Here is a little step-by-step guide to create a soft mirror from the root file system, known as an encapsulation of the system's disk. This will provide full protection against one disk failure, and complete redundancy. In the same time, this will have the effect to speed read requests (since there exists multiple backing devices hosting the same data), but write performance is generally degraded. First, know your running system, particularly on which disk it is currently installed and which other device is available for the second mirror side.

# df -hF ufs
Filesystem             size   used  avail capacity  Mounted on
/dev/dsk/c1d0s0        7.9G   5.2G   2.6G    67%    /
# swap -lh
swapfile             dev    swaplo   blocks     free
/dev/dsk/c1d0s1     102,65       4K     4.0G     4.0G
#
# echo | format
Searching for disks...done

AVAILABLE DISK SELECTIONS:
       0. c1d0 
          /pci@0,0/pci-ide@8/ide@0/cmdk@0,0
       1. c2d0 
          /pci@0,0/pci-ide@8/ide@1/cmdk@0,0
[...]

Well, we will use the c2d0 as the second submirror. So, we need to default to one Solaris partition that uses the whole disk and make it bootable (we are using GRUB in this case). The slice for the second submirror must have a slice tag of root and the root slice must be slice 0 (so, we will duplicate the label's content from the boot disk to the mirror disk).

# fdisk -B /dev/rdsk/c2d0p0
# fdisk /dev/rdsk/c2d0p0
             Total disk size is 36483 cylinders
             Cylinder size is 16065 (512 byte) blocks

                                               Cylinders
      Partition   Status    Type          Start   End   Length    %
      =========   ======    ============  =====   ===   ======   ===
          1       Active    Solaris2          1  36482    36482    100

SELECT ONE OF THE FOLLOWING:
   1. Create a partition
   2. Specify the active partition
   3. Delete a partition
   4. Change between Solaris and Solaris2 Partition IDs
   5. Exit (update disk configuration and exit)
   6. Cancel (exit without updating disk configuration)
Enter Selection:
#
# prtvtoc /dev/rdsk/c1d0s2 | fmthard -s - /dev/rdsk/c2d0s2
fmthard:  New volume table of contents now in place.
#
# /sbin/installgrub /boot/grub/stage1 /boot/grub/stage2 /dev/rdsk/c2d0s0
stage1 written to partition 0 sector 0 (abs 16065)
stage2 written to partition 0, 260 sectors starting at 50 (abs 16115)

Create replicas of the metadevice state database:

# metadb -a -c 3 -f c1d0s4 c2d0s4
# metadb
        flags           first blk       block count
     a        u         16              8192            /dev/dsk/c1d0s4
     a        u         8208            8192            /dev/dsk/c1d0s4
     a        u         16400           8192            /dev/dsk/c1d0s4
     a        u         16              8192            /dev/dsk/c2d0s4
     a        u         8208            8192            /dev/dsk/c2d0s4
     a        u         16400           8192            /dev/dsk/c2d0s4

Flag -f is needed because it is the first invocation/creation of metadb(1m).

Set up the RAID-0 metadevices (stripe or concatenation volumes) corresponding to the / file system and the swap space, and automatically configure system files (/etc/vfstab and /etc/system) for the root metadevice.

# metainit -f d10 1 1 c1d0s0
d10: Concat/Stripe is setup
# metainit -f d11 1 1 c1d0s1
d11: Concat/Stripe is setup
# metainit d20 1 1 c2d0s0
d20: Concat/Stripe is setup
# metainit d21 1 1 c2d0s1
d21: Concat/Stripe is setup
# metainit d0 -m d10
d0: Mirror is setup
# metainit d1 -m d11
d1: Mirror is setup
#
# cp /etc/vfstab /etc/vfstab.beforesvm
# sed -e 's@/dev/dsk/c1d0s1@/dev/md/dsk/d1@' /etc/vfstab.beforesvm > /etc/vfstab
# metaroot d0
# diff /etc/vfstab /etc/vfstab.beforesvm
6,7c6,7
< /dev/md/dsk/d1   -                 -   swap   -   no   -
< /dev/md/dsk/d0   /dev/md/rdsk/d0   /   ufs    1   no   -
---
> /dev/dsk/c1d0s1  -                 -   swap   -   no   -
> /dev/dsk/c1d0s0  /dev/rdsk/c1d0s0  /   ufs    1   no   -

Flag -f is needed because the file systems created on the slice we want to initialize a new metadevice are currently mounted (in use).

Reboot on the metadevices: the operating system will now boot encapsulated, on a one-side mirror. Last, attach the second part of the mirror and adapt the system dump configuration.

# lockfs -af && shutdown -y -g 0 -i 6
[...]
# metattach d0 d20
d0: submirror d20 is attached
# metattach d1 d21
d1: submirror d21 is attached
#
# metastat -p
d1 -m /dev/md/rdsk/d11 /dev/md/rdsk/d21 1
d11 1 1 /dev/rdsk/c1d0s1
d21 1 1 /dev/rdsk/c2d0s1
d0 -m /dev/md/rdsk/d10 /dev/md/rdsk/d20 1
d10 1 1 /dev/rdsk/c1d0s0
d20 1 1 /dev/rdsk/c2d0s0
# metastat | grep %
    Resync in progress: 41 % done
    Resync in progress: 46 % done
#
# rmdir /var/crash/*
# mkdir /var/crash/`hostname`
# chmod 700 /var/crash/`hostname`
# dumpadm -s /var/crash/`hostname` -d /dev/md/dsk/d1
      Dump content: kernel pages
       Dump device: /dev/md/dsk/d1 (swap)
Savecore directory: /var/crash/bento
  Savecore enabled: yes

Last, define the alternative boot path in the menu.lst GRUB configuration file: the Solaris/BSD slice 0 on the first fdisk partition on the second BIOS disk.

cat << EOF >> /boot/grub/menu.lst
title Solaris Nevada snv_65 X86 (Alternate Boot Path)
root (hd1,0,a)
kernel$ /platform/i86pc/kernel/$ISADIR/unix
module$ /platform/i86pc/$ISADIR/boot_archive
EOF
#
# bootadm list-menu 
The location for the active GRUB menu is: /boot/grub/menu.lst
default 0
timeout 10
0 Solaris Nevada snv_65 X86
1 Solaris failsafe
2 Solaris Nevada snv_65 X86 (Alternate Boot Path)

For further (and deeper) information on this subject, please refer to the excellent Sun Microsystems Documentation on Solaris Volume Manager, and particularly x86: Creating a RAID-1 Volume From the root (/) File System.