Saw this issue with UDB backup.  It justed looked at the size of the disk spool 
space at the start of backup.
On Wednesday, February 09, 2005, at 09:08AM, Robin Sharpe <Robin_Sharpe AT 
BERLEX DOT COM> wrote:

>Has anyone encountered the subject message?
>
>We saw it today in a Oracle RMAN backup...  what I think happened is, at
>the time the backup script started, there was not enough space in the
>ORACLE_DISK pool, nor in the ORACLE_LTO2 pool which is next in the
>heirarchy.  Other sessions of the same job were still working, and we added
>scratches and ran a migration while this backup job was still running.  The
>script terminated much later, I think when the other sessions finished...
>but why couldn't it recover and use the newly available disk space or
>scratch tapes?
>
>There isn't a whole lot of detail in the TDP manual about how server space
>gets reserved and allocated...  but it seems from experience that TDP looks
>for enough space to hold the entire full backup.  Can anyone confirm this?
>
>TIA
>
>Robin Sharpe
>Berlex Labs
>
>