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We´re setting up RMAN to backup a database over NFS and ZFS, for which I have to create shares and mounting points (4 shares in my case).

I am trying to anticipate the size of the shares, which should be the maximum size of the backup. We´re going to proceed with an initial backup level 0 and then we´ll do daily level 1 backups.

Is there a mathematical formula for me to know the maximum backup size at the end, using x as the database size?

Thank you,


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1 Answer 1

In short: Not really.

An RMAN backupset (e.g. BACKUP DATABASE) will backup up any blocks in your datafiles that have never been used. This is different from empty blocks. There's no way that I am aware of to differentiate between blocks which are currently empty and blocks which are unused.

From: http://docs.oracle.com/cd/B28359_01/backup.111/b28270/rcmcncpt.htm#BRADV89481:

"RMAN always skips blocks that have never been used"

If you were to do an image copy (e.g. BACKUP AS COPY DATABASE), then this is essentially a direct copy of the datafiles. So if the datafile making up your system tablespace was called system.dbf, RMAN essentially copies this whole file. This means your backup will be the same size as your datafiles.

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What about incremental backup? Everyday we´ll add up, right? How to anticipate the size of it? –  Sebas Jun 25 at 16:21
RMAN will backup the entire block that has been changed even by a single byte, so it depends highly on what is being changed. In my experience, even if you can do an estimate based on data change rates (e.g. you know that 10% of your data changes daily, so 10% of your blocks may change), but this is at best a guess and is rarely accurate. –  Phil Sumner Jun 26 at 9:57

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