1.6. Describing Channel Allocation

Channel allocation is a method of connecting RMAN and the target database while also determining the type of I/O device that the server process will use to perform the backup or restore operation. Figure 1.2 illustrates this situation. The I/O device can be either tape or disk. Channels can be allocated manually or automatically.

Figure 1.2. Channel allocation

Manual channel allocation is performed any time you issue the command ALLOCATE CHANNEL. A manual command for allocating a channel is ALLOCATE CHANNEL channel name TYPE DISK. This is used for writing to a disk file system. The command ALLOCATE CHANNEL channel name TYPE 'SBT_TAPE' is another manual method used for writing to a tape backup system. These are the most common channel allocation usages. Allocating a channel is initiated with the ALLOCATE CHANNEL command, which starts a server process on the server of the target database.

Automatic channel allocation is performed by setting the RMAN configuration at the RMAN command prompt. This is done by using the CONFIGURE DEFAULT DEVICE or CONFIGURE DEVICE command. Automatic channel allocation is automatically used when executing the BACKUP, RESTORE, or DELETE commands. The complete listing of automatic channel allocation is as follows:

CONFIGURE DEVICE TYPE DISK backup|clear|parallelism n
CONFIGURE DEFAULT DEVICE TYPE to|clear
CONFIGURE CHANNEL DEVICE TYPE disk|equal
CONFIGURE CHANNEL n DEVICE TYPE disk|equal

There are some default naming conventions for the devices ORA_MAINT_DISK_n and ORA_SBT_TAPE_n. The following example shows that the default device type is set to disk and parallelism is set to 1. This means that if you don't allocate a channel manually, the parameters will be listed as follows:

RMAN> show all;

RMAN configuration parameters are:
CONFIGURE RETENTION POLICY TO REDUNDANCY 1; # default

CONFIGURE BACKUP OPTIMIZATION OFF; # default
CONFIGURE DEFAULT DEVICE TYPE TO DISK; # default
CONFIGURE CONTROLFILE AUTOBACKUP OFF; # default
CONFIGURE CONTROLFILE AUTOBACKUP FORMAT FOR DEVICE TYPE DISK TO '%F'; # default
CONFIGURE DEVICE TYPE DISK PARALLELISM 1 BACKUP TYPE TO BACKUPSET; # default
CONFIGURE DATAFILE BACKUP COPIES FOR DEVICE TYPE DISK TO 1; # default
CONFIGURE ARCHIVELOG BACKUP COPIES FOR DEVICE TYPE DISK TO 1; # default
CONFIGURE MAXSETSIZE TO UNLIMITED; # default
CONFIGURE ARCHIVELOG DELETION POLICY TO NONE; # default
CONFIGURE SNAPSHOT CONTROLFILE NAME TO 'C:ORACLEPRODUCT10.1.0DB_1DATABASES
NCFORA101T.ORA'; # default

RMAN>

Multiple Backup Types on Your Tapes?

Your tape backup device could be supporting multiple backups that may be RMAN-based as well as normal file system backups. Most ATLs and the software support file system backups and RMAN backups are backed up with the same tape media. That means you could have each type of backup on a tape, especially since digital linear tapes (DLTs) support large storage volumes of 200GB or more per tape, meaning file system backups and RMAN backups could be interspersed on a single tape.

You should be aware that there are potentially two types of backups on the same tape. If you are heavily dependent on tapes for recovery operations, make sure that the tape cycle that your organization uses supports the requirements of the file system backup or the RMAN backup that is needed for the longest period of time.

For example, file system backups are needed for only one week until the next complete backup file system backup is taken on the weekend. RMAN backups may be needed up to a month to support business requirements. Therefore, you should store all the tapes for up to a month and if possible, procure additional tapes so you can keep backups separated.


There are also channel control options or commands that are used whether channels are allocated manually or automatically. Channel control options or channel control commands basically control the operating system resources that RMAN uses when performing RMAN operations. Channel control options or commands perform the functions described in Table 1.3.

Table 1.3. Channel Commands and Options
Channel Control Option or CommandFunctions
ALLOCATE CHANNEL RATE or CONFIGURE CHANNEL RATELimits the I/O bandwidth in kilobytes, megabytes, or gigabytes
ALLOCATE CHANNEL or CONFIGURE CHANNEL MAXPIECESIZELimits the size of the backup pieces
ALLOCATE CHANNEL or CONFIGURE CHANNEL MAXSETSIZELimits the size of the backup sets
SENDSends vendor-specific commands to the Media Manager
ALLOCATE CHANNEL CONNECT or CONFIGURE CHANNEL CONNECTInstructs a specific instance to perform an operation
ALLOCATE CHANNEL PARMS or CONFIGURE CHANNE PARMSSends vendor-specific parameters to the Media Manager

New with Oracle 10g is an automated channel failover for backup and restore operations. In order for this feature to function, multiple channels must be allocated. If there is a failure in the backup or restore operations, RMAN will complete the operation with the available channels. This can commonly happen when multiple backups attempt to use a one tape device or when there is an MML problem of some sort.

Error messages are reported in the V$RMAN_OUTPUT dynamic view. Error information is also logged to the screen or the RMAN log file when MML comes across any problems.

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