Creating a RAID-1 Volume

How to Create a RAID-1 Volume From Unused Slices

This procedure shows you how to create a two-way mirror. If you want to create a three-way mirror or a four-way mirror, use the same procedure.

Before You Begin

Check Prerequisites for Creating Solaris Volume Manager Components and Creating and Maintaining RAID-1 Volumes.

Create two stripes or concatenations. These components become the submirrors.
See How to Create a RAID-0 (Stripe) Volume or How to Create a RAID-0 (Concatenation) Volume.
To create the mirror, use one of the following methods:
- From the Enhanced Storage tool within the Solaris Management Console, open the Volumes node, then choose Action⇒Create Volume and follow the onscreen instructions. For more information, see the online help.
- Use the following form of the metainit command to create a one-way mirror:
```
# metainit volume-name -m submirror-name
```
  volume-name
  
  Specifies the name of the volume to create
  
  -m
  
  Specifies to create a mirror
  
  submirror-name
  
  Specifies the name of the component that will be the first submirror in the mirror
  
  See the following examples and the metainit(1M) man page for more information.
To add the second submirror, use one of the following methods:
- From the Enhanced Storage tool within the Solaris Management Console, open the Volumes node, then choose the mirror you want to modify. Choose Action⇒Properties, then the Submirrors. Follow the onscreen instructions to attach the submirror. For more information, see the online help.
- Use the following form of the metattach command:
```
# metattach volume-name submirror-name
```
  volume-name
  
  Specifies the name of the RAID-1 volume on which to add the submirror
  
  submirror-name
  
  Specifies the name of the component that will be the second submirror attached to the mirror
  
  See the following examples and the metattach(1M) man page for more information.

Example 11-1 Creating a Two-Way Mirror

# metainit d51 1 1 c0t0d0s2
d51: Concat/Stripe is setup
# metainit d52 1 1 c1t0d0s2
d52: Concat/Stripe is setup
# metainit d50 -m d51
d50: Mirror is setup
# metattach d50 d52
d50: Submirror d52 is attached

This example shows you how to create the two-way mirror, d50. The metainit command creates two submirrors (d51 and d52), which are RAID-0 volumes. The metainit -m command creates the one-way mirror from the d51 RAID-0 volume. The metattach command attaches d52, creating a two-way mirror and causing a resynchronization. Any data on the attached submirror is overwritten by the other submirror during the resynchronization.

Example 11-2 Creating a Two-Way Mirror Without Resynchronization

# metainit d51 1 1 c0t0d0s2
d51: Concat/Stripe is setup
# metainit d52 1 1 c1t0d0s2
d52: Concat/Stripe is setup
# metainit d50 -m d51 d52
metainit: d50: WARNING: This form of metainit is not recommended.
The submirrors may not have the same data.
Please see ERRORS in metainit(1M) for additional information.
d50: Mirror is setup

This example shows the creation a two-way mirror, d50. The metainit command creates two submirrors (d51 and d52), which are RAID-0 volumes. The metainit -m command is then run with both submirrors to create the mirror. When you create a mirror using the metainit command rather than the metattach command , no resynchronization operations occur. As a result, data could become corrupted when Solaris Volume Manager assumes that both sides of the mirror are identical and can be used interchangeably.

How to Create a RAID-1 Volume From a File System

Use this procedure to mirror an existing file system. If the file system can be unmounted, the entire procedure can be completed without a reboot. For file systems that cannot be unmounted, such as /usr and /swap, the system must be rebooted to complete the procedure.

When creating a RAID-1 volume from an existing file system built on a slice, only the single slice may be included in the primary RAID-0 volume (submirror). If you are mirroring system-critical file systems, all submirrors must consist of only a single slice.

For the procedures associated with mirroring the root (/) file system, see SPARC: How to Create a RAID-1 Volume From the root (/) File System and x86: How to Create a RAID-1 Volume From the root (/) File System.

In the example used in this procedure, the existing slice is c1t0d0s0. The second slice, c1t1d0s0, is available for the second half of the mirror. The submirrors are d1 and d2, respectively, and the mirror is d0.

Caution - Be sure to create a one-way mirror with the metainit command then attach the additional submirrors with the metattach command. When the metattach command is not used, no resynchronization operations occur. As a result, data could become corrupted when Solaris Volume Manager assumes that both sides of the mirror are identical and can be used interchangeably.

Before You Begin

Check Prerequisites for Creating Solaris Volume Manager Components and Creating and Maintaining RAID-1 Volumes.

