RMAN Performance Maladies

We have just implemented RMAN against our largest Siebel implementation (don't laugh - but it's less then 1/2 terrabyte). We did so because now that we have upgraded to Oracle 10gR2 (10.2.0.2) we can take advantage of flash recovery features and full backup compression similar to which we achived with our old backup product (BMC Sqlbacktrack). We customarily take full backups nightly, as per our SLA, and at this point, incrementals are not an option. However, the actual runtime to complete a full database backup with RMAN compared to SqlBackTrack has gone from 30 minutes to over 4 1/2 hours!

I have seen anecdotal reports of long runtimes for full RMAN backups on this list, but I've seen an overwhelming number of guffaws from the more elite and experienced poster indicating that this must be user error or some other database performance problem that any competent DBA should easily be able to solve without posting to this list. However, even after opening an SR with Oracle support, who seems to believe our runtime is in the expected range, we still believe something can be done to overcome the huge IO spike and slow performance we are witnessing during the backup, and we are not that experienced with RMAN from a performance and tuning perspective, and would appreciated anyone's insights or experiences.

My question is - has anyone actually run RMAN against a large (> 200g) database right out of the box without tuning?

Has anyone spent a great deal of time tuning?

We are running 4 channels, have an 8 cpu Sunsparc solaris with 16g of memory, and have experimented with recommended parameter settings for maxsetsize and maxfilesperset without success. Any information or assistance or comment would be appreciated, thanks in advance!

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