Data Guard High SCN Oracle Database Tips by Donald BurlesonDecember 9, 2015

Oracle Data Guard - See high SCN

olumn Type Format a12
Column Status Format a40 Trunc
Column High_Scn Format 999999999
Select
   Type,
   Status,
   High_Scn
   See code depot for full scripts
From
   V$LOGSTDBY;

A sample output from the script is shown below:

TYPE         STATUS                                     HIGH_SCN
------------ ---------------------------------------- ----------
COORDINATOR  ORA-16116: no work available
READER       ORA-16127: stalled waiting for additiona
BUILDER      ORA-16127: stalled waiting for additiona    2187196
PREPARER     ORA-16116: no work available                2187196
ANALYZER     ORA-16117: processing                       2187196
APPLIER      ORA-16117: processing                       2187156

The HIGH_SCN column will give an estimate of the progress of the individual process. From the above output, it is apparent that the APPLIER process is falling behind.

Statistics Related to Log Mining Engine and Memory Usage

A dynamic performance view called v$logstdby_stats, provides useful information about the log mining engine and memory usage during the SQL apply operations. Understanding the data presented by this view will help isolate the performance related issues. This view will not have any statistics when the SQL apply service is not running.

Query the view, logstdby_stats.sql, from the code depot and congregate the results into four bands. These bands are: LogMiningEngine; Memory Wait; Unsuccessful Handling of Low Memory; and Total Pageouts. Based on the output of this script, the determination can be made as to whether the log-mining engine or the shared_pool_size on a logical Oracle instance need to be tuned.

            logstdby_stats.sql

-- *************************************************
-- Copyright © 20012 by Rampant TechPress
-- *************************************************

-- Script to gather statistics from v$logstdby_stats DPT
Set linesize 82
Column logMiningEngine format 9999999999
Column MemWait format 9999999999
Column UnsuccHandleLowMem format 99999999
Column PageOuts format a10

Select
   a.Value logMiningEngine,
   b.Value MemWait,
   c.Value UnsuccHandleLowMem,
   d.Value PageOuts

From
   (Select
       sum(Value) Value
    From
       V$LOGSTDBY_STATS
    See code depot for full scripts
   (Select
       sum(Value) Value   
From
       V$LOGSTDBY_STATS
        From
       V$LOGSTDBY_STATS v1,
       V$LOGSTDBY_STATS v2
   (Select
       Value
    From
       V$LOGSTDBY_STATS;

A sample output of the script is shown below:

LOGMININGENGINE     MEMWAIT UNSUCCHANDLELOWMEM PAGEOUTS
--------------- ----------- ------------------ ---------
           1234         263                  8 7
 

A high value in LOGMININGENGINE suggests there are plenty of transactions ready for the APPLIER process, so the log-mining engine is not a bottleneck. The other three columns in the output of the logstdby_stats.sql script present data related to memory usage from the shared pool of the logical standby database. More about tuning the log apply service will be provided in next the section.

The above text is an excerpt from: the book "Oracle Data Guard", ISBN 0-9745993-8-7, by Bipul Kumar.  Order it directly from the publisher link and save 30%.