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Rman Parallelize Datafile

February 17, 2012

Parallel Backup of the Same Datafile:

You probably already know that you can parallelize the backup by declaring more than one channel so that each channel becomes a RMAN session. However, very few realize that each channel can back up only one datafile at a time. So even through there are several channels, each datafile is backed by only one channel, somewhat contrary to the perception that the backup is truly parallel.

In Oracle Database 11g RMAN, the channels can break the datafiles into chunks known as “sections.” You can specify the size of each section. Here’s an example:

RMAN> run {
2>      allocate channel c1 type disk format ‘/backup1/%U’;
3>      allocate channel c2 type disk format ‘/backup2/%U’;
4>      backup
5>      section size 500m
6>      datafile 6;
7> }
This RMAN command allocates two channels and backs up the users’ tablespace in parallel on two channels. Each channel takes a 500MB section of the datafile and backs it up in parallel. This makes backup of large files faster.

When backed up this way, the backups show up as sections as well.

RMAN> list backup of datafile 6;

<snipped>

List of Backup Pieces for backup set 901 Copy #1
BP Key  Pc# Status      Piece Name
——- — ———– ———-
2007    1   AVAILABLE   /backup1/9dhk7os1_1_1
2008    2   AVAILABLE   /backup2/9dhk7os1_1_1
2009    3   AVAILABLE   /backup1/9dhk7os1_1_3
2009    3   AVAILABLE   /backup2/9dhk7os1_1_4

Note how the pieces of the backup show up as sections of the file. As each section goes to a different channel, you can define them as different mount points (such as /backup1 and /backup2), you can back them to tape in parallel as well.

However, if the large file #6 resides on only one disk, there is no advantage to using parallel backups. If you section this file, the disk head has to move constantly to address different sections of the file, outweighing the benefits of sectioning.

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