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 Oracle Memory Advisors (SGA and PGA)
Oracle Tips by Burleson Consulting

Advanced Oracle Utilities: The Definitive Reference by Rampant TechPress is written by the top Oracle database experts (Bert Scalzo, Donald Burleson, and Steve Callan).  The following is an excerpt from the book.

These advisors are straightforward. There is some debate as to the validity of the findings they produce, both in a tabular format showing increase or decrease in reads as memory is increased or decreased. The tabular/graph-like data can be queried directly via canned scripts. An example of what the SGA Advisor generates in a graph is shown in Figure 8.8.

Figure 8.8:  Memory Advice

Sample queries for SGA target advice include those which replicate what a graph shows and what the history has been at a certain size (does the SGA size advice vary during the day?) . In this example, it can be seen that the advice has held steady at 508MB for the snapshots shown.

 

select snap_id, sga_size

from dba_hist_sga_target_advice

 where sga_size_factor = 1

order by snap_id asc

 

  SNAP_ID   SGA_SIZE

---------- ----------

         4        508

         5        508

         6        508

         7        508

         8        508

         9        508

        10        508

        11        508

 

The relevant data dictionary views have ADVICE in them and it is a trivial matter to query those based on a snapshot ID to see what the advice was at that time.

Undo Advisor

The Automated Undo Advisor is also simple to access and query. This advisor evaluates the space needed to ensure a guaranteed retention time. The retention time comes into play when considering flashback. Since flashback goes back in time, a sufficient amount of undo space, which supports the amount of time, needs to be set aside for the undo tablespace. Undo data cannot be overwritten in order for flashback to work.

Figure 8.9:  Undo Tablespace Growth Line

Calculating the current optimal setting can be done via a query like that below:

 

col "ACTUAL UNDO SIZE [MByte]" for 999999999

col "UNDO RETENTION [Sec]" for a20

col "OPTIMAL UNDO RETENTION [Sec]" for 999999999

SELECT d.undo_size/(1024*1024) "ACTUAL UNDO SIZE [MB]",

       SUBSTR(e.value,1,25) "UNDO RETENTION [Sec]",

       (TO_NUMBER(e.value) * TO_NUMBER(f.value) *

       g.undo_block_per_sec) / (1024*1024)

      "NEEDED UNDO SIZE [MB]"

  FROM (

       SELECT SUM(a.bytes) undo_size

         FROM v$datafile a,

              v$tablespace b,

              dba_tablespaces c

        WHERE c.contents = 'UNDO'

          AND c.status = 'ONLINE'

          AND b.name = c.tablespace_name

          AND a.ts# = b.ts#

       ) d,

      v$parameter e,

       v$parameter f,

       (

       SELECT MAX(undoblks/((end_time-begin_time)*3600*24))

         undo_block_per_sec

         FROM v$undostat

       ) g

 WHERE e.name = 'undo_retention'

  AND f.name = 'db_block_size';

 

The results for a small system running on a laptop:

 

ACTUAL UNDO SIZE [MB] UNDO RETENTION [Sec] NEEDED UNDO SIZE [MB]

--------------------- -------------------- ---------------------

                  190 900                                43.5234375 

MTTR  Advisor

The MTTR Advisor (Mean Time To Recover Advisor) is enabled when two parameters are set to certain values: STATISTICS_LEVEL and FAST_START_MTTR_TARGET. The required values for STATISTICS_LEVEL are TYPICAL or ALL. When the FAST_START_MTTR_TARGET is set to a non-zero value, combined with STATISTICS_LEVEL being appropriately set, the advisor is started.

 

Several other parameters should not be set because they will interfere with mechanisms used to meet the value or setting of FAST_START_MTTR_TARGET.  These other parameters are:

  • FAST_START_IO_TARGET

  • LOG_CHECKPOINT_INTERVAL

  • LOG_CHECKPOINT_TIMEOUT

The possible settings for FAST_START_MTTR_TARGET range between 0 and 3600 seconds. Any setting above 3600 defaults back to 3600.

 

Due to what the parameter represents, the two-phase instance recovery process of roll forward and roll back, some amount of time is going to be spent on startup to apply committed transactions not yet written to datafiles and to roll back the uncommitted ones. Therefore, a setting of 0 and obviously anything less than that is not realistic. Whether the time is 10 seconds, 30 seconds, or 2 minutes, examples depend, to a degree, on how the redo logs are sized.

 

So, in conjunction with the redo log size and the time it takes for the database to start up regardless of what the logs have to deal with, a query from V$INSTANCE_RECOVERY can be used to calibrate the optimal or practical time for the mean time to recovery.  The results of the following query show the system is pretty much calibrated.

 

SELECT TARGET_MTTR, ESTIMATED_MTTR

FROM V$INSTANCE_RECOVERY;

 

TARGET_MTTR ESTIMATED_MTTR

----------- --------------

         37             35

 

Another ADVICE type view, V$MTTR_TARGET_ADVICE, shows statistics and advisories collected by this advisor. If the advisor is not started, the following graph will not appear under the Instance Recovery section of the Recovery Settings page in OEM.

 

Figure 8.10:   MTTR Advice

     


For more details on Oracle utilities, see the book "Advanced Oracle Utilities" by Bert Scalzo, Donald K. Burleson, and Steve Callan.

You can buy it direct from the publisher for 30% off directly from Rampant TechPress.

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