Call now: 252-767-6166  
Oracle Training Oracle Support Development Oracle Apps

 
 Home
 E-mail Us
 Oracle Articles


 Oracle Training
 Oracle Tips

 Oracle Forum
 Class Catalog


 Remote DBA
 Oracle Tuning
 Emergency 911
 RAC Support
 Apps Support
 Analysis
 Design
 Implementation
 Oracle Support


 SQL Tuning
 Security

 Oracle UNIX
 Oracle Linux
 Monitoring
 Remote s
upport
 Remote plans
 Remote
services
 Application Server

 Applications
 Oracle Forms
 Oracle Portal
 App Upgrades
 SQL Server
 Oracle Concepts
 Software Support

 Remote S
upport  
 Development  

 Implementation


 Consulting Staff
 Consulting Prices
 Help Wanted!

 


 Oracle Posters
 Oracle Books

 Oracle Scripts
 Ion
 Excel-DB  

Don Burleson Blog 


 

 

 


 

 

 
 

Oracle Physical read vs. Direct Read Tips

Oracle Tips by Burleson Consulting

November 15, 2012

Question:  What is the difference between a physical read and a direct read operation?  Is a direct read like using direct I/O?

Answer:  You can think that Oracle has three types of I/O:

  • Physical read:  A physical read is a vanilla disk read.
  • Direct I/O:  A direct I/O is a read that bypasses the Journalled file system (JFS) buffer cache on the server.
  • Direct Path Read:  The direct path read Oracle metric occurs during Direct Path operations when the data is asynchronously read from the database files into the PGA instead of into the SGA data buffer.  Direct reads occur under these conditions:
    • - When reading from the TEMP tablespace (a sort operation)

    • - When reading a parallel full-table scan (parallel query factotum (slave) processes)

    • - Reading a LOB segment

You can plot the difference between physical reads, logical reads (reads from the data buffer) and direct reads by querying the AWR tables.

--- Hourly based comparison between Physical reads and Direct Reads
select
s1.h_date,
trunc(s1.v_avg,2) pyh_reads,
trunc(s2.v_avg,2) dir_reads,
trunc(s1.mb_sec,2) pyh_mb_s,
trunc(s2.mb_sec,2) dir_mb_s,
trunc((s2.v_avg/s1.v_avg)*100,2) R_PCT
from
--S1-B-----------
(
select
trunc(b_snap_date,'HH') h_date,
sum(snap_value) svalue,
sum(snap_value/snap_secs) v_avg,
sum(snap_value/snap_secs)*v_db_block_size/1024/1024 mb_sec
from
(select
s.INSTANCE_NUMBER,
cast (s.END_INTERVAL_TIME as date) e_snap_date,
cast (s.BEGIN_INTERVAL_TIME as date) b_snap_date,
(cast(s.END_INTERVAL_TIME as date) - cast(s.BEGIN_INTERVAL_TIME as date))*24*60*60 snap_secs,
t.VALUE,
(t.VALUE-LAG (t.VALUE) OVER (ORDER BY s.INSTANCE_NUMBER, s.BEGIN_INTERVAL_TIME)) snap_value
from
DBA_HIST_SNAPSHOT s,
DBA_HIST_SYSSTAT t
where 1=1
and s.SNAP_ID = t.SNAP_ID
and s.DBID = t.DBID
and s.INSTANCE_NUMBER = t.INSTANCE_NUMBER
and s.DBID = (select DBID from V$DATABASE)
and t.STAT_NAME = 'physical reads'
) pr,
(select VALUE v_db_block_size from v$parameter where name = 'db_block_size')
where snap_value > 0
group by trunc(b_snap_date,'HH'),v_db_block_size
) S1,
--S2-B-----------
(
select
trunc(b_snap_date,'HH') h_date,
sum(snap_value) svalue,
sum(snap_value/snap_secs) v_avg,
sum(snap_value/snap_secs)*v_db_block_size/1024/1024 mb_sec
from
(select
s.INSTANCE_NUMBER,
cast (s.END_INTERVAL_TIME as date) e_snap_date,
cast (s.BEGIN_INTERVAL_TIME as date) b_snap_date,
(cast(s.END_INTERVAL_TIME as date) - cast(s.BEGIN_INTERVAL_TIME as date))*24*60*60 snap_secs,
t.VALUE,
(t.VALUE-LAG (t.VALUE) OVER (ORDER BY s.INSTANCE_NUMBER, s.BEGIN_INTERVAL_TIME)) snap_value
from
DBA_HIST_SNAPSHOT s,
DBA_HIST_SYSSTAT t
where 1=1
and s.SNAP_ID = t.SNAP_ID
and s.DBID = t.DBID
and s.INSTANCE_NUMBER = t.INSTANCE_NUMBER
and s.DBID = (select DBID from V$DATABASE)
and t.STAT_NAME = 'physical reads direct'
) pr,
(select VALUE v_db_block_size from v$parameter where name = 'db_block_size')
where snap_value > 0
group by trunc(b_snap_date,'HH'),v_db_block_size
) S2
--S2-E-----------
where 1=1
and s1.h_date = s2.h_date (+)
order by
s1.h_date;

 
Get the Complete
Oracle SQL Tuning Information 

The landmark book "Advanced Oracle SQL Tuning  The Definitive Reference"  is filled with valuable information on Oracle SQL Tuning. This book includes scripts and tools to hypercharge Oracle 11g performance and you can buy it for 30% off directly from the publisher.

 

 

Burleson is the American Team

Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

Verify experience! Anyone considering using the services of an Oracle support expert should independently investigate their credentials and experience, and not rely on advertisements and self-proclaimed expertise. All legitimate Oracle experts publish their Oracle qualifications.

Errata?  Oracle technology is changing and we strive to update our BC Oracle support information.  If you find an error or have a suggestion for improving our content, we would appreciate your feedback.  Just  e-mail:  and include the URL for the page.


                    









Burleson Consulting

The Oracle of Database Support

Oracle Performance Tuning

Remote DBA Services


 

Copyright ? 1996 -  2014

All rights reserved by Burleson

Oracle ? is the registered trademark of Oracle Corporation.


 

  
 

 
 
 
 
 

 
 
 
 
 
 
Oracle training Excel
 
Oracle performance tuning software