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Don Burleson Blog 







Troubleshooting a hung Oracle database

Oracle Database Tips by Donald BurlesonAugust 12, 2015

Oracle hangs only when he is waiting for a resource.  It might be a software resource (a latch or lock), or you could be hung waiting for server resources (CPU, RAM).  Other possibilities for Oracle hanging include:

  • Pathing hang - Make sure that your Oracle environment variables are properly set ($ORACLE_HOME).

  • External hang - The network being down, Kerberos security issues, SSO or a firewall issue can cause an Oracle connection to hang.  One way to test this is to set sqlnet.authentication_services=(none) in your sqlnet.ora file and retry connecting.

  • Listener down/hanging - Start by checking the listener (check lsnrctl stat).  Also, see my notes on diagnosing Oracle network connectivity issues.

  • RAM shortage hang - Over allocation of server resources, usually RAM, whereby there is not enough RAM to spawn another connection to Oracle.

  • Data hanging - It is not uncommon for an end-user session to "hang" when they are trying to grab a shared data resource that is held by another end-user.

 There are several ways to find the root cause of an Oracle hanging issue:

  • oradebug - See below for complete steps for diagnosing a database when you cannot connect to SQL*Plus.

  • Automatic Session History - Check ASH during the hang (if you have purchased the license):

  • Report - Run a couple of STATSPACK or AWR snapshot during the hang and get an elapsed time report.

  • Check for locking - Whenever Oracle is has a session hangs waiting on a resource, this information can be found in the v$session view in the row_wait_file# and row_wait_block#. 

Column host format a6;
Column username format a10;
Column os_user format a8;
Column program format a30;
Column tsname format a12;
   b.machine host,
   b.username username,
   b.osuser os_user,
   b.program program,
   a.tablespace_name ts_name,
   row_wait_file# file_nbr,
   row_wait_block# block_nbr,
   dba_data_files a,
   v$session      b,
   dba_extents    c
see code depot for full scripts
b.row_wait_file# = a.file_id
c.file_id = row_wait_file#
row_wait_block# between c.block_id and c.block_id + c.blocks - 1
row_wait_file# <> 0


Debugging a hung database

Back in Oracle 10g a hung database was a real problem, especially if the DBA could not connect via SQL*Plus to release the source of the hanging.  In these cases, the DBA had few options other than to force-down the instance and warmstart it.

There is a new feature in Oracle 11g SQL*Plus called the "prelim" option. This option is very useful for running oradebug and other utilities that do not require a real connection to the database.

root> sqlplus –prelim



SQL> set _prelim on

SQL> connect / as sysdba

At this point, you are free to run the oradebug commands to diagnose a hung database issue using the new hanganalyze utility:

  1. SQL> oradebug hanganalyze 3
  2. Wait at least 2 minutes to give time to identify process state changes.
  3. SQL> oradebug hanganalyze 3
  4. Open a separate SQL session and immediately generate a system state dump.
  5. SQL> alter session set events 'immediate trace name SYSTEMSTATE level 10';

For more complete details on diagnosing a hung database with oradebug, see the book "Advanced Oracle Utilities: The Definitive Reference".



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