About RMAN Channels

Automatic and Manual Channel Allocation

You can use the automatic channel allocation feature to configure a set of persistent, automatic channels for use in all RMAN sessions. You can use the manual channel allocation feature to specify channels for commands used within a RUN block.

RMAN allocates automatic channels according to the settings in these commands:

• CONFIGURE DEVICE TYPE ... PARALLELISM

• CONFIGURE DEFAULT DEVICE TYPE

• CONFIGURE CHANNEL

For example, you can issue the following commands at the RMAN prompt:

# since you do not manually allocate channels, RMAN uses preconfigured channels
BACKUP DATAFILE 3;
RESTORE TABLESPACE users; 

When you run a command that requires channels, and no channels have been allocated using the ALLOCATE command, RMAN automatically allocates channels according to values set with the CONFIGURE command in the following cases:

• You use commands such as BACKUP, RESTORE, or DELETE outside of a RUN block.

• You use commands within a RUN block but do not allocate any channels within the RUN block.

You can override automatic channel allocation settings by manually allocating channels within a RUN block. Manual channels always override automatic channels. For example, you override automatic channel allocation when you issue a command as follows:

RUN 
{
  ALLOCATE CHANNEL c1 DEVICE TYPE sbt;
  BACKUP DATABASE PLUS ARCHIVELOG;
}

RMAN optimizes automatic channel allocation by leaving automatic channels allocated so long as each new command requires exactly the same channel configuration as the previous command. For example, RMAN can use the same preallocated channels for the following series of commands:

BACKUP DATAFILE 1;
BACKUP CURRENT CONTROLFILE;
BACKUP ARCHIVELOG ALL;

If you issue a command such as ALLOCATE or CONFIGURE, then RMAN automatically releases the preallocated channels.

Automatic Channel Device Configuration and Parallelism

The CONFIGURE DEVICE TYPE ... PARALLELISM command specifies the number of automatic channels to allocate for a specified device type. For example, if you configure parallelism to 3 for a device type, then RMAN allocates three channels for the device type when using automatic channels.

You can change a parallelism setting by issuing another CONFIGURE DEVICE TYPE ... PARALLELISM command. This example configures PARALLELISM 2 and then changes it to 3:

CONFIGURE DEVICE TYPE DISK PARALLELISM 2;
CONFIGURE DEVICE TYPE DISK PARALLELISM 3;

The parallelism setting defines the number of channels for a device that RMAN allocates in parallel. It does not have to correspond to the actual number of channels configured for the device. For example, you can manually configure four different sbt channels and set PARALLELISM for sbt to 2, 1, or 10.

You can view the default setting for parallelism by running the SHOW DEVICE TYPE command. For example:

RMAN> SHOW DEVICE TYPE;
RMAN configuration parameters are:
CONFIGURE DEVICE TYPE DISK PARALLELISM 1 BACKUP TYPE TO BACKUPSET; #default

As always when the SHOW command is used to view the value of a parameter, RMAN includes a "#default" comment at the end of the line if the RMAN default value has not been overridden.

The following example configures the default device to sbt and then displays the resulting configuration using the SHOW DEVICE TYPE command:

RMAN> CONFIGURE DEFAULT DEVICE TYPE TO sbt;
new RMAN configuration parameters:
CONFIGURE DEFAULT DEVICE TYPE TO 'sbt';
new RMAN configuration parameters are successfully stored

RMAN> SHOW DEVICE TYPE;
RMAN configuration parameters are:
CONFIGURE DEVICE TYPE SBT PARALLELISM 1; # default
CONFIGURE DEVICE TYPE DISK PARALLELISM 1 BACKUP TYPE TO BACKUPSET; #
default

Automatic Channel Default Device Types

Run the CONFIGURE DEFAULT DEVICE TYPE command to specify a default device type for automatic channels. For example, you may make backups to tape most of the time and only occasionally make a backup to disk. In this case, configure channels for disk and tape devices, but make sbt the default device type:

CONFIGURE DEVICE TYPE DISK PARALLELISM 1; # configure device disk
CONFIGURE DEVICE TYPE sbt PARALLELISM 2; # configure device sbt
CONFIGURE DEFAULT DEVICE TYPE TO sbt;

Now, RMAN will, by default, use sbt channels for backups. For example, if you run the following command:

BACKUP TABLESPACE users;

RMAN only allocates channels of type sbt during the backup because sbt is the default device.

