Monday, October 28, 2013

Database -- RMAN Multiplexing

Multiplexing term for  Rman is defined basically for the representation of Rman 's ability to read multiple files concurrently and write to a single output file by multiplexing its inputs.. (except Image copies, as multiplexing is against the logic of image copying...)
Multiplexing is on the basis of channels..

The level of multiplexing is determined by the following factor.

The level of multiplexing is the lesser of the MAXOPENFILES(default 8) and the FILESPERSET (default 64) in each backup set.

As an example, you want to backup a database which has 100 datafiles, with one channel. FILEPERSET is set to 60 ..  So the minimum of the FILEPERSET and MAXOPENFILES (default 8 ) is 8. So multiplexity level is 8.
Thus, one channel simultaneously writes blocks from 8 datafiles into each backup piece.

So depending on the multiplexing level, Rman  Input Buffer Size's are differ.
When the level is less than or equal to 4, RMAN allocates 16 buffers of size 1..
When 8 > the level  > 4, RMAN allocates a variable number of disk buffers of size 512 KB so that the total buffer size for all the input files is less than 16 MB.
When the level > 8 , RMAN  allocatest disk buffers of 128 KB for each file.. (For each input file 512)

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