Identify the slice that contains the existing file system to be mirrored This example uses the slice c1t0d0s0.
Create a new RAID-0 volume on the slice from the previous step by using one of the following methods:
- From the Enhanced Storage tool within the Solaris Management Console, open the Volumes node, then choose Action⇒Create Volume. Follow the onscreen instructions . For more information, see the online help.
- Use the following form of the metainit command:
```
# metainit -f volume-name number-of-stripes components-per-stripe component-name
```
  -f
  
  Forces the command to continue. You must use this option when the slice contains a mounted file system.
  
  volume-name
  
  Specifies the name of the volume to create. For information on naming volumes, see Volume Names.
  
  number-of-stripes
  
  Specifies the number of stripes to create.
  
  components-per-stripe
  
  Specifies the number of components each stripe should have.
  
  component-names
  
  Specifies the names of the components that are used. This example uses the root slice, c0t0d0s0.
Create a second RAID-0 volume (concatenation) on an unused slice (c1t1d0s0 in this example) to act as the second submirror. The second submirror must be the same size as the original submirror or larger. Use one of the following methods:
- From the Enhanced Storage tool within the Solaris Management Console, open the Volumes node, then choose Action->Create Volume and follow the instructions on screen. For more information, see the online help.
- Use the following form of the metainit command.
```
# metainit volume-name number-of-stripes components-per-stripe component-name
```
  Note - See Step 2 for an explanation of the options.
Create a one-way mirror by using one of the following methods:
- From the Enhanced Storage tool within the Solaris Management Console, open the Volumes node, then choose Action⇒Create Volume. Follow the onscreen instructions . For more information, see the online help.
- Use the following form of the metainit command.
```
# metainit volume-name -m submirror-name
```
  volume-name
  
  Specifies the name of the volume to create.
  
  -m
  
  Specifies to create a mirror.
  
  submirror-name
  
  Specifies the name of the component that will be the first submirror in the mirror. In this example, it is the RAID-0 volume that contains the root slice.
See the metainit(1M) man page for more information.

Caution - When you create a mirror from an existing file system, you must follow the next two steps precisely to avoid data corruption.
Edit the /etc/vfstab file so that the file system mount instructions refer to the mirror, not to the block device. For more information about the/etc/vfstab file, see Mounting File Systems in System Administration Guide: Devices and File Systems.
For example, if the /etc/vfstab file contains the following entry for the file system:
```
/dev/dsk/slice /dev/rdsk/slice /var ufs 2 yes -
```
Change the entry to read as follows:
```
/dev/md/dsk/mirror-name /dev/md/rdsk/mirror-name /var ufs 2 yes -
```
Remount your newly mirrored file system according to one of the following procedures:
- If you are mirroring a file system that can be unmounted, then unmount and remount the file system.
```
# umount /filesystem
# mount /filesystem
```
- If you are mirroring a file system that cannot be unmounted, then reboot your system.
```
# reboot
```
Use the following form of the metattach command to attach the second submirror.
```
# metattach volume-name submirror-name
```
volume-name

Specifies the name of the RAID-1 volume on which to add the submirror

submirror-name

Specifies the name of the component that will be the second submirror attached to the mirror

See the metattach(1M) man page for more information.

Example 11-3 Creating a Two-Way Mirror From a File System That Can Be Unmounted

# metainit -f d1 1 1 c1t0d0s0
d1: Concat/Stripe is setup
# metainit d2 1 1 c1t1d0s0
d2: Concat/Stripe is setup
# metainit d0 -m d1
d0: Mirror is setup
# umount /master
(Edit the /etc/vfstab file so that the file system references the mirror)
# mount /master
# metattach d0 d2
d0: Submirror d2 is attached

In this example, the -f option forces the creation of the first concatenation, d1, which contains the mounted file system /master on /dev/dsk/c1t0d0s0. The second concatenation, d2, is created from /dev/dsk/c1t1d0s0. This slice must be the same size as , or larger than the size of d1.) The metainit command with the -m option creates the one-way mirror, d0, from d1.

Next, the entry for the file system should be changed in the /etc/vfstab file to reference the mirror. The following line in the /etc/vfstab file initially appears as follows:

/dev/dsk/c1t0d0s0 /dev/rdsk/c1t0d0s0 /var ufs 2 yes -

The entry is changed to the following:

/dev/md/dsk/d0 /dev/md/rdsk/d0 /var ufs 2 yes -

Finally, the file system is remounted and submirror d2 is attached to the mirror, causing a mirror resynchronization. The system confirms that the RAID-0 and RAID-1 volumes are set up, and that submirror d2 is attached.

