RMAN Backup Options: Naming, Sizing, and Speed

Using FORMAT with RMAN BACKUP AS COPY

As with backup pieces,FORMAT variables are also used to specify the names of image copies. The default format %U is defined differently for image copies than for backup pieces. RMAN produces image copies of three types of files: datafiles, control files, and archived logs. The following table describes the meaning of %U for each type of file.

Using DB_FILE_NAME_CONVERT With BACKUP AS COPY

When creating image copies, you can also name the output copies with the DB_FILE_NAME_CONVERT option of the BACKUP command. This parameter works identically to the initialization parameter DB_FILE_NAME_CONVERT. Pairs of filename prefixes are provided to change the names of the output files. If a file is not converted by any of the pairs, then RMAN uses the FORMAT specification; if no FORMAT is specified, then RMAN uses the default format %U.

For example, you can run the following command to copy the datafiles whose filename is prefixed with /maindisk/oradata/users so that they are prefixed with /backups/users_ts:

BACKUP AS COPY TABLESPACE users
  DB_FILE_NAME_CONVERT=('/maindisk/oradata/users','/backups/users_ts');

Default RMAN Backup Tag Format

If you do not specify a tag, then RMAN creates a default tag for backups (except for control file autobackups) in the format TAGYYYYMMDDTHHMMSS, where YYYY is the year, MM is the month, DD is the day, HH is the hour (in 24-hour format), MM is the minutes, and SS is the seconds. For example, a backup of datafile 1 may receive the tag TAG20030208T133437. The date and time refer to when RMAN started the backup, in the time zone of the instance performing the backup. If multiple backup sets are created by one BACKUP command, then each backup piece is assigned the same default tag.

How Tags Are Applied

When applied to a backup set, a tag applies to a specific copy of the backup set. If you do not duplex a backup set, that is, make multiple identical copies of it, then the backup set has just one tag. For example,

BACKUP COPIES 1 DATAFILE 7 TAG foo; 

creates one backup set with tag FOO. Tags are stored in uppercase, regardless of the case used when entering them. However, you can back up this backup set and give this new copy of the backup set the tag BAR. So, the backup set has two identical copies: one tagged FOO and the other tagged BAR.

When applied to image copies, a tag applies to each individual image copy. For example, you run the following command:

# Back up as image copies the datafiles of tablespaces users and tools
# all copies get the TAG 'users_tools' 
BACKUP AS COPY TAG users_tools TABLESPACE users, tools;

You can also copy an image copy with a specific tag, and give the output copy a different tag, as in the following example:

# Create new copies of all image copies of the database that have the tag 
# 'full_cold_copy', and give the new copies the tag 'new_full_cold_copy'
BACKUP AS COPY 
  COPY OF DATABASE 
    FROM TAG=full_cold_copy 
    TAG=new_full_cold_copy;

# Create backup sets of the image copy of tablespace users that has the tag
# 'monday_users', and of tablespace SYSTEM which has the tag 'monday_system'.
# All these new backup # sets receive the tag 'for_audit'.
BACKUP AS BACKUPSET TAG for_audit 
  COPY OF TABLESPACE users FROM TAG monday_users 
    TABLESPACE SYSTEM FROM TAG monday_system;

Uniqueness of Backup Tags

Tags do not need to be unique, so multiple backup sets or image copies can have the same tag, for example, weekly_backup. When you specify that a datafile should be restored from backups that have a specific tag, and more than one backup of the requested file has the desired tag, RMAN restores the most recent backup that has the desired tag (within any other constraints of the restore command, of course, such as a point in time).

Tags can indicate the intended purpose or usage of different classes of backups or copies. For example, you can tag datafile copies that you intend to use in a SWITCH differently (for_switch_only) from file copies that should be used only for RESTORE (for_restore_only).

Factors Affecting the Number and Size of Backup Sets

In determining the characteristics of the RMAN backup sets, the internal algorithm is influenced by the following factors:

• The number of input files specified in each backupSpec clause

• The number of channels that you allocate

• The default number of files in each backup set (4 for datafiles and 16 for archived logs)

• The default number of files read simultaneously by a single channel (8)

• The MAXSETSIZE parameter (specified on the CONFIGURE and BACKUP commands), which specifies a maximum backup set size

The most important rules in the algorithm for backup set creation are:

• Each allocated channel that performs work in the backup job—that is, each channel that is not idle—generates at least one backup set.

  Note:

  RMAN writes backup pieces sequentially; striping a backup piece across multiple output devices is not supported. For example, RMAN does not simultaneously write half of a backup piece to one device and the other half to another device, nor can it write the first piece of a backup set to one device andthe second piece to another device.

• RMAN always tries to divide the backup load so that all allocated channels have roughly the same amount of work to do.

• The default number of datafiles in each backup set is determined by an internal RMAN limit (16 for archived logs, 4 for datafiles).

• The number of datafiles multiplexed in a backup set is limited by the lesser of the number of files in each backup set and the default number of files read by a single channel simultaneously (8).

• The maximum size of a backup set is determined by the MAXSETSIZE parameter of the CONFIGURE or BACKUP command.

  See Also:

  Oracle Database Backup and Recovery Reference to learn the syntax for the backupSpec clause, and Chapter 11, "Tuning Backup and Recovery" to learn about RMAN buffer management

Overview of the MAXSETSIZE Parameter

To specify the maximum size of each backup set, use the MAXSETSIZE parameter in the CONFIGURE or BACKUP command. By limiting the overall size of the backup set, the parameter indirectly limits the number of files in the set and can possibly force RMAN to create additional backup sets.

BACKUP DATABASE MAXSETSIZE = 10M;