Do you need to put tablespaces hot backup mode when using RMAN

To understand why RMAN does not require extra logging or backup mode, you must first understand why those features are required for non-RMAN online backups.

A non-RMAN online backup consists of a non-Oracle tool, such as cp or dd, backing up a datafile at the same time that DBWR is updating the file. Oracle can’t prevent the tool from reading a particular block at the exact same time that DBWR is updating that block. When that happens, the non-Oracle tool might read a block in a half-updated state, so that the block which is copied to the backup media might only have been updated in its first half, while the second half contains older data. This is called a “fractured block”. If this backup needs to be restored later, and that block needs to be recovered, recovery will fail because that block is not usable.

The ‘alter tablespace begin backup’ command is oracle solution for the fractured block problem. When a tablespace is in backup mode, and a change is made to a data block, instead of logging just the changed bytes to the redo log, oracle also log a copy of the entire block image before the change, so that it can reconstruct this block if media recovery finds that this block was fractured. That block image logging is what causes extra redo to be generated while files are in backup mode.

The reason that RMAN does not require extra logging is that it guarantees that it will never back up a fractured block. it can make that guarantee because we know the format of Oracle data blocks, and it verify that each block that it read is complete before it copy it to the backup. If it read a fractured block, it will read the block again to obtain a complete block before backing it up. non-Oracle tools are not able to do the same thing because they do not know how to verify the contents of an Oracle data block.

Backup mode has another effect, which is to ‘freeze’ the checkpoint in the header of the file until the file is removed from backup mode. Oracle does this because it cannot guarantee that the third-party backup tool will copy the file header prior to copying the data blocks. RMAN does not need to freeze the file header checkpoint because it knows the order in which it will read the blocks, which enables it to capture a known good checkpoint for the file.


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