Example 11-4 Creating a Two-way Mirror From a File System That Cannot Be Unmounted

# metainit -f d12 1 1 c0t3d0s6
d12: Concat/Stripe is setup
# metainit d22 1 1 c1t0d0s6
d22: Concat/Stripe is setup
# metainit d2 -m d12
d2: Mirror is setup
(Edit the /etc/vfstab file so that /usr references the mirror)
# reboot
...
# metattach d2 d22
d2: Submirror d22 is attached

This example creates a two-way mirror using a slice containing the /usr file system. The -f option forces the creation of the first concatenation, d12, which contains the mounted file system /usr on /dev/dsk/c0t3d0s6. The second concatenation, d22, is created from /dev/dsk/c1t0d0s6. This slice must be the same size as, or larger than the size of d12. The metainit command with the -m option creates the one-way mirror d2 using the concatenation that contains the /usr file system. Next, the /etc/vfstab file must be edited to change the entry for /usr to reference the mirror.

The /etc/vfstab file contains the following entry for the /usr file system:

/dev/dsk/c0t3d0s6 /dev/rdsk/c0t3d0s6 /usr ufs 1 yes -

Change the entry to read as follows:

/dev/md/dsk/d2 /dev/md/rdsk/d2 /usr ufs 1 yes -

After a reboot, the second submirror d22 is attached to the mirror, causing a mirror resynchronization.

Example 11-5 Creating a Mirror From the /swap Space

# metainit -f d11 1 1 c0t0d0s1
d11: Concat/Stripe is setup
# metainit d21 1 1 c1t0d0s1
d21: Concat/Stripe is setup
# metainit d1 -m d11
d1: Mirror is setup
(Edit the /etc/vfstab file so that swap references the mirror)
# reboot
...
# metattach d1 d21
d1: Submirror d21 is attached

In this example, the -f option forces the creation of the first concatenation, d11, which contains the mounted file system swap on /dev/dsk/c0t0d0s1. The second concatenation, d21, is created from /dev/dsk/c1t0d0s1. This slice must be the same size as, or larger than the size of d11. The metainit command with the -m option creates the one-way mirror d1 using the concatenation that contains swap. Next, if there is an entry for swap in the /etc/vfstab file, it must be edited to reference the mirror.

The /etc/vfstab file contains the following entry for the swap space:

/dev/dsk/c0t0d0s1 - - swap - no -

Change the entry to read as follows:

/dev/md/dsk/d1 - - swap - no -

After a reboot, the second submirror d21 is attached to the mirror, causing a mirror resynchronization.

To save the crash dump when you have mirrored the swap space, use the dumpadm command to configure the dump device as a volume. For instance, if the swap device is named /dev/md/dsk/d2, use the dumpadm command to set this device as the dump device.

SPARC: How to Create a RAID-1 Volume From the root (`/`) File System

The process for mirroring the root (/) file system on a SPARC platform is similar to mirroring any other file system that you cannot unmount. The procedure differs in that the metaroot command is run instead of manually editing the /etc/vfstab file. Mirroring the root (/) file system also requires recording the path to the alternate boot device. This device reboots the system if the submirror fails.

Before You Begin

Check Prerequisites for Creating Solaris Volume Manager Components and Creating and Maintaining RAID-1 Volumes.

Identify the slice that contains the existing root (/) file system to be mirrored. This example uses the slice c1t0d0s0.
Create a new RAID-0 volume on the slice from the previous step by using one of the following methods. Only the single slice can be included in the RAID-0 volume.
- From the Enhanced Storage tool within the Solaris Management Console, open the Volumes node, then choose Action⇒Create Volume. Follow the onscreen instructions . For more information, see the online help.
- Use the following form of the metainit command:
```
# metainit -f volume-name number-of-stripes components-per-stripe component-name
```
  -f
  
  Forces the command to continue. You must use this option when the slice contains a mounted file system.
  
  volume-name
  
  Specifies the name of the volume to create. For information on naming volumes, see Volume Names.
  
  number-of-stripes
  
  Specifies the number of stripes to create.
  
  components-per-stripe
  
  Specifies the number of components each stripe should have.
  
  component-names
  
  Specifies the names of the components that are used. This example uses the root slice, c0t0d0s0.
Create a second RAID-0 volume on an unused slice (c1t1d0s0 in this example) to act as the second submirror. The secondary submirror must be the same size as the original submirror, or larger. Use one of the following methods:
- From the Enhanced Storage tool within the Solaris Management Console, open the Volumes node, then choose Action->Create Volume and follow the instructions on screen. For more information, see the online help.
- Use the following form of the metainit command.
```
# metainit volume-name number-of-stripes components-per-stripe component-name
```
  Note - See Step 2 for and explanation of the options.
Create a one-way mirror by using one of the following methods:
- From the Enhanced Storage tool within the Solaris Management Console, open the Volumes node, then choose Action⇒Create Volume. Follow the onscreen instructions . For more information, see the online help.
- Use the following form of the metainit command.
```
# metainit volume-name -m submirror-name
```
  volume-name
  
  Specifies the name of the volume to create.
  
