RMAN spread a backup job between many RAC instances in parallel to increase throughput

There are two options to allocate RMAN channels on different RAC instances to increase the throughput.

I will start with the option, that assures all RAC instances get one channel. Regarding the other option, it does load balance but in a random way, so with a small number of channels, you may see that all of them are allocated in one instance. So you decide which option is better for you.

The test is done on a 2-node cluster.

1. Configure parallelism and two channels in RMAN. Indicate a connect string, one per instance:

$ RMAN target /
RMAN> CONFIGURE DEVICE TYPE disk PARALLELISM 2;
RMAN> CONFIGURE CHANNEL 1 DEVICE TYPE DISK CONNECT 'sys/Oracle123@ORCL1 as sysdba';
RMAN> CONFIGURE CHANNEL 2 DEVICE TYPE DISK CONNECT 'sys/Oracle123@ORCL2 as sysdba';

2. Define ORCL1 and ORCL2 aliases on each database node under $ORACLE_HOME/network/admin/tnsnames.ora:

ORCL1=
(DESCRIPTION=
           (ADDRESS= (PROTOCOL=tcp) (HOST=rac1.example.com) (PORT=1522))
           (CONNECT_DATA =
              (SERVER = DEDICATED)
              (SERVICE_NAME = orcl)
           )
      )
 
ORCL2=
(DESCRIPTION=
           (ADDRESS= (PROTOCOL=tcp) (HOST=rac2.example.com) (PORT=1522))
           (CONNECT_DATA =
              (SERVER = DEDICATED)
              (SERVICE_NAME = orcl)
           )
      )

Please note, in my case 1522 is a local listener port.

3. Run backup:

RMAN> backup database;
 
Starting backup at 22-MAR-22
using target database control file instead of recovery catalog
allocated channel: ORA_DISK_1
channel ORA_DISK_1: SID=507 instance=orcl1 device type=DISK
allocated channel: ORA_DISK_2
channel ORA_DISK_2: SID=753 instance=orcl2 device type=DISK
channel ORA_DISK_2: SID=753 instance=orcl2 device type=DISK
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00001 name=+DATA/ORCL/DATAFILE/system.257.1098460673
input datafile file number=00004 name=+DATA/ORCL/DATAFILE/undotbs1.259.1098460743
channel ORA_DISK_1: starting piece 1 at 22-MAR-22
channel ORA_DISK_2: starting full datafile backup set
channel ORA_DISK_2: specifying datafile(s) in backup set
input datafile file number=00003 name=+DATA/ORCL/DATAFILE/sysaux.258.1098460717
input datafile file number=00005 name=+DATA/ORCL/DATAFILE/undotbs2.265.1098461311
input datafile file number=00007 name=+DATA/ORCL/DATAFILE/users.260.1098460743

As you see, two files were backed up by the 1^st channel (1^st instance) and the other three files by the 2^nd channel (2^nd instance).

Now let’s explain another possible variant:

1. Configure one TNS string with load balance parameter:

ORCL_BALANCE=
     (DESCRIPTION=
           (TRANSPORT_CONNECT_TIMEOUT=3) (RETRY_COUNT=6)(LOAD_BALANCE=on)
           (ADDRESS= (PROTOCOL=tcp) (HOST=rac1.example.com) (PORT=1522))
           (ADDRESS= (PROTOCOL=tcp) (HOST=rac2.example.com) (PORT=1522))
           (CONNECT_DATA =
              (SERVER = DEDICATED)
              (SERVICE_NAME = orcl)
           )
      )

Both node addresses are defined and Oracle will pick each address randomly.

2. Configure parallelism and one channel with ORCL_BALANCE string:

Please note, I did this test case on the same server where I’ve already defined CHANNEL 1 and CHANNEL 2, so I had to clear them:

RMAN> CONFIGURE CHANNEL DEVICE TYPE DISK clear;
RMAN> CONFIGURE CHANNEL 1 DEVICE TYPE DISK clear;
RMAN> CONFIGURE CHANNEL 2 DEVICE TYPE DISK clear;

Define channel and parallelism:

RMAN> CONFIGURE DEVICE TYPE disk PARALLELISM 2;
RMAN> CONFIGURE CHANNEL 1 DEVICE TYPE DISK CONNECT 'sys/Oracle123@ORCL_BALANCE as sysdba';

RMAN> backup database;
 
Starting backup at 22-MAR-22
allocated channel: ORA_DISK_1
channel ORA_DISK_1: SID=752 instance=orcl1 device type=DISK
allocated channel: ORA_DISK_2
channel ORA_DISK_2: SID=138 instance=orcl1 device type=DISK

Both channels were allocated on orcl1. Try one more time, or better configure parallelism 3, CHANNEL parameter is already defined and it is permanent until changed:

RMAN>  CONFIGURE DEVICE TYPE disk PARALLELISM 3;
 
RMAN> backup database;
 
Starting backup at 22-MAR-22
allocated channel: ORA_DISK_1
channel ORA_DISK_1: SID=752 instance=orcl1 device type=DISK
allocated channel: ORA_DISK_2
channel ORA_DISK_2: SID=138 instance=orcl1 device type=DISK
allocated channel: ORA_DISK_3
channel ORA_DISK_3: SID=753 instance=orcl2 device type=DISK

As you see it really did a random choice. But two channels were allocated on the 1st node and the last one on the 2nd node.

I think here random algorithm is not a good option, but you better know which variant is appropriate in your case.