Question

Which is better to use for backup, Oracle's RMAN or the Unix Operating System shell commands such as the cp or dd commands? Please explain the reasons why you would choose use one over the other.

Answer 1

They are completely different animals that better fit on different file or device manipulation cases:

dd
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This command was created as a "copy and convert" utility, originally intended for converting files between the ASCII, little-endian, byte-stream world of DEC computers and the EBCDIC, big-endian, appearing at the first time on Unix Version 5. It became the de facto command utility to manipulate everything that can be mapped as a file inside your Unix-like operating system(clone a disc, backup of mbr, clone a disk as file, copy some blocks of one device file, write an image to a usb stick..), and piped to other commands, the sky is the limit. One alternative to this software is the dcfldd command.

cp
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Make copy of files and directories. This is a more "higher" level of abstraction, where you can copy directories recursively, without caring about block size, file conversion, etc. It is a better tool to deal with "1-to-many" cases of file copy, ownership, symbolic link follow, recursively copy, and verbosity. However, it has it's limitations like dealing with file changes, remote copy, and those things better handled by rsync.

RMAN
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Using RMAN, you can automatically backup the database to tape.

Using RMAN Block Media Recovery, you can recover your database in the data block level. If some data blocks of huge datafiles are corrupted, you do not need to restore and recover the whole datafile. In this case, by using Block Media Recovery, only corrupted data blocks are recovered.

Most production databases are big in size and change frequently. It is not the right option to backup the whole database each time (every day). By using the incremental backup functionality, you only take a backup of changed data blocks by reducing time of backup and future recover process.

Using compression, you can easily create binary compressed backup sets. Although more CPU will be used while taking such backups, the backup pieces will be smaller, reducing the requirement for disk/tape space to hold the backups.
Using the Encryption feature, you can create encrypted backup sets which will make this backup more secure. By using the correct key, these backup sets will be decrypted automatically during restore and recovery operations.

Using the cross platform tablespace conversion functionality, you can convert the tablespace which was created in one OS to another.Because RMAN does not take a backup of the temporary tablespace, during recovery it is created automatically by RMAN. It makes the DBA's work easier.

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