Oracle RAC question

DB: 11.2.0.3, db size: 21 TB (allocated), used: 7 GB, 3 nodes cluster, almost more than 12 services are running, but they have placed only one service on one node means:

1. service_a is only running on node1
2. service_b is on node2
3. service_c is on node3

when I asked it was told that because most of the activity is INSERT only and if service will be running on multiple nodes, there will be a cache fusion wait or interconnect wait.

the configuration is something like this:

for service_a:
Preferred instances: orclprdi1
Available instances: orclprdi2,orclprdi3

My question, is this correct understanding that if there is one service doing mostly INSERTs, we should assign them to only one node, not 2 or all the 3, if we do so, then we might encounter INTERCONNECT waits or cache fusion waits.

I'm not that strong on RAC, that's why putting this question here ... please assist/guide/advise ... much appreciated and thanks in advance.

Also looking for some advise on this allocation & used space, there is huge difference and due to this backup is taking for ever, looking for some expert thoughts there too.

Best regards.
Mushfique KhanDirector OperationsAsked:
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Mark GeerlingsDatabase AdministratorCommented:
Normally in RAC, selects, updates and deletes are more likely to cause interconnect or cache fusion waits than inserts.  But, inserts could also cause these waits in a RAC environment if these insert transactions are usually small (like just a record or two or three).  If the inserts are usually for more records than fit into a single database block, then they aren't as likely to cause interconnect waits.  

Indexes on the tables where the inserts happen could also contribute to interconnect waits, if these inserts are directed to different nodes in the cluster.

RMAN seems to be quite intelligent these days in skipping over allocated but unused database blocks.  I was concerned when our Oracle11.2 database grew from 7.5Tb to 11.5 Tb over a weekend recently when we implemented an upgrade that required quite a bit of space.  Then, after the upgrade, much of that new space was released (only inside the data files, not from the O/S or storage system perspective).  I was pleasantly surprised to see that our full RMAN backups after the upgrade take almost no more time than before the upgrade, even though our total database size is now 50% larger than before the upgrade.

Which tool do you use for backups?  Is that Oracle's RMAN, or something else?

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Mushfique KhanDirector OperationsAuthor Commented:
thanks markgeer, we are using RMAN with NetBackup, and writing directly to tape, and our last backup completed with errors took 70 hours, where as you can see the used size of the db.

Please advise, in your opinion how to handle this situation?
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