Backup Sets and Image Copies:
The backup command lets you make two types of RMAN backups: backup sets and image copies. By default, all RMAN backups are in the form of backup sets. Each backup set contains one or more backup pieces, which are files in an RMAN-specific format. Backup sets are the default backup type for both disk- and tape-based backups.
A backup set is a logical structure that consists of a minimum of one backup piece, which is a physical, RMAN-specific format file that actually contains the backed-up data. A backup set can contain data from one or more datafiles, archived redo log files, or control files. By default, a backup set contains just one backup piece. However, you can limit the size of a backup piece by using the maxpiecesize parameter. If you do this and the backup set size is larger than the backup piece size specified by the maxpiecesize parameter, there’ll be multiple backup pieces within that backup set.
Each of the objects you back up with the backup command—database, tablespace, archived redo logs, and so on—will result in at least one backup set if you specify backup set as the backup type. RMAN determines the number of backup sets for a backup according to an internal algorithm. However, you can limit the size of a backup set by specifying the maxsetsize parameter. You can also indirectly control the number of backup sets made by RMAN for each backup by specifying the filesperset parameter, which limits the number of input files (datafiles, archived redo log files, and so on) that can be backed up into a single backup set.
The key difference between an image copy and a backup set is that RMAN can write blocks from many files into the same backup set (known as multiplexing) but can’t do so in the case of an image copy—an image copy is identical, byte by byte, to the original datafile,control file, or archived redo log file. An RMAN image copy and a copy you make with an operating system copy command such as dd (which makes image copies) are identical.
Since RMAN image copies are identical to copies made with operating system copy commands, you may use user-made image copies for an RMAN restore and recovery operation after first making the copies “known” to RMAN by using the catalog command. After this point, there’s no difference between those image copies made by you and those made by RMAN. During a restore operation, if you have both image copies and backup sets from the same time period, RMAN prefers to use an image copy over a backup set. This is because there is more overhead involved in sorting through a backup set to get the files to restore. In addition, image copies offer yet another benefit during a restore and recovery operation. If you need to restore a current datafile and happen to have an image copy of that datafile available, you can use the switch command to simply point the database to the replacement file instead of the original datafile. This eliminates the need to restore the datafile, thus speeding up database recovery considerably.
RMAN Backup Modes:
A control file or an archived redo log file is always backed up completely and in a consistent fashion. A datafile, however, may be backed up partly or completely. You can also make consistent or inconsistent backups with datafiles. The various backup types are as follows:
Full vs. incremental backups: A full backup is a backup of a datafile that includes every allocated block in that file. Note that an image copy backup of a datafile will always include every block in that file. A backup of a datafile as a backup set, however, may skip data blocks that aren’t in use. An incremental backup can be one of two different levels: a level 0 backup including all blocks in the datafile except those blocks compressed because they have never been used or a level 1 backup including only those blocks that have changed since the parent backup.
Consistent vs. inconsistent backups: A backup taken after a database was shut down gracefully (as opposed to using the shutdown abort command or a shutdown following an abrupt database crash) and restarted in mount state is said to be consistent. A consistent backup doesn’t require recovery after you restore the database. A backup taken while the
database is online or after it was brought into mount state after being shut down abruptly is called an inconsistent backup. An inconsistent backup always needs recovery to make the backup consistent.
If you’re running in archivelog mode, the target database must be mounted or be open before you can issue an RMAN backup command. If you’re running the database in noarchivelog mode, the database must first be shut down cleanly and started up in mount state before you can use RMAN for backups. If the database was abruptly shut down and restarted, RMAN can’t make the backups. You mustn’t back up a database running in noarchivelog mode while the database is open.
By default, all RMAN backups—whole database, tablespace level, and so on—are full backups. That is, all data blocks in the datafiles that were ever used, even if they are currently empty, are included in the backup. You can specify the command backup full database, for example, to start a whole-database backup, but it’s not necessary to do so. Just use the command backup database to do the same thing. However, when you are performing an incremental RMAN backup, you must specify the keyword incremental in your backup commands since it isn’t the default backup type.
Types of Files That RMAN Can Back Up:
RMAN lets you back up all the files you’d need for a database recovery, such as the following:
• Datafiles
• Control files
• Archived redo logs
• Image copies of datafiles and control files, including those made by RMAN
• Backup pieces that contain RMAN backups
The Oracle database uses three types of “live” files during its operation: datafiles, online redo log files, and control files. Of these three types of files, RMAN backs up only the datafiles and the control files. You can’t use RMAN to back up the online redo log files. If you’re operating in noarchivelog mode, then you won’t need the online redo logs, since the database files are always consistent when you back up the database using the only permitted modes of backing up a database in noarchivelog mode, which are closed whole backups. You won’t need the online redo log backups if you’re operating in archivelog mode either, since RMAN is continually backing up all your archived redo logs. However, you must make sure you always multiplex the online redo log so you won’t lose all members of a group and thus all the committed changes as yet unrecorded in the datafiles.
In addition to the previously mentioned types of files, RMAN also can back up the server parameter file, or spfile, which contains the initialization parameter for starting up your database. You can’t, however, back up the following types of files using RMAN:
• External files
• Network configuration files
• Password files
• Any Oracle home-related files
Use normal operating system copy utilities to back up any of these four types of files.
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