RMAN and Incremental Backups.

[ErrolDC]

When performing incremental backups with RMAN, how does RMAN determine which blocks have changed since the last backup? Is this reliant upon the actual backup peices or the metadata related to this stored in the recovery catalog or control file?

Thanks.

 

[Matsul]

I can only answer part of this. As, to improve incremental backup performance, Ora 10i has a new feature whereby block tracking is kept in a table V$BLOCK_CHANGE_TRACKING which is created in the target database I assume neither the control file of the catalog database contain the actual block changes.

In ora 9i Rman checks the blocks to see which have changed but I am not sure if it looks at the actual datafile and the incremental backups to deduce this or just the datafile.

 

[karluk]

Matsul is right. 9i incrementals have to read through every block on every datafile to determine which blocks have changed. Therefore the increase in speed over a full backup isn't that large. However, you do save a lot on writes, so you should get some increase in performance.

If you are using block change tracking in 10g, the incrementals should be much faster. I recently performed some benchmarks, and the incrementals were at least twice as fast using block change tracking. It's not a free lunch, however. Oracle needs to constantly keep track of block changes, so your online performance should suffer a little.