You can override the default device for backups by specifying the target device as part of the command. For example:

BACKUP DEVICE TYPE sbt DATABASE;

If the default device type is DISK, then the preceding command overrides this default and uses the sbt channel configuration. Note that this command fails unless you have configured the sbt device or configured sbt channels.

When restoring files, RMAN allocates all automatic channels according to the settings configured for each device type. The default device type configuration is irrelevant. For example, if you configure PARALLELISM to 3 for the default sbt device and PARALLELISM to 2 for DISK, then RMAN automatically allocates three sbt channels and two DISK channels during the restore.

Automatic Channel Naming Conventions

RMAN uses the following convention for channel naming: ORA_devicetype_n, where devicetype refers to the user's device type (such as DISK or sbt_tape) and n refers to the channel number.

For example, RMAN names the first DISK channel ORA_DISK_1, the second ORA_DISK_2, and so forth. RMAN names the first sbt channel ORA_SBT_TAPE_1, the second ORA_SBT_TAPE_2, and so forth. When you parallelize channels, RMAN always allocates channels in numerical order, starting with 1 and ending with the parallelism setting (CONFIGURE DEVICE TYPE ... PARALLELISM n), as in this example:

ORA_SBT_TAPE_1
ORA_SBT_TAPE_2
ORA_SBT_TAPE_3

Automatic channel allocation also applies to maintenance commands. If RMAN allocates an automatic maintenance channel, then it uses the same naming convention as any other automatically allocated channel. If you manually allocate a maintenance channel using ALLOCATE CHANNEL FOR MAINTENANCE, then RMAN uses the following convention for channel naming: ORA_MAINT_devicetype_n, where devicetype refers to the user's device type (for example, DISK or sbt) and n refers to the channel number. For example, RMAN uses these names for two manually allocated disk channels:

ORA_MAINT_DISK_1
ORA_MAINT_DISK_2

Note that if you run the CONFIGURE DEVICE TYPE command to configure default settings for a device type and do not run CONFIGURE CHANNEL for this device type, then RMAN allocates all channels without other channel control options. For example, assume that you configure the sbt device and run a backup as follows:

CONFIGURE DEVICE TYPE sbt PARALLELISM 1;
BACKUP DEVICE TYPE sbt DATABASE;

In effect, RMAN does the following:

RUN 
{
  ALLOCATE CHANNEL ORA_SBT_TAPE_1 DEVICE TYPE sbt;
  BACKUP DATABASE;
}

Channel names beginning with the ORA_ prefix are reserved by RMAN for its own use. You cannot manually allocate a channel with a name that begins with ORA_.

Automatic Channel Generic Configurations

The CONFIGURE CHANNEL DEVICE TYPE command configures generic settings that are used for all automatic channels of the specified device type. In other words, the command creates a template of settings that RMAN uses for all channels allocated on the device. For example, you can configure disk and tape channels as follows:

CONFIGURE CHANNEL DEVICE TYPE sbt PARMS='ENV=(NSR_SERVER=bksvr1)';
CONFIGURE CHANNEL DEVICE TYPE DISK RATE 5M FORMAT="?/oradata/%U";

Because you do not specify channel numbers for these channels, the channel settings are generic to all automatic channels of the specified type. The configuration acts as a template. For example, if you set PARALLELISM for DISK to 10, and the default device type is DISK, then RMAN allocates ten disk channels using the settings in the CONFIGURE CHANNEL DEVICE TYPE DISK command.

Automatic Channel-Specific Configurations

You can also configure parameters that apply to a specific automatic channel. If you are using a media manager that requires different settings on each channel, then you may find it useful to configure individual channels.

