I want to know if it's better to hast DB server by SAS hard disk than SATA hard disk?
And it's also better to use RAID1.
I don't know much about RAID.
If the server is RAID6 default. Can I change it to RAID1? How?
Depends. I am very happy witha system of 8 SATA discs (Velociraptor) and 2 SSD + operating system discs. Better is relative - also depends on needed speed. The window for SAS is narrow, though - need too much speed and SSD are cheaper. I use a SAS backplane, though. THe Server has now a total of 22 discs, 24 slots, the next upgrade puts it into a new case for 72 discs.
Better than WHAT?
Read the documentation.
Depends on the server. Hardware raid? Did I mention it is a decent idea to read the documentation? Software - what OS?
In general, database servers either are trivial or require someone with knowledge what he does to plane the IO layout / budget properly.