It is recommended that you use a separate disc for OS and your three 1TB discs as raidz1 configuration. The separate disc can be a IDE disc, or a USB disc.
The thing is that ZFS likes to have whole discs and not partitions. To use ZFS to it's full extent, use whole discs. With enough RAM (4GB or so), OpenSolaris will not read much from the system disc. -- This message posted from opensolaris.org
