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Friday, April 9, 2010
RMAN Backup multiplexing
When creating backup sets, RMAN can simultaneously read multiple files from disk
and then write their blocks into the same backup set. For example, RMAN can read
from two datafiles simultaneously, and then combine the blocks from these datafiles
into a single backup piece. The combination of blocks from multiple files is called
backup multiplexing. Image copies, by contrast, are never multiplexed.
RMAN multiplexing is determined by several factors. For example, the FILESPERSET
parameter of the BACKUP command determines how many datafiles to put in each
backup set. The MAXOPENFILES parameter of ALLOCATE CHANNEL or CONFIGURE
CHANNEL defines how many datafiles RMAN can read from simultaneously. 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