Motive of checkpoint and SCN using with DBWr and LOGWr processes ??

Question

What checkpoint has to do with log writer process i am not getting exactly ?..
Like see i fire 1 update query and apparently it is generating some redo blocks which in turn will come to my redo log files now in tihs whole cycle where the checkpoint will occur and why??
1)My update query
2) take locks
3)generate redo
4)generate undo
5)Blocks are modified but they are still in redo log buffer...
now this blocks eventually comes to redo log files in this whole way where check pointing take place and why??
checkpoint also takes place when Datablocks are flushed to datafiles again the same reason why??
Same way around the same question the what checkpointing has to do with DBWr process also i am not clear...
Apart from this whole picture SCN is generated when user issue comitts..and we can say SCN can be used to identify that transaction is committed or not.?
So what is the motive of SCN to update in Control file...MAy b to get the latest transaction committed..??
Sorry one thread with so much questionss..but this all things are creating a fuzzy picture i want to make it clear thnx for your help in advance ..
I read documentation but they havent mentioned in depth for checkpointing..??
THANKS
Kamesh
Edited by: 851733 on Apr 12, 2011 7:57 AM

Answer 1

851733 wrote:
What checkpoint has to do with log writer process i am not getting exactly ?..And where exactly did you read that it has anything to do with it? How did you come up to the relation anyways? The time checkpointing would come into the play with the log files would be when there would be a log switch and this would induce a checkpoint, causing/triggering the DBWR to write the dirty buffers to the datafile and allowing the redo log group to be reused. That's about it.
Like see i fire 1 update query and apparently it is generating some redo blocks which in turn will come to my redo log files now in tihs whole cycle where the checkpoint will occur and why??
1)My update query
2) take locks
3)generate redo
4)generate undo
5)Blocks are modified but they are still in redo log buffer...
now this blocks eventually comes to redo log files in this whole way where check pointing take place and why??Read my reply above, at the time of writing the change vectors in the log file, there won't be any checkpointing coming into the picture.
checkpoint also takes place when Datablocks are flushed to datafiles again the same reason why??Wrong, the checkpoint event would make the dirty buffers written to the dataflile. Please spend some time reading the Backup and Recovery guide and in that, instance recovery section. In order to make sure that there wont be much time spent in the subsequent instance recovery, it would be required to move the dirty buffers periodically to the data file. THis would be caused by the incremental checkpoint . Doing so would constantly write the content out of the buffer cache thus leaving few buffers only as the candidate for the recovery in the case of the instance crash.
Same way around the same question the what checkpointing has to do with DBWr process also i am not clear...Read the oracle documentation's Concept guide again and again as long as it doesn't start getting in sync in with you(and it may take time). One of the events , when DBWR writes , is the occurance of the Checkpoint. Whenever there would be a checkpoint, the DBWR would be triggered to write the buffers (dirty) to the datafile.
Apart from this whole picture SCN is generated when user issue comitts..and we can say SCN can be used to identify that transaction is committed or not.? Not precisely since there would be a SCN always there , even when you query , for that too. But yes, with the commit, there would be a commit SCN that would be generated including a commit flag entered in the redo stream telling that the transaction is finally committed. The same entry would be updated in the transcation table as well mentioning that the tranaction is committed and is now over.
So what is the motive of SCN to update in Control file...MAy b to get the latest transaction committed..??Where did you read it?
Sorry one thread with so much questionss..but this all things are creating a fuzzy picture i want to make it clear thnx for your help in advance ..
I read documentation but they havent mentioned in depth for checkpointing..??
Read the book, Expert one on one by Tom Kyte and also, from documentation, version 11.2's Concept guide. These two would be more than enough to get the basics correct.
HTH
Aman....