I tried the second level backup (the incremental backup)

It took about 1 minute with B 1 and now I have the hourly backup :-)   That's great.

As nbackup was working, there was no performance issues this time.

I couldn't see any performance degradation.

But I guess, B 0 level (full backups) should be used with caution if the db size is huge with many users on it and the some users like me use the same disk array for a backup! :)   

I'll schedule a full backup weekly on sunday nights.

Thanks a lot.