Paul Sherman wrote 'I tried an 'alter system check datafiles'
I had never come across that command before and looked at the documentation which states that 'in a distributed database system, such as an Oracle Parallel Server environment, updates an instance's SGA from the database control file to reflect information on all online datafiles'. Global actions against all nodes , local against the current instance only.
I did check out the OPS install/config guide but did not found anything more in there. I was failing to understand in which circumstances the command would be required, knowing that we are running a number of OPS set ups and I had never used it. Looking further on Metalink I found the following note (Note:1071756.6)
When running "ALTER SYSTEM CHECK DATAFILES" it delivers a "STATEMENT PROCESSED"
but there is no recorded activity. There is no alert file even though the book says there is.
You can do an ALTER SYSTEM CHECK DATAFILES if the database is only mounted.
But the command does not return an error if a datafile is missing.
Solution Description:
The ALTER SYSTEM CHECK DATAFILES command was designed to solve a
particular problem of file accessibility. This most often has to do
with OPS, and disks that are shared among several machines. There are
cases where a disk may be accessible on one machine, but not another,
because of configuration or OS problems.
Imagine that you have a datafile that is marked OFFLINE. If the disk
where that datafile lives is accessible to a particular machine in an
OPS installation, then you can bring it ONLINE. If, however, another
machine can't get to that disk at the time you bring it ONLINE, then
that instance won't be able to verify the datafile. Hence in that
instance, the controlfile will have the datafile listed as ONLINE, but
the datafile still will not be properly accessible and entered in the SGA.
Explanation:
The ALTER SYSTEM CHECK DATAFILES command was designed to allow
you to correct the problem from the OS perspective and make the disk
accessible. This would then trigger the instance into recognizing this fact,
verifying the datafile, and thereby making it available to Oracle in that instance.
So the command does not quite do what might be implied by the current
documentation. It does not verify access to all online datafiles,rather,
it only looks at those that are now online, but which were not previously
verified. Once a datafile has been verified, it is considered verified for all
time, until you OFFLINE the file or until the database is dismounted in that
instance.
I hope someone founds that useful, at least I have learnt a bit more by checking it out.
Thanks Paul for mentioning it, if only in passing.
John
Oracle DBA
MMO2
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Received on Wed Jan 30 2002 - 05:01:10 CST