You can mix a CONFIGURE CHANNEL command that creates a generic configuration with a CONFIGURE CHANNEL command that creates a specific configuration. A generic automatic channel creates a configuration that can be used for any channel that is not explicitly configured.

For example, assume that you run these commands:

CONFIGURE DEVICE TYPE DISK PARALLELISM 3;
CONFIGURE CHANNEL DEVICE TYPE DISK MAXPIECESIZE = 2M;
CONFIGURE CHANNEL 3 DEVICE TYPE DISK MAXPIECESIZE = 900K;

In this scenario, RMAN allocates ORA_DISK_1 and ORA_DISK_2 with option MAXPIECESIZE = 2M, using the settings for the DISK channel with no number. RMAN allocates ORA_DISK_3 with MAXPIECESIZE = 900K because this channel was manually assigned a channel number. RMAN always allocates the number of channels specified in the parallelism parameter.

Clearing Automatic Channel Settings

You can specify the CLEAR option for any CONFIGURE command. The CLEAR option returns the specified configuration to its default value. Assume you run these commands:

CONFIGURE DEVICE TYPE DISK CLEAR;     # returns DISK to default PARALLELISM 1
                                      # and backup type to BACKUPSET
CONFIGURE DEFAULT DEVICE TYPE CLEAR;    # returns to default device type of DISK
CONFIGURE CHANNEL DEVICE TYPE sbt CLEAR; # removes all options for sbt channel
CONFIGURE CHANNEL 3 DEVICE TYPE DISK CLEAR; # removes configurations for 3rd ch.

Each CONFIGURE...CLEAR command removes the user-entered settings and returns the configuration to its default value.

The only way to find out the default setting for parameters set through CONFIGURE is to use CONFIGURE... CLEAR to un-set the parameter, so that it takes on the default value, and then run SHOW ALL to view all parameters. For any parameter for which the value is currently set to RMAN'S default, RMAN includes a "#default" comment at the end of that line of the output from SHOW ALL.

Determining Channel Parallelism to Match Hardware Devices

RMAN can perform the I/O required for many commands in parallel, to make optimal use of your hardware resources. To perform I/O in parallel, however, the I/O must be associated with a single RMAN command, not a series of commands. For example, it can be more efficient to back up three datafiles using a command such as:

BACKUP DATAFILE 5,6,7;

rather than issuing the commands

BACKUP DATAFILE 5;
BACKUP DATAFILE 6;
BACKUP DATAFILE 7;

When all three datafiles are backed up in one command, RMAN recognizes the opportunity for parallelism and can use multiple channels to do the I/O in parallel. When three separate commands are used, RMAN can only perform the backups one at a time, regardless of available channels and I/O devices.

The number of channels available (whether allocated in a RUN block or configured in advance) for use with a device at the moment that you run a command determines whether RMAN will read from or write to that device in parallel while carrying out the command. Failing to allocate the right number of channels adversely affects RMAN performance during I/O operations.

As a rule, the number of channels used in carrying out an individual RMAN command should match the number of physical devices accessed in carrying out that command. If manually allocating channels for a command, allocate one for each device; if configuring automatic channels, configure the PARALLELISM setting appropriately.

When backing up to tape, you should allocate one channel for each tape drive. When backing up to disk, allocate one channel for each physical disk, unless you can optimize the backup for your disk topography by using multiple disk channels. Each manually allocated channel uses a separate connection to the target or auxiliary database.

The following script creates three backups sequentially: three separate BACKUP commands are used to back up one file each. Only one channel is active at any one time because only one file is being backed up in each command.

RUN
{
  ALLOCATE CHANNEL c1 DEVICE TYPE sbt;
  ALLOCATE CHANNEL c2 DEVICE TYPE sbt;
  ALLOCATE CHANNEL c3 DEVICE TYPE sbt;
  BACKUP DATAFILE 5;
  BACKUP DATAFILE 6;
  BACKUP DATAFILE 7;
}

The following statement uses parallelization on the same example: one RMAN BACKUP command backs up three datafiles, with all three channels in use. The three channels are concurrently active—each server session copies one of the datafiles to a separate tape drive.

