Hi Dmitry, first thank you for your satisfying answers.

I realized i had some mistakes while trying to ask, so I still have some minor things in my mind.

First, as you said, it's a mistake in my question, it should be 

I can't see the default option for NBACKUP'S Direct I/O SETTING in my system.

Does nbackup, take the behavior of firebird server's "forced writes", if it's not given as a parameter? Or is it turned on by default, or off?

I've used the same raid array to store the backup. Yes, it's a good idea to use an other drive. I was not expecting this kind of slow down. For first time nbackup users like me, it can be a good thing if it writes a confirmation message to the console saying, "using another physical disk is strongly recommended, do you wish to continue?"  Just an idea.

Now, I'm hasitating to start a level 1 backup, because if I start a B 1 level backup and if it takes about 6-7 minutes and slow down the operations tremendously again, they will cut my head off :)