When you choose the slices that you want to use to mirror a file system, consider the following guidelines.
Any file system, including root (/), swap, and /usr, can use a mirror. Any application, such as a database, also can use a mirror.
Make sure that your submirror slices are of equal size. Submirrors of different sizes result in unused disk space.
If you have a mirrored file system in which the first submirror attached does not start on cylinder 0, all additional submirrors you attach must also not start on cylinder 0. If you attempt to attach a submirror starting on cylinder 0 to a mirror in which the original submirror does not start on cylinder 0, the following error message is displayed:
can't attach labeled submirror to an unlabeled mirror
You must ensure that all submirrors you plan to attach to a mirror either all start on cylinder 0, or that none of them start on cylinder 0.
Starting cylinders do not have to be identical across all submirrors, but all submirrors must either include or not include cylinder 0.