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Asked: 2011-02-02 10:56:22 +0800 CST

Oracle 10gR2 RMAN: Channel allocation

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Oracle 10gR2, Win2003.

I know enough about RMAN to perform basic functions. I'm trying to learn more.

We have an RMAN backup process that is used to duplicate a database nightly. The backup is generated on a single disk. The script allocates 2 channels for that backup, both pointing to the same directory. It works fine, generating 2 backupsets (backupsets? I think. I'm a bit confused about terminology: a backup consists of one or more backupsets?). Anyway, the duplicate process picks up these backupsets and restores a copy of the database from them. I noticed today that it allocates 3 channels, but only two appear to be used. Is that because the backup process only used 2? Which brings me to the major point: given my configuration, is there any advantage to allocating more channels for backup and restore? This is an 80GB database, and the restores takes over 2 hours to complete which seems slow to me - I can do a hot backup in that amount of time, and I thought RMAN had performance advantages over simple scripted backups.

To summarize my questions:

  1. Does an RMAN backup consist of one or more backupsets?
  2. Can a restore operation only use as many channels as were used in the backup?
  3. Is there any advantage to increasing the number of channels for both given a backup to a single disk?
oracle oracle10g rman

1 Answers

    1. The RMAN backupset style (there is also an imagecopy backup style) consists of multiple files. When you look at the set with "list backupset" in rman you will see all of the files associated with the single backupset.
    2. The channels are just a degree of parallelism (which also enabled access to other non-disk devices for receiving backups). When reading or writing backupset files, the number of channels makes no difference with respect to functionality, only speed and resource usage.
    3. For reads from a single disk, backing up to a single disk, you may not find very much performance increase from multiple channels, but this is one of those YMMV situations.
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