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 1st channel (1st instance) and the other three files by the 2nd channel (2nd 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.