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Hung Database Analysis Tips

Expert Oracle Database Tips by Donald BurlesonConsulting

March 9, 2015

 

Question:  :  :  I had a hung database last week, and after three minutes and hundreds of hung users, I killed the Oracle background processes and re-started the database.  I got no trace or dump file.  How can I get diagnostic information from a hung instance before I re-start the database? 

I cannot inconvenience my end-user community for capturing diagnostics for more than a few seconds.  Does Oracle have a way to dump an SGA and diagnostics prior to bouncing a hung database?

Answer: :  The SQL*Plus utility has an undocumented argument invoked as (-prelim) to allow you to connect to a hung Oracle database that is not accepting new connections. The sqlplus prelim option allows you to access a hung database to quickly run an oradebug trace report. 

Also see onnecting to a hung instance using -prelim option. 

This prelim utility is not the same as the hang manager (hangman) utility, a tool introduced in Oracle 11g to detect database bottlenecks.  An extension of the dba_waiters and dba_blockers views, the hangman tables have a "hang chain" that allow the DBA to find the source of hangs, such as the "deadly embrace" where mutually blocking locks or latches hang a process.       

Note:  Using the sqlplus -prelim should be done very quickly since you will want to quickly re-start your instance so as to minimize the downtime of your user community.  You never want to keep hundreds of users waiting, and a 2 minute bounce (nuke and re-start) is always faster than trying to resurrect connectivity to a hung instance.  

Here is how to quickly generate a hang analysis immediately prior to re-starting a hung production instance:

/u01/app/oracle/bin>sqlplus system/mypass -prelim /nolog
SQL*Plus: Release 10.2.0.2.0 - Production  

> oradebug setmypid
Statement processed.  

> oradebug hanganalyze 12
Hang Analysis in /u01/app/oracle/admin/mysid/udump/mysid_ora_1234.trc

You can also use the -prelim argument to capture Automated Session  History (ASH) data for a hung database:

/u01/app/oracle/bin>sqlplus system/mypass -prelim /nolog
SQL*Plus: Release 10.2.0.2.0 - Production  

>oradebug setmypid
Statement processed.  

>oradebug dump ashdump 10
Statement processed.  

>alter session set events 'immediate trace name ashdump level 10';

 
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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.

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