Doc ID:  Note:30681.1 
Subject:  EVENT: ADJUST_SCN - Quick Reference 
Type:  REFERENCE 
Status:  PUBLISHED 
 Content Type:  TEXT/PLAIN 
Creation Date:  20-OCT-1997 
Last Revision Date:  04-AUG-2000 
Language:  USAENG 
 

ADJUST_SCN Event
~~~~~~~~~~~~~~~~
*** WARNING ***
   This event should only ever be used under the guidance
   of an experienced Oracle analyst.
   If an SCN is ahead of the current database SCN, this indicates
   some form of database corruption. The database should be rebuilt
   after bumping the SCN. 
****************

    The ADJUST_SCN event is useful in some recovery situations where the
    current SCN needs to be incremented by a large value to ensure it 
    is ahead of the highest SCN in the database. This is typically 
    required if either:
      a. An ORA-600 [2662] error is signalled against database blocks
    or
      b. ORA-1555 errors keep occuring after forcing the database open
         or ORA-604 / ORA-1555 errors occur during database open.
         (Note: If startup reports ORA-704 & ORA-1555 errors together
                then the ADJUST_SCN event cannot be used to bump the
                SCN as the error is occuring during bootstrap.
                Repeated startup/shutdown attempts may help if the SCN
                mismatch is small)
    or
      c. If a database has been forced open used _ALLOW_RESETLOGS_CORRUPTION
         (See <Parameter:Allow_Resetlogs_Corruption> )


    The ADJUST_SCN event acts as described below.

  **NOTE: You can check that the ADJUST_SCN event has fired as it
  	  should write a message to the alert log in the form
	  "Debugging event used to advance scn to %s".
	  If this message is NOT present in the alert log the event
	  has probably not fired.


  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  If the database will NOT open:
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Take a backup.
    You can use event 10015 to trigger an ADJUST_SCN on database open:

	startup mount;

	alter session set events '10015 trace name adjust_scn level 1';

        (NB: You can only use IMMEDIATE here on an OPEN database. If the 
	     database is only mounted use the 10015 trigger to adjust SCN, 
	     otherwise you get ORA 600 [2251], [65535], [4294967295] )

	alter database open;

	If you get an ORA 600:2256 shutdown, use a higher level and reopen.

    Do *NOT* set this event in init.ora or the instance will crash as soon
    as SMON or PMON try to do any clean up. Always use it with the 
    "alter session" command.

  ~~~~~~~~~~~~~~~~~~~~~~~~~~
  If the database *IS* OPEN: 
  ~~~~~~~~~~~~~~~~~~~~~~~~~~
    You can increase the SCN thus:

	alter session set events 'IMMEDIATE trace name ADJUST_SCN level 1';

    LEVEL:  Level 1 is usually sufficient - it raises the SCN to 1 billion
				            (1024*1024*1024)
	    Level 2 raises it to 2 billion etc...

	    If you try to raise the SCN to a level LESS THAN or EQUAL to its
	    current setting you will get <OERI:2256>    - See below.
	    Ie: The event steps the SCN to known levels. You cannot use
		the same level twice.

  Calculating a Level from 600 errors:
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	To get a LEVEL for ADJUST_SCN:

	a) Determine the TARGET scn:
 	    ora-600 [2662]    See <OERI:2662>  Use TARGET >= blocks SCN
	    ora-600 [2256]    See <OERI:2256>  Use TARGET >= Current SCN

  	b) Multiply the TARGET wrap number by 4. This will give you the level 
	   to use in the adjust_scn to get the correct wrap number.
	c) Next, add the following value to the level to get the desired base
	   value as well :

        Add to Level         Base
        ~~~~~~~~~~~~ ~~~~~~~~~~~~
                   0            0
                   1   1073741824
                   2   2147483648
                   3   3221225472