Wednesday, July 17, 2019

How to understand AWR report in Oracle11g?

Let we discuss about How to understand AWR report in oracle11g?
* We already know the steps to generate AWR reports , but after you did this how can you read it .
To understand AWR report:
To check "DB Time"

Snap Id Snap Time Sessions Cursors/Session
Begin Snap: 1235 23 May 2017 10:30 191 6.7
End Snap: 1236 23 May 2017  11:31    173 7.4
Elapsed: 59.23 (mins)
DB Time: 710.73 (mins) 

In above scenario " DB time " metric is higher than elapsed time so it meaning session are waiting for something
Note:if the Dbtime is higher than elapsed time so the session must be waiting for something

To View Instance Efficiency

 Instance Efficiency Percentages (Target 100%)
Buffer Nowait %: 100.00 Redo NoWait %: 100.00
Buffer Hit %: 98.67 In-memory Sort %: 100.00
Library Hit %: 98.60 Soft Parse %: 99.69
Execute to Parse %: 5.26 Latch Hit %: 99.31
Parse CPU to Parse Elapsd %: 12.78 %Non-Parse CPU: 99.10 
Note: As per the thumb rule, Instance Efficieny Percentages should be ideally above 90%.

To Review Shared Pool Statistics

 Shared Pool Statistics
Begin End
Memory Usage %: 83.49 80.93
% SQL with executions>1: 42.46 82.96
% Memory for SQL w/exec>1: 47.77 81.03 
In above the memory Usage of shared pool statistics has been shown here now it seems 83 %
If suppose its higher than 90 % the conflict in the shared pool

To View Top 5 Events

 Top 5 Timed Events
Event Waits Time(s) Avg Wait(ms) % Total Call Time Wait Class
db file sequential read 4,076,086 28,532 7 66.9 User I/O
CPU time 11,214 26.3
Backup: sbtbackup 4 4,398 1,099,452 10.3 Administrative
log file sync 37,365 2,421 65 5.7 Commit
log file parallel write 37,928 1,371 36 3.2 System I/O 
Here, the significant wait is the db file sequential read which contributes to 67% of DB Time.

Then , SQL Statistics can be checked.

SQL Statistics
SQL ordered by Elapsed Time
SQL ordered by CPU Time
SQL ordered by Gets
SQL ordered by Reads 
SQL Statistics section would have commonly the above four sections.
Each section shows the list of SQLs based on the order of the respective metric.
For example, SQL ordered by Elapsed Time section shows the list of SQLs in the order of the Elapsed Time.
High resource consuming SQLs can be spotted out and meant for tuning.
Note: All the above four sections of SQL Statistics show the list of SQLs in descending order.
i.e, For ex: Highest elapsed time is shown as first.

Then comes the IO Stats section.
This shows the IO Statistics for each tablespaces in the database.

Note:As the thumb rule, the Av Rd(ms) [Average Reads in milliseconds] should not cross beyond 30, add myself(not greater that 30) which is considered to be IO bottleneck.

Tablespace IO Stats
ordered by IOs (Reads + Writes) desc 

Tablespace Reads     Av Reads/s Av Rd(ms) Av Blks/Rd Writes Av Writes/s Buffer Waits Av Buf Wt(ms)
TEMP        3,316,082 933         4.91      1.00              28,840 8 0 0.00
DATA1      520,120     146 16.06 1.21 185,846 52 902 13.00
DATA3      93,411       26 42.82 2.98 13,442 4 16 23.13
DATA2      98,171       28 91.97 7.97 5,333 2 325 34.89 

In the above example, the Av Rd(ms) is high in all tablespaces indicating the IO contention.

To verfiy Advisory Statistics

This section shows the following:-

Buffer Pool Advisory
PGA Aggr Summary
PGA Aggr Target Stats
PGA Aggr Target Histogram
PGA Memory Advisory
Shared Pool Advisory
SGA Target Advisory
Streams Pool Advisory
Java Pool Advisory 
It is very commonly used to check the advisories for the most important SGA structures like shared pool, buffer cache etc and PGA.

Then finally, init.ora Parameters is shown which shows the list of parameters set at instance level.

init.ora Parameters
All the above said sections except the DB Time can be checked from Statspack report also.
The statspack snapshots are not generated automatically as in AWR.It has to be generated during the problem period as follows:-
Take 2 snapshots between 60 minutes interval during the problem and generate the statspack report

exec statspack.snap
wait for 60 minutes
exec statspack.snap

Please run $ORACLE_HOME/rdbms/admin/spreport.sql
and specify BEGIN and END ID's of the snapshots taken during the problem.

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