[Networker] Need help with Oracle backup channels
2012-12-07 15:04:18
Hi,
Looking around on Google, I've found a number of Oracle documents that
advise/warn against using media manager multiplexing for Oracle RMAN
backups. They suggest that only RMAN should be doing any multiplexing.
If I understand this correctly, they're saying that we should *not* be
using more than one channel for the RMAN backup script when sending the
data to a sequential device, i.e. tape. I suppose one exception would be
if we had multiple nodes, each writing to a different tape drive, or
maybe we were duplicating it to other tape drives, but that's not the
case. I could find no information in the EMC NMO document regarding
this, 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 Oracle
backup pool and one clone pool. We use the NMO to do the backups. Our
full backups are not that big, so we can ride on a single tape for quite
a while. We do clone the backups. Nothing else writes to this pool other
than the Oracle backups. When we originally set this up, the RMAN script
used four channels. As a result, the backups always consist of four save
sets, followed by the control file. We have successfully tested
recovery. The parallelism for the drives can easily handle the number of
channels we're currently using. As a result, the backups always write to
a single tape, so it's not like we have the channels set so high that it
exceeds the parallelism on the drive and forces another tape to be
loaded 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 single
channel instead. In this case, I guess we'd end up with a single save
set. That would make it easier to query (mminfo), but I'm concerned that
this might slow the drive speed way down, and the fulls might take a lot
longer. They also point out that while multiplexing the data on the
media 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 that
multiple channels creates two or more save sets, which then interleaves
the data on the tape thus making it take longer to recover it?
This was the only thing that seemed obvious but I could find nothing
specific.
2. What are others doing? Should we change the RMAN script to use only
one channel to SBT_TAPE?
In other words, keep it to the fewest number of channels that we can
possibly live with in terms of drive speed.
3. I don't understand how multiplexing is done in RMAN versus whatever
the default would be. I've read through a bunch of stuff, and it's very
confusing and nebulous.
The idea of multiple save sets being interleaved to a single tape when
running normal NetWorker backups makes sense to me. But if you have a
single channel, is RMAN wrapping multiple files together when it uses
that channel? With standard NetWorker flat file backups, are multiple
files being woven together when a single save set is running? Just not
sure how to correlate these two (RMAN's definition of multiplexing
versus NetWorker's), and what setting or variables we need to set on the
Oracle side to do this. I can work closely with the DBA to make any
necessary changes. That's not an issue.
Thanks.
George
--
George Sinclair
Voice: (301) 713-3284 x210
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