1 2 Previous Next 18 Replies Latest reply: Feb 17, 2009 8:41 AM by Aman.... RSS

    dbwr for redo log buffer.

    685267
      hi,

      Is there any situation, when dbwr writes anything in log buffer other than writing into data files.
        • 1. Re: dbwr for redo log buffer.
          Pavan Kumar
          Hi,

          Can you re-phrase your Question or Query...!! Bit hard to Understand.

          - Pavan Kumar N
          • 2. Re: dbwr for redo log buffer.
            591309
            Never ever.
            In computers,things are done only as per programmed.
            DBWR has no relation to redo log buffer and LGWR has no relation with buffer cache.
            • 3. Re: dbwr for redo log buffer.
              Pavan DBA
              hi dbwr process is having only one way action and its job is to write dirty blocks from db buffer cache to db files. log buffer cache has no relation with dbwr.
              • 4. Re: dbwr for redo log buffer.
                Aman....
                user_7000006 wrote:
                hi,

                Is there any situation, when dbwr writes anything in log buffer other than writing into data files.
                No, the dbwr is not going to read from the log buffer. Yes, it may happen that it can ping to lgwr or vice versa, but it won't read through log buffer and write to the log files.

                That said, its always better if you come up with some reference of any site/book/paper where you have read it.At times, things taken out of the context mean differently and that may lead to a different answer for a query.
                HTH
                Aman....
                • 5. Re: dbwr for redo log buffer.
                  Timur Akhmadeev
                  Well, never say never...
                  SQL> select sid from v$session s, v$process p where s.program like '%DBW%' and p.addr = s.paddr;
                   
                         SID
                  ----------
                         662
                   
                  SQL> select name, value from v$sesstat s, v$statname n where s.statistic# = n.statistic#
                    2  and name like 'redo%' and sid = 662
                    3  /
                   
                  NAME                                                                  VALUE
                  ---------------------------------------------------------------- ----------
                  redo synch writes                                                         0
                  redo synch time                                                           0
                  redo blocks read for recovery                                             0
                  redo entries                                                        3763645
                  redo size                                                         422412184
                  redo buffer allocation retries                                        20052
                  redo wastage                                                              0
                  redo writer latching time                                                 0
                  redo writes                                                               0
                  redo blocks written                                                       0
                  redo write time                                                           0
                  redo log space requests                                                3415
                  redo log space wait time                                                  0
                  redo log switch interrupts                                                0
                  redo ordering marks                                                       1
                  redo subscn max counts                                                    5
                   
                  16 rows selected
                  • 6. Re: dbwr for redo log buffer.
                    26741
                    The "redo blocks read for recovery" isn't done by DBWR.
                    • 7. Re: dbwr for redo log buffer.
                      Timur Akhmadeev
                      Yes, but what about 'redo size'? In what DBWR activities redo is being generated? Maybe it is blocks cleanout?
                      • 8. Re: dbwr for redo log buffer.
                        26741
                        Yes, block cleanout DOES generate redo. But it is LGWR that writes the redo that is generated.
                        • 9. Re: dbwr for redo log buffer.
                          Aman....
                          Timur Akhmadeev wrote:
                          Yes, but what about 'redo size'? In what DBWR activities redo is being generated? Maybe it is blocks cleanout?
                          Yes in the block clean out , there would be redo generation. DBWR does use an IPC connetion with LGWR but that doesn't mean that it will write from the LOg buffer to the log files, for that , there is a never.

                          Aman....
                          • 10. Re: dbwr for redo log buffer.
                            Timur Akhmadeev
                            Probably I misunderstood OP's question. Sorry.
                            • 11. Re: dbwr for redo log buffer.
                              Aman....
                              No probs!
                              Aman....
                              • 12. Re: dbwr for redo log buffer.
                                Jonathan Lewis
                                Timur Akhmadeev wrote:
                                Well, never say never...
                                redo entries                                                        3763645
                                redo size                                                         422412184
                                redo buffer allocation retries                                        20052
                                To allow for the newer "two-pass recovery" the database writer writes "block written records" (BWR) to the log buffer. This is just a short note for each block written which comprises the block address and the block SCN. Since these BWRs are not critical data (i.e. if you lose one it slows down recovery, but doesn't make it fail) DBWR doesn't send a synch message to the log writer to get them written to disc, they just go with the next log write.


                                Regards
                                Jonathan Lewis
                                http://jonathanlewis.wordpress.com
                                http://www.jlcomp.demon.co.uk

                                "For every expert there is an equal and opposite expert"
                                Arthur C. Clarke

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                                • 13. Re: dbwr for redo log buffer.
                                  26741
                                  We learn something new every day.
                                  • 14. Re: dbwr for redo log buffer.
                                    Pavan Kumar
                                    Hi,

                                    Jonathan Can you get little more insight in to that topic. I will be thankful to you. That's why I request to prepare one seminar in INDIA for us. we wil be lucky..!!

                                    @Hemanth, you are right. :-)

                                    - Pavan Kumar N
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