Emre Baransel - Oracle Blog

I wanted to summirize some complicated RMAN concepts, using the "11gR2 Backup and Recovery User's Guide" definitions.

The number of channels available for use with a device when you run a command determines whether RMAN reads from or write to this device in parallel while performing the command. When the work is done in parallel, the backup of the files is done by more than one channel. Each channel may back up more than one file, but unless a multisection backup is performed, no file is backed up by more than one channel.

The number of channels available for a device type when you run a command determines whether RMAN reads or writes in parallel. As a rule, the number of channels used in executing a command should match the number of devices accessed. Thus, for tape backups, allocate one channel for each tape drive. For disk backups, allocate one channel for each physical disk, unless you can optimize the backup for your disk subsystem architecture with multiple channels. Failing to allocate the right number of channels adversely affects RMAN performance during I/O operations.

RMAN always allocates the number of channels specified in PARALLELISM, using specifically configured channels if you have configured them and generic channels if you have not. If you configure specific channels with numbers higher than the parallelism setting, then this setting prevents RMAN from using them.

An RMAN backup set in which each backup piece contains a file section, which is a contiguous range of blocks in a datafile. A multisection backup set contains multiple backup pieces, but a backup set never contains only a part of a datafile.You create a multisection backup by specifying the SECTION SIZE parameter on the BACKUP command. An RMAN channel can process each file section independently, either serially or in parallel. Thus, in a multisection backup, multiple channels can

back up a single file.

When creating backup sets, RMAN can simultaneously read multiple files from disk and then write their blocks into the same backup set.

Note: If RMAN creates a multisection backup of a datafile, then the datafile is not multiplexed with any other datafile or file section.

The basic multiplexing algorithm is as follows:

■ Number of files in each backup set

This number is the minimum of the FILESPERSET setting and the number of files read by each channel. The FILESPERSET default is 64.

■ The level of multiplexing

This is the number of input files simultaneously read and then written into the same backup piece. The level of multiplexing is the minimum of MAXOPENFILES and the number of files in each backup set. The MAXOPENFILES default is 8.

If FILESPERSET is lower then the level of multiplexing, then multiplexing will be FILESPERSET value.

RMAN multiplexing of backup sets is different from media manager multiplexing.

Caution: Oracle recommends that you never use media manager multiplexing for RMAN backups.