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Performance Troubleshooting Series : Understanding your database load from Metrics – Part 2

Hello,

This is the second part of the series of Performance Troubleshooting, Whilst the first one is from AWR/Statspack tables.

And what if , if you don't have both in your environment, That made me to write up this second post.

Those who does not know about the metrics tables a short notes here, After that I will continue this post,

From 10g onwards, we have metrics collected as like v$sysstat and I believe this is the basis of AWR related stuff.

v$sysstat - RAW Live metrics (Contains 679 database performance metrics)

v$sysmetric-RAW Live metrics (Contains 205 metrics derived from sysstat or somewhere else)

v$sysmetric_history- Last one hour (Contains 10249 metrics for each metric in v$sysmetric for about 15sec or 60 sec interval)

v$sysmetric_summary-Last one hour (contains 158 metrics with aggregated values of history of one hour for each metric, i.e max,min,avg)

dba_hist* - Last seven days collection.

Note: the counts are as of 11gr2

Well, the question is what this metrics provides us?

Wonderful wealthy information, like How many reads per sec, CPU per sec, service times, network volume per sec, almost all information how your database is performing.

I hope the above information is enough for now, lets get back to post, This metrics are collected irrespective you are in standard edition/enterprise edition and also and your statistics level set to typical.  I have myself verified the dba_hist* view which points to WRH* tables and derived from v$sysmetric views, and how many times I query those tables dba_features_usage does not reflect the same. I hope you also test to use this before using the following when you don't have AWR license.

For Previous & Historical : db_load_by_metrics.sql

Update: 10-Oct-2015 for rac instances (thanks to venkat) -  db_load_by_metrics_rac.sql

For Current & Running Instance:-  db_load_by_metrics_current.sql

 

The screenshot of the report and the explanation of each column and how to interpret the report. (Click to enlarge)

http://db.geeksinsight.com/wp-content/uploads/2015/10/db_load_by_metrics.jpg

Now lets interpret the report,

Question

Column

to Verify

Additional

Related Columns

to Verify

 Description or notes while troubleshooting

check DB load

at so and so time

 AAS

DB Time,

Redo,

PReads,

PWrites,

Sessions

As you see AAS is high on certain period that is where the database is loaded, average active sessions

should be compared with your CPU_Count for the database and if its higher than that, then your database is loaded

Evidently, you can find additional information why the DB is loaded, is it excessive

Commits, DB Block Changes i.e DML, Reads, Writes, Locks, Wait in other columns which can easily spotted

Sessions/Process

Reached max limit

 Sess/Logon  

By looking at this columns you can identify the sessions/process spike in database and can suggest whether there is

logon spike during that snap
DB is slow

AAS

SQL Resp Time

CPU, DB Time,

Reads/Writes/

PGA/Temp

SharedPool/HParses

Commits/DB BlockChanges

As told AAS is the basic metric you can see really the DB is slow or not, if under control below than your CPU count then it may else impacting your DB (i.e Single Statement but the DB is okay)

CPU , DB Time high numbers than other snapshot tells it is loaded

PReads/Writes tells whether any I/O issue at that snap

SPoolfree%/HParse/Execs , tells during that snap is there any excessive executions than before, or any excessive hard parses than before, If so any you may see the spoolfree% is drop down and that is evident.

Further, excessive commits and many db block changes can also hamper the performance with sessions waiting high log file sync which can be seen by the respective columns

I/O Spikes

PReads

PWrites

Total Preads/Writes

Executions

User Calls

Ftablescans

When you receive any I/O Spikes issue, check the columns to determine any I/O Spike during that snap period,

and the contributing factors can be many executions, many calls or Ftable scans

If its more calls/executions than before that means application is doing some nasty or running more load

If its ftablescans more than earlier, could be statistics issues, or again someone running full counts(*) or sorts or doing nasty
Check DB response time

SQL Resp

Time

 User Calls

This question is commonly asked by Application/Business teams always, DB response

To derive DB response time , simple metric can be SQL Service Response time per user call.

As you see above sometime the SQL Service Response time is 3 i.e in centiseconds and apparently there are many user calls

So you can clearly state that many user calls are degrading the sql service response time.

If not there may other factors like CPU waiting, More I/O etc, other columns may help to understand who is contributing to it.
unable to allocate temp Temp Used PGA You can check during the period how much temp is used for each snap, if PGA is exhausting then it may use over temp so may be its worth to check the pga column is PGA is normally used. However, although PGA is undercontrol excessive sorts may be cause of it. But this column provides you the temp utilisation
Out of Process Memory PGA  

Every hour snap of PGA utilisation by sessions which gives a glance of total pga allocation to database.

If you see more utilisation worth while to consider the pga advisory
Log File Sync commits

redo mb per sec

db block changes

 When you see sessions waiting for high log file sync, you may check any excessive commits during period and relavant db block changes and redo generated MB per sec, which gives you clue about the same
Check Locks during that time Lock Waits

Db block changes

commits

 When you have been asked to verify any locks during that time, the lock waits columns tells how many waits for locks during that snap is happening and if you see more yes, there is contention for lock , you need to go further troubleshoot with ASH to identify which sessions causing locks etc.
Network Spikes NWorkMB   Is the database or user experiencing delays due to more network utilisation, that way also you can check in network utilized mb per sec.

So, after reading this, the best possible questions and answers are derived from single query above rather generating multiple reports etc.

Again this report was derived from metric tables.

Questions & Suggestions are welcome. Hope this helps

-Geek DBA

October 9th, 2015 | Tags: , , , | Category: 12c Database

5 comments to Performance Troubleshooting Series : Understanding your database load from Metrics – Part 2