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…Multiplexing
Pros
- Keep output device streaming
- “Feed” it lots of data
- Especially helpful with incremental where few blocks have changed
Cons
- Can impede throughput if input is already striped on disk
- Can increase restore time
- Must read blocks of file “X” to restore file “Y”
Notes:
A benefit of multiplexing can be realized when the output device is not receiving enough data to keep it streaming. If the tape device is continually starting, stopping, rewinding and starting again, it is not streaming. THIS IS AN ISSUE YOU WILL WANT TO ADDRESS. This can happen, for example, if we are conducting an incremental backup but only a few blocks have changed. RMAN still has to read the entire input file(s), but only writes a few blocks – thus the output device is often found idle, waiting for data. Multiplexing files together increases the chances that the incremental backup will find changed blocks to write out – and keep the output device streaming.
Multiplexing has a dark side. Too much multiplexing, especially in environments where the data is already striped across many disks (preferably in 1MB stripes), will decrease throughput. Additionally, if a backup piece has 10 datafiles multiplexed within it, yet only 1 needs to be restored, your forcing RMAN to read entirely through a large file containing all 10 files during the restore operation.