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Parallelism
Number of active channels defines degree of parallelization
Common guideline:
- One channel per output device (tape or disk)
- Can deviate, especially for disk
- Test!
- See our test, next slide…
Channels and sets
- Each channel creates 1 or more backup sets
- Too many sets can be hard to manage
Notes:
When using RMAN, the number of allocated channels (manually or configured) determines the degree of parallelization. Parallelism in RMAN is the number of server processes (i.e. channels) that are reading and writing Oracle data. If one channel is allocated, the degree of parallelization is one; if two channels are allocated the degree of parallelization is two, etc.
The general guideline is to allocate one channel per output device. This is because allocating more channels than output devices will often just create a bottleneck by sending more data (issuing more I/O requests) than the device can handle, thus the additional channels/processes will wait. While one channel per output device is common, this is not always the case. Backup to disk devices that have high transfer rates might warrant more channels. The best advice is to test. See the next page for an example of how to test the performance of an RMAN operation.
A point to remember is that each channel allocated to a backup creates a backup set. Too many backup sets can become a management headache.