For maximum data availability. The trade-off is that a mirror requires twice the number of slices (disks) as the amount of data to be mirrored.
DiskSuite enables you to create up to a three-way mirror (a mirror of three submirrors). However, two-way mirrors usually provide sufficient data redundancy for most applications, and are less expensive in terms of disk drive costs.
Why should I always create a one-way mirror then attach additional submirrors?
This ensures that a mirror resync is performed so that data is consistent in all submirrors.