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Multiplexing & Memory
Level of multiplexing controls memory request / allocation
PGA used if BACKUP_DISK_IO_SLAVES = 0
SGA used if BACKUP_DISK_IO_SLAVES > 0
Notes:
The level of multiplexing has an effect on the size and number of memory buffers allocated. When less than or equal to 4, RMAN creates 16 1MB buffers, which work really nicely with the 1MB stripes recommended in the SAME methodology.
This memory is usually allocated from the PGA (because disks usually support asynchronous I/O by default and the BACKUP_DISK_IO_SLAVES parameter is set to 0). Using the Oracle9i automatic PGA tuning feature will help here. This feature is enabled by setting initialization parameters WORKAREA_SIZE_POLICY=AUTO and PGA_AGGREGATE_TARGET=“n”, where n is a number, in bytes, that you want the database to set as the upper limit for all combined work areas. Oracle will dynamically allocate memory to active queries, attempting to keep the total at or below the upper limit. Use the PGA advisory to tune the PGA_AGGREGATE_TARGET parameter.