Sundaramurthy Blog

September 27, 2010

RMAN Buffer size

Filed under: RMAN Internals — sundar5 @ 11:34 pm

RMAN has two packages to controle entire backup and restore activities .  It is  avaialble under $ORACLE_HOME/rdbms/admin directory


Package names are:

1. dbms_backup_restore

2. dbms_rcvman


Number of Files Read Per Channel Buffer Size
Multiplexing 4 Each buffer    = 1MB, total buffer size for channel is up to 16MB.
Number of buffers per file will depend on the number of files.
4 >Multiplexing 8 Each buffer = 512KB, total buffer size for channel is up to 16MB.
Numbers of buffers per file will depend on number of files.
Multiplexing > 8                Each file will have 4 512KB buffers.

The number and size of the buffers used for each RMAN job can be found in the


Oracle Note:

If restoring to ASM you may find setting the following two parameters useful:


The default is 16 for _BACKUP_KSFQ_BUFCNT and 1 MB for _BACKUP_KSFQ_BUFSZ.
These defaults are acceptable for ASM diskgroups with fewer than 32 disks and when ASM stripe size is 1 MB otherwise to set:

_BACKUP_KSFQ_BUFCNT to the number of disks in the ASM disk group and _BACKUP_KSFQ_BUFSZ to the size of the ASM stripe.

The number and size of input buffers can be changed by using the undocumented parameters _backup_ksfq_bufcnt and _backup_ksfq_bufsz. When using these parameters to increase backup I/O throughput, it is recommended that _backup_ksfq_bufsz be first set to the stripe size of the files being read and to test the effect that setting has on performance.

Controlefile section:

(size = 1108, compat size = 1108, section max = 50, section in-use = 0,
last-recid= 0, old-recno = 0, last-recno = 0)
(extent = 1, blkno = 283, numrecs = 50)


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