[Networker] Need help with Oracle backup channels

Hi,

Looking around on Google, I've found a number of Oracle documents thatadvise/warn against using media manager multiplexing for Oracle RMANbackups. They suggest that only RMAN should be doing any multiplexing.If I understand this correctly, they're saying that we should *not* beusing more than one channel for the RMAN backup script when sending thedata to a sequential device, i.e. tape. I suppose one exception would beif we had multiple nodes, each writing to a different tape drive, ormaybe we were duplicating it to other tape drives, but that's not thecase. I could find no information in the EMC NMO document regardingthis, and perhaps that's not surprising since that's not an Oracle document.

We have several tape drives in a single tape library and one Oraclebackup pool and one clone pool. We use the NMO to do the backups. Ourfull backups are not that big, so we can ride on a single tape for quitea while. We do clone the backups. Nothing else writes to this pool otherthan the Oracle backups. When we originally set this up, the RMAN scriptused four channels. As a result, the backups always consist of four savesets, followed by the control file. We have successfully testedrecovery. The parallelism for the drives can easily handle the number ofchannels we're currently using. As a result, the backups always write toa single tape, so it's not like we have the channels set so high that itexceeds the parallelism on the drive and forces another tape to beloaded in a second drive. A full currently takes about 3 hours.Incrementals are very fast, however.

I'm wondering if we should change the RMAN script to use a singlechannel instead. In this case, I guess we'd end up with a single saveset. That would make it easier to query (mminfo), but I'm concerned thatthis might slow the drive speed way down, and the fulls might take a lotlonger. They also point out that while multiplexing the data on themedia manager's end may speed up backup times, it's not recommended.Instead, RMAN should be set to do the multiplexing.

1. What is the main concern that Oracle raises? Is it the fact thatmultiple channels creates two or more save sets, which then interleavesthe data on the tape thus making it take longer to recover it?

This was the only thing that seemed obvious but I could find nothingspecific.

2. What are others doing? Should we change the RMAN script to use onlyone channel to SBT_TAPE?

In other words, keep it to the fewest number of channels that we canpossibly live with in terms of drive speed.

3. I don't understand how multiplexing is done in RMAN versus whateverthe default would be. I've read through a bunch of stuff, and it's veryconfusing and nebulous.

The idea of multiple save sets being interleaved to a single tape whenrunning normal NetWorker backups makes sense to me. But if you have asingle channel, is RMAN wrapping multiple files together when it usesthat channel? With standard NetWorker flat file backups, are multiplefiles being woven together when a single save set is running? Just notsure how to correlate these two (RMAN's definition of multiplexingversus NetWorker's), and what setting or variables we need to set on theOracle side to do this. I can work closely with the DBA to make anynecessary changes. That's not an issue.


Thanks.

George

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George Sinclair
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