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4 Replies Latest reply: Aug 30, 2013 9:19 AM by blavoie RSS

OCP: Question regarding critical files, control files and crash

blavoie Newbie
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Hello,

 

I'm currently studying for OCP certification exam 1z0-053.

 

For this I bought the All-in-One Exam Guide from Oracle Press (http://www.mcgrawhill.ca/professional/products/9780071629188/)

 

While studying, I like to test, prove things and simulate operation to get more familiar as possible with important concepts.

 

The study guide covers the fact that when a critical file is damaged or lost, the instance will terminate. Listed critical files are : any copy of controlfile, a datafile that is part of the SYSTEM tablespace, a datafile that is part of the current undo tablespace.

 

As said here is the quote:

Some files are critical. Damage to a critical file will mean that the database instance will terminate if it is open, and cannot be reopened until the damage is repaired.

[...]

Virtually the moment the damage occurs, the instance will terminate. As ever, the DBA’s first reaction to a crashed instance should be to attempt a startup. This will fail, in NOMOUNT mode, with an appropriate error message. The alert log will state which controlfile copy is missing, and also—in the section listing the nondefault initialization parameters—how many controlfile copies there actually are, and where they are.

 

 

In a end-of-chapter self test there is a question:

1. Loss of which of these files will cause an open database to crash? (Choose all correct answers.)

A. A multiplexed controlfile

B. A multiplexed online log file

C. A multiplexed archive log file

D. An active undo tablespace datafile

E. An active temporary tablespace tempfile

F. A datafile from the SYSAUX tablespace

G. A datafile from the SYSTEM tablespace

H. A datafile containing critical user data

 

[...]

 

Response:

1.   A, D, and G. Damage to any controlfile copy will terminate the instance, as will damage to the critical tablespaces: SYSTEM and the current undo tablespace.

 

So, under 11.2.0.3 when I delete or corrupt a control file copy, the instance isn't crashing and the database continue to be working. Only alert log is regularly notifynig the controlfile missing:

 

Fri Aug 30 10:16:02 2013

Errors in file /u01/app/oracle/diag/rdbms/ocp1/ocp1/trace/ocp1_m000_26767.trc:

ORA-00210: cannot open the specified control file

ORA-00202: control file: '/u01/app/oracle/oradata/ocp1/control03.ctl'

ORA-27041: unable to open file

Linux-x86_64 Error: 2: No such file or directory

Additional information: 3

 

 

I'm not really undestanding.

Do I need to stick on what is said in the book or the experienced facts?

 

Thanks

Bruno

  • 1. Re: OCP: Question regarding critical files, control files and crash
    matthew_morris Expert
    Currently Being Moderated

    The information in the Exam Guide is not correct as written.  The Oracle instance won't terminate on the loss of a single control file.  The Oracle documentation on managing control files is here:

     

    Managing Control Files

     

    It states:  "If a control file is damaged due to a disk failure, the associated instance must be shut down. Once the disk drive is repaired, the damaged control file can be restored using the intact copy of the control file from the other disk and the instance can be restarted."

     

    'Must be shut down' means that the instance has become unstable.  Unstable instances act differently depending on the situation so the book does not (can not) document the specific behavior that might occur.  While loss of a control file means that the DBA should abort the instance, this is a very different thing from the instance terminating on its own.

     

    The ultimate source is always the Oracle documentation. I checked the errata page at McGraw-Hill for that book.  I don't see that one listed.  Submit this to them and get your name listed on the page as a contributor.

  • 2. Re: OCP: Question regarding critical files, control files and crash
    blavoie Newbie
    Currently Being Moderated

    Thanks,

     

    I hope that the official exam questions answer are not as imprecise as that.

     

    By knowing these facts from the official documentation, is the control file always considered as a critical file? I'll suppose that yes.

     

    In my testings, I discovered that an can't shutdown cleanly the instance with a missing controlfile. We must use shutdown abort and recreate a valid controlfile from a multiplexed copy. Also, impossible to connect to rman when any controlfile missing/damaged.

  • 3. Re: OCP: Question regarding critical files, control files and crash
    matthew_morris Expert
    Currently Being Moderated

    I hope that the official exam questions answer are not as imprecise as that.

     

    By knowing these facts from the official documentation, is the control file always considered as a critical file? I'll suppose that yes.

     

    In my testings, I discovered that an can't shutdown cleanly the instance with a missing controlfile. We must use shutdown abort and recreate a valid controlfile from a multiplexed copy. Also, impossible to connect to rman when any controlfile missing/damaged.

     

    The exam questions and answers tend to be very clear cut in my experience.  That said, there are sometimes misleading wrong answers (known as red herrings), especially on the expert-level exams designed to pull you away from the correct answer.

     

    The control file is definitely a critical file.  What is incorrect is the implication that losing one copy of a multiplexed file causes the instance to terminate.

     

    The link to the documentation I sent specifically notes that you must perform a shutdown abort.  This is because both a shutdown immediate and shutdown normal must write to the control files as part of the shutdown process.  Only an abort skips this step.

  • 4. Re: OCP: Question regarding critical files, control files and crash
    blavoie Newbie
    Currently Being Moderated

    Thanks a lot for your time....

    Bruno

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