RUN
{
  ALLOCATE CHANNEL c1 DEVICE TYPE sbt;
  ALLOCATE CHANNEL c2 DEVICE TYPE sbt;
  ALLOCATE CHANNEL c3 DEVICE TYPE sbt;
  BACKUP DATAFILE 5,6,7;
}

Channel Control Options for Manual and Automatic Channels

Whether you allocate channels manually or automatically, you can use channel control commands and options to do the following:

• Control the operating system resources RMAN uses when performing RMAN operations

• Affect the degree of parallelism for a backup or restore command

• Set limits on I/O bandwidth consumption in kilobytes, megabytes, or gigabytes (ALLOCATE CHANNEL ... RATE, CONFIGURE CHANNEL ... RATE)

• Set limits on the size of backup pieces (the MAXPIECESIZE parameter specified on the CONFIGURE CHANNEL and ALLOCATE CHANNEL commands)

• Set limits on the size of backup sets (the MAXSETSIZE parameter specified on the BACKUP and CONFIGURE commands)

• Send vendor-specific commands to the media manager (SEND)

• Specify vendor-specific parameters for the media manager (ALLOCATE CHANNEL ... PARMS, CONFIGURE CHANNEL ... PARMS)

• Specify which instance performs the operation (ALLOCATE CHANNEL ... CONNECT, CONFIGURE CHANNEL ... CONNECT)

In releases 8.1.5 and later of the database, the ALLOCATE CHANNEL command causes RMAN to contact the media manager whenever the type specified is other than DISK. In earlier releases, the ALLOCATE CHANNEL command does not cause RMAN to contact the media manager; RMAN does not call the media manager until a BACKUP, RESTORE, or RECOVER command is issued.

Because RMAN has one preconfigured automatic DISK channel, you do not have to manually allocate a maintenance channel when running CHANGE, CROSSCHECK, or DELETE against a disk file (that is, an ARCHIVELOG, DATAFILECOPY, or CONTROLFILECOPY).

A maintenance channel is useful only for a maintenance task; you cannot use it as an input or output channel for a backup or restore.

Channel Failover

A BACKUP command is decomposed into multiple independent backup steps by RMAN. Each independent step can be executed on any channel allocatedfor the type of device used in the command. If you have multiple channels allocated, and one channel fails or encounters a problem during a backup step, then RMAN attempts to complete the work on another channel. Typically, such retriable errors can occur when a media manager encounters problems with one of several tape drives, or when an instance fails in a RAC environment.

RMAN reports a message in V$RMAN_OUTPUT and in the output to the interactive session or log file when it encounters such problems, as in the following example (refer to bold text):

channel ORA_SBT_TAPE_1: backup set failed, re-triable on other channel 
ORA-19506: failed to create sequential file, name="/bkup/63d3c3og_1_1", parms="" ORA-27028: skgfqcre: sbtbackup returned error 
ORA-19511: Error received from media manager layer, error text: failed to open 
           file /bkup/63d3c3og_1_1 for backup, errno = 2 
channel ORA_SBT_TAPE_2: finished piece 1 at 06-SEP-01 piece handle=5ld3blun_1_1 
                        comment=API Version 2.0,MMS Version 3.2.0.0 
channel ORA_SBT_TAPE_2: backup set complete, elapsed time: 00:00:04 
retrying ORA_SBT_TAPE_1 failed backup step on ORA_SBT_TAPE_2 
channel ORA_SBT_TAPE_2: starting full datafile backupset 
channel ORA_SBT_TAPE_2: specifying datafile(s) in backupset input datafile 
                        fno=00004 name=/oracle/dbs/tbs_12.f input datafile 
                        fno=00017 name=/oracle/dbs/tbs_14.f
channel ORA_SBT_TAPE_2: starting piece 1 at 06-SEP-01 piece handle=5ld3buds_1_1 
                        comment=API Version 2.0,MMS Version 3.2.0.0 
channel ORA_SBT_TAPE_2: backup set complete, elapsed time: 00:00:06

Note that if RMAN is executing a script, then the next command in the script will not be executed if there were any errors in the previous command.