If you have multiple disks dedicated to Use them for Backups, why to wait until the first disk get full in order to use the second one?. With RMAN you can use them at the same time using the spreading feature of RMAN. In this article I will show you how to spread a database backup across 2 disks:

OPTION 1

We will use the following parameters and they have a default value:

RMAN> show device type;

RMAN configuration parameters for database with db_unique_name ORCL are:
CONFIGURE DEVICE TYPE DISK PARALLELISM 1 BACKUP TYPE TO BACKUPSET;

RMAN> show channel;

RMAN configuration parameters for database with db_unique_name ORCL are:
RMAN configuration has no stored or default parameters

Configuring 2 Channels:

RMAN> CONFIGURE CHANNEL 1 DEVICE TYPE DISK FORMAT '/home/oracle/backup1/%U';

new RMAN configuration parameters:
CONFIGURE CHANNEL 1 DEVICE TYPE DISK FORMAT '/home/oracle/backup1/%U';
new RMAN configuration parameters are successfully stored

RMAN> CONFIGURE CHANNEL 2 DEVICE TYPE DISK FORMAT '/home/oracle/backup2/%U';

new RMAN configuration parameters:
CONFIGURE CHANNEL 2 DEVICE TYPE DISK FORMAT '/home/oracle/backup2/%U';
new RMAN configuration parameters are successfully stored

Confirming the parameter:

RMAN> show channel;

RMAN configuration parameters for database with db_unique_name ORCL are:
CONFIGURE CHANNEL 1 DEVICE TYPE DISK FORMAT '/home/oracle/backup1/%U';
CONFIGURE CHANNEL 2 DEVICE TYPE DISK FORMAT '/home/oracle/backup2/%U';

Create a database Backup:

RMAN> backup database;

Confirming the result:

[oracle@a1 ~]$ ls -ltr /home/oracle/backup*
/home/oracle/backup2:
total 0

/home/oracle/backup1:
total 924292
-rw-r----- 1 oracle asmadmin 156942336 Jan 22 18:28 a9ptcja0_1_1
-rw-r----- 1 oracle asmadmin 36438016 Jan 22 18:28 aaptcja5_1_1
-rw-r----- 1 oracle asmadmin 368164864 Jan 22 18:28 abptcja9_1_1
-rw-r----- 1 oracle asmadmin 361185280 Jan 22 18:28 acptcjac_1_1
-rw-r----- 1 oracle asmadmin 19587072 Jan 22 18:28 adptcjaf_1_1
-rw-r----- 1 oracle asmadmin 3203072 Jan 22 18:28 aeptcjai_1_1
[oracle@a1 ~]$

As you can see the backup wasn't spread across "backup1" and "backup2", the reason it is because we are using Parallelism=1. Let me show you what oracle says about this:

"The PARALLELISM setting is not constrained by the number of specifically configured channels. For example, if you back up to 20 different tape devices, then you can configure 20 different SBT channels, each with a manually assigned number (from 1 to 20) and each with a different set of channel options. In such a situation, you can set PARALLELISM to any value up to the number of devices, in this instance 20.

RMAN always numbers parallel channels starting with 1 and ending with the PARALLELISM setting.

RMAN always allocates the number of channels specified in PARALLELISM, using specifically configured channels if you have configured them and generic channels if you have not. Note that if you configure specific channels with numbers higher than the parallelism setting, RMAN will not use these channels."

http://docs.oracle.com/cd/B28359_01/backup.111/b28270/rcmconfa.htm#BRADV89456

Let's Change the Parallelism to 2

RMAN> CONFIGURE DEVICE TYPE DISK PARALLELISM 2;

using target database control file instead of recovery catalog
old RMAN configuration parameters:
CONFIGURE DEVICE TYPE DISK PARALLELISM 1 BACKUP TYPE TO BACKUPSET;
new RMAN configuration parameters:
CONFIGURE DEVICE TYPE DISK PARALLELISM 2 BACKUP TYPE TO BACKUPSET;
new RMAN configuration parameters are successfully stored

RMAN> show device type;

RMAN configuration parameters for database with db_unique_name ORCL are:
CONFIGURE DEVICE TYPE DISK PARALLELISM 2 BACKUP TYPE TO BACKUPSET;

Creating a backup: 

RMAN> backup database;

Confirming the result:

[oracle@a1 ~]$ ls -ltr /home/oracle/backup*
/home/oracle/backup2:
total 391828
-rw-r----- 1 oracle asmadmin 36438016 Jan 22 18:31 ahptcjg1_1_1
-rw-r----- 1 oracle asmadmin 361185280 Jan 22 18:31 ajptcjg4_1_1
-rw-r----- 1 oracle asmadmin 3203072 Jan 22 18:31 alptcjgc_1_1

/home/oracle/backup1:
total 532472
-rw-r----- 1 oracle asmadmin 156942336 Jan 22 18:31 agptcjg1_1_1
-rw-r----- 1 oracle asmadmin 368164864 Jan 22 18:31 aiptcjg4_1_1
-rw-r----- 1 oracle asmadmin 19595264 Jan 22 18:31 akptcjgc_1_1
[oracle@a1 ~]$

OPTION 2

There is another way to Spread the backups allocating channels in a RMAN run block, let me show you:

Checking the current parameters and its values: I left the parallelism set to 2 since we already know that with parallelism=1 it will not work.

RMAN> show channel;   <<<<---- The channels were cleared. 

RMAN configuration parameters for database with db_unique_name ORCL are:
RMAN configuration has no stored or default parameters

RMAN> show device type ;

RMAN configuration parameters for database with db_unique_name ORCL are:
CONFIGURE DEVICE TYPE DISK PARALLELISM 2 BACKUP TYPE TO BACKUPSET;

Allocating channels manually:

RMAN> run{
2> ALLOCATE CHANNEL c1 DEVICE TYPE DISK FORMAT '/home/oracle/backup1/%U'; <<- This does the magic to /backup1
3> ALLOCATE CHANNEL c2 DEVICE TYPE DISK FORMAT '/home/oracle/backup2/%U'; <<- This does the magic to /backup2
4> backup database;  <<--Creating a Backup
5> }

Checking the result:

[oracle@a1 ~]$ ls -ltr /home/oracle/backup*
/home/oracle/backup2:
total 391828
-rw-r----- 1 oracle asmadmin 36438016 Jan 22 18:50 avptckin_1_1
-rw-r----- 1 oracle asmadmin 361185280 Jan 22 18:50 b1ptckiq_1_1
-rw-r----- 1 oracle asmadmin 3203072 Jan 22 18:50 b3ptckj2_1_1

/home/oracle/backup1:
total 532464
-rw-r----- 1 oracle asmadmin 156942336 Jan 22 18:50 auptckin_1_1
-rw-r----- 1 oracle asmadmin 368164864 Jan 22 18:50 b0ptckiq_1_1
-rw-r----- 1 oracle asmadmin 19587072 Jan 22 18:50 b2ptckj2_1_1
[oracle@a1 ~]$