Thanks very much to all those who responded- it was very useful
-----Original Message-----
Sent: 03 July 2002 19:19
To: Multiple recipients of list ORACLE-L
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
- John.Hallas_at_vodafone.co.uk 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"
> <Fawzia.Malik_at_bskyb.com> 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
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> >
>
> >
> > --
> > Please see the official ORACLE-L FAQ:
> > http://www.orafaq.com
> > --
> > Author: Malik, Fawzia
> > INET: Fawzia.Malik_at_bskyb.com
> >
> > Fat City Network Services -- (858)
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Received on Fri Jul 05 2002 - 07:23:29 CDT