RE: Query Database while shutdown in progress

  • From: "Randy Johnson" <randyjo@xxxxxxxxxxxxx>
  • To: "'oracle-l'" <oracle-l@xxxxxxxxxxxxx>
  • Date: Thu, 28 Jun 2007 18:29:45 -0500

Okay, I stand corrected. That's what happens when you reply from memory. 
 
My bad.
 
 
CHECKPOINT TUNING AND ERROR HANDLING
 
1.  What is a Checkpoint?
   
   A Checkpoint is a database event which synchronizes the modified data
blocks in memory with the datafiles on disk.  It offers Oracle the means for
ensuring the consistency of data modified by transactions.  The mechanism of
writing modified blocks on disk in Oracle is not synchronized with the
commit of the corresponding transactions.
 
    A checkpoint has two purposes: (1) to establish data consistency, and
(2) enable faster database recovery.   How is recovery faster?  Because all
database changes up to the checkpoint have been recorded in the datafiles,
making it unnecessary to apply redo log entries prior to the checkpoint. The
checkpoint must ensure that all the modified buffers in the cache are really
written to the corresponding datafiles to avoid the loss of data
    which may occur with a crash (instance or disk failure).
  
    Oracle writes the dirty buffers to disk only on certain conditions:
 
      - A shadow process must scan more than one-quarter of the
db_block_buffer
         parameter.
      - Every three seconds.                               <----<<
      - When a checkpoint is produced.
 
    A checkpoint is realized on five types of events:
 
      - At each switch of the redo log files.
      - When the delay for LOG_CHECKPOINT_TIMEOUT is reached.
      - When the size in bytes corresponding to :
         (LOG_CHECKPOINT_INTERVAL* size of IO OS blocks)
         is written on the current redo log file.
      -  Directly by the ALTER SYSTEM SWITCH LOGFILE command.
      - Directly with the ALTER SYSTEM CHECKPOINT command.
     
 
    During a checkpoint the following occurs:
     -  The database writer (DBWR) writes all modified database
        blocks in the buffer cache back to datafiles,
     -  Log writer (LGWR) updates both the controlfile and
        the datafiles to indicate when the last checkpoint
        occurred (SCN)
 
 
 
 
 

   _____  

From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx]
On Behalf Of Jared Still
Sent: Thursday, June 28, 2007 1:46 PM
To: randyjo@xxxxxxxxxxxxx
Cc: oracle-l
Subject: Re: Query Database while shutdown in progress



On 6/27/07, Randy Johnson <HYPERLINK
"mailto:randyjo@xxxxxxxxxxxxx"randyjo@xxxxxxxxxxxxx> wrote: 

  
BTW, unless I'm mistaken, checkpoints occur automatically at the least every
3 seconds.
 


Gee, I hope not.

LGWR on the other hand...  :)

-- 
Jared Still
Certifiable Oracle DBA and Part Time Perl Evangelist 



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