John, I actually have never sat down tested to see when Oracle actually does the redo kick. I do have a OPS and RAC test environment so I may have to see how big the GAP gets before Oracle starts doing log archive switches on the idle node.
The only reason I have heard why Oracle does this kick is to keep on-line redo logs archiving off of idle instances, while other active instances generate redo and archive on-line logs. The intent is to keep archive streams from all participating instances close to each other in time, so that a set of archived logs for all redo streams/instances can easily be identified and managed for backup or recovery. Also used in executing the ALTER SYSTEM ARCHIVE LOG CURRENT command, which is used to cause all instances to archive their current logs. I know Anjo has some detail on this feature. So he may be able to provide more information. Scott --- [EMAIL PROTECTED] wrote: > Thank you Scott for supplying a well-written summary > of SCN usage within OPS > nodes. > > I was interested in "If the gap in the log sequence > gets to big Oracle will > force the idle node to do log switches " > > I was not aware of that and whilst I believe you, I > cannot see what the > reason behind it is. Have you any thoughts on why it > is set like that > I suppose it is quite easy to test if you happen to > have a OPS development > setup handy (which I do not at the moment) > > John > > -----Original Message----- > Sent: 02 July 2002 19:44 > To: Multiple recipients of list ORACLE-L > > > Fawzia, What are you querying to get the SCN > information. I would also be curious where you read > about the recovery issues in OPS environment? Oracle > maintains what is called a snapshot SCN and global > SCN. The snapshot SCN is usually maintained in the > SGA > and the global SCN is maintained by the DLM. All > transactions committed in an OPS environment use the > global SCN and that SCN is always incrementing even > in > if both instances are idle. In an OPS environment it > is possible for multiple transactions on different > nodes to commit with the same SCN but each > transaction > will commit with an SCN that is greater whether > these > transactions are on the same node or different > nodes. > The bottom line is that Oracle basically treats > SCN's > with committed transactions the same whether it is a > single instance or OPS. > > The snapshot SCN is used for queries and the > snapshot > SCN could be older than current or global SCN. This > SCN only effects queries not DML. The only issue > here > is using this snapshot SCN could give read > consistency > on transactions that where committed on the on the > other node. > > an example > > Time 10:00:00 - Node2 issues DML and commits updates > deptno from 10 to 11 committed with current SCN > 1000, > current SCN is now incremented to 1001 > > Time 10:00:01 - Node1 issues a select on deptno and > sees 10 (should see 11 because the transaction was > commmited) because the snapshot SCN is 998 and the > transaction was committed with newer SCN we must > perform a read consistent read. > > PMON is the process that synchs current or global > SCN > with the snapshot SCN and PMON usually does this > every > 3 seconds > > so if you issue the query > Time 10:00:05 (5 seconds after the commit) - Node1 > issues the select on deptno and now sees 11. This is > because PMON sync-ed up the snapshot SCN with the > current or global SCN and now my SCN for my query is > newer than the SCN the transaction you committed > with > so there is no need to perform a CR you can just use > the current version. > > The above issue is really only a problem is a system > if you have a lot of DML on 1 node and a lot of > selects on the other node. It this is an issue you > can > set MAX_COMMIT_PROPAGATION_DELAY=0 and this will > force > the queries to always use the current or global SCN > and now you should see the committed data as soon as > it is committed because both the DML and queries are > use the current or global SCN. > > I find odd that you have SCN of 500 and 2000. The > way > Oracle maintains SCN's in an OPS environment > shouldn't > allow for this large of a gap in SCN's. This looks > more like log sequence numbers not SCN's. If these > are > indeed log sequence then this gap is not an issue. > Recovery is based more on SCN's not log sequence > numbers. > > If the gap in the log sequence gets to big Oracle > will > force the idle node to do log switches and if you > are > archiving you will see archive logs 1 OS block in > size. When Oracle does this you should a KK > "Redo > log Kick" lock being allocated. > > Hope this helps, > > Scott > > --- "Malik, Fawzia" > <[EMAIL PROTECTED]> wrote: > > > > > > Hi gurus, > > > > I have a query..Basically we have an OPS set up > here > > (8.0.6) and I have > > noticed that the scn on node A is 500 and the > scn > on > > node B is 2000. I am > > concerned about this w.r.t recovery- surely > this > > would be an issue ??I > > logged a call with oracle and they said it > wasnt > an > > issue, but then I read > > in a document that it WAS an issue and could > > potentially lose everything > > between 500 abd 2000..and that the workaround > would > > be to add more logs to A > > or to modify the check point interval.... > > > > Please can you advise?? > > > > Rgds > > > > Fawzia > > > > > > > ********************************************************************** > > Information in this email is confidential and > may > be > > privileged. > > It is intended for the addressee only. If you > have > > received it in error, > > please notify the sender immediately and delete > it > > from your system. > > You should not otherwise copy it, retransmit it > or > > use or disclose its > > contents to anyone. > > Thank you for your co-operation. > > > ********************************************************************** > > > > -- > > Please see the official ORACLE-L FAQ: > > http://www.orafaq.com > > -- > > Author: Malik, Fawzia > > INET: [EMAIL PROTECTED] > > > > Fat City Network Services -- (858) 538-5051 > FAX: > > (858) 538-5051 > > San Diego, California -- Public Internet > > access / Mailing Lists > > > -------------------------------------------------------------------- > > To REMOVE yourself from this mailing list, send > an > > E-Mail message > > to: [EMAIL PROTECTED] (note EXACT spelling > of > > 'ListGuru') and in > > the message BODY, include a line containing: > UNSUB > > ORACLE-L > > (or the name of mailing list you want to be > removed > > from). You may > > also send the HELP command for other > information > === message truncated === __________________________________________________ Do You Yahoo!? Sign up for SBC Yahoo! Dial - First Month Free http://sbc.yahoo.com -- Please see the official ORACLE-L FAQ: http://www.orafaq.com -- Author: Scott INET: [EMAIL PROTECTED] Fat City Network Services -- (858) 538-5051 FAX: (858) 538-5051 San Diego, California -- Public Internet access / Mailing Lists -------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB ORACLE-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).
