|Delta' format 9,999


break on c1

select
   to_char(s.begin_interval_time,'mm-dd hh24') c1,
   sql.sql_id c2,
   sql.executions_delta c3,
   sql.buffer_gets_delta c4,
   sql.disk_reads_delta c5,
   sql.iowait_delta c6,
   sql.apwait_delta c7,
   sql.ccwait_delta c8
from
   see code depot for full scripts
   dba_hist_sqlstat sql,
   dba_hist_snapshot s
where
s.snap_id = sql.snap_id
order by
c1,
c2
;

 

 
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