When backing up to tape, the guideline is that the number of tape channels equals the number of tape devices divided by the number of duplexed copies (see Example 2-8). If RMAN is writing to a striped file system or an ASM disk group, however, then multiple channels can improve performance. When making backups to disk, the guideline is to allocate one channel for each output device (see Example 2-7). Allocating multiple channels simultaneously allows a single job to read or write multiple backup sets or disk copies in parallel, with each channel operating on a separate backup set or copy.
You can control the degree of parallelism within a job by the number of channels that you allocate.
You can allocate up to 255 channels each channel can read up to 64 files in parallel. You can override automatic channel configurations by manually allocating channels within a RUN command, but you cannot use BACKUP DEVICE TYPE or RESTORE DEVICE TYPE to use automatic channels after specifying manual channels with ALLOCATE CHANNEL.
Automatic channels apply to any RMAN job in which you do not manually allocate channels. Manually allocated channels are distinct from automatically allocated channels specified with CONFIGURE.
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