  -m
  
  Specifies to create a mirror.
  
  submirror-name
  
  Specifies the name of the component that will be the first submirror in the mirror. In this example, it is the RAID-0 volume that contains the root slice.
Remount your newly mirrored file system. Run the metaroot volume-name command, replacing volume-name with the name of the mirror you have created. Then, reboot your system.
```
# metaroot volume-name
# reboot
```
For more information, see the metaroot(1M) man page.
Use the following form of the metattach command to attach the second submirror.
```
# metattach volume-name submirror-name
```
volume-name

Specifies the name of the RAID-1 volume on which to add the submirror

submirror-name

Specifies the name of the component that will be the second submirror attached to the mirror

See the metattach(1M) man page for more information.
Record the alternate boot path.
1. Determine the path to the alternate root device. Use the ls -l command on the slice that is being attached as the second submirror to the root (/) file system mirror.
```
# ls -l /dev/dsk/c1t1d0s0
lrwxrwxrwx 1  root root  55 Mar 5 12:54  /dev/rdsk/c1t1d0s0 -> \ 
../../devices/sbus@1,f8000000/esp@1,200000/sd@3,0:a
```
2. Record the string that follows the /devices directory: /sbus@1,f8000000/esp@1,200000/sd@3,0:a.
  Note - Because the system might not be available, this information should also be written down somewhere other than on the system. See Recovering From Boot Problems for details on booting from the alternate boot device.
3. Edit the string to change the major name (sd, in this case) to disk, resulting in /sbus@1,f8000000/esp@1,200000/disk@3,0:a. If the system uses an IDE bus, the original full path might look like
```
$ ls -l /dev/dsk/c1t1d0s0 
lrwxrwxrwx 1 root root 38 Mar 13 15:03 /dev/dsk/c0t0d0s0 -> \
../../devices/pci@1f,0/ide@d/dad@0,0:a
```
  After changing the major name dad to disk, you would have /pci@1f,0/ide@d/disk@0,0:a
4. Use the OpenBoot^TM PROM nvalias command to define a “backup root” device alias for the secondary root (/) file system mirror. For example:
```
ok  nvalias backup_root /sbus@1,f8000000/esp@1,200000/disk@3,0:a
```
5. Redefine the boot-device alias to reference both the primary and secondary submirrors, in the order in which you want them to be used, and store the configuration.
```
ok printenv boot-device
boot-device =         disk net
ok setenv boot-device disk backup_root net
boot-device =         disk backup_root net
ok nvstore
```
Note - In the event that the primary submirror fails, the system would automatically boot to the second submirror. Or, if you boot manually, rather than using autoboot, you would enter:
```
ok boot backup_root
```

Example 11-6 SPARC: Creating a Mirror From the root (/) File System

# metainit -f d1 1 1 c0t0d0s0
d1: Concat/Stripe is setup
# metainit d2 1 1 c0t1d0s0
d2: Concat/Stripe is setup
# metainit d0 -m d1
d0: Mirror is setup
# metaroot d0
# lockfs -fa
# reboot
...
# metattach d0 d2
d0: Submirror d2 is attached
# ls -l /dev/dsk/c0t1d0s0
lrwxrwxrwx   1 root     root          88 Feb  8 15:51 /dev/rdsk/c1t3d0s0 ->
../../devices/pci@1f,0/pci@1,1/ide@3/dad@0,0:a
# init 0
.
.
.
ok nvalias backup_root /pci@1f,0/pci@1,1/ide@3/disk@0,0:a
ok setenv boot-device disk backup_root net
ok nvstore

In this example, the -f option forces the creation of the first RAID-0 volume, d1, which contains the mounted root (/) file system on /dev/dsk/c0t0d0s0. The second concatenation, d2, is created from /dev/dsk/c0t1d0s0. This slice must be the same size as, or larger than the size of d1. The metainit command with the -m option creates the one-way mirror d0 using the concatenation that contains root (/).

Next, the metaroot command edits the /etc/vfstab and /etc/system files so that the system can be booted with the root (/) file system on a volume. It is a good idea to run the lockfs -fa command before rebooting. For more information, see the lockfs(1M) man page.

Do not attach the second submirror before the system is rebooted. You must reboot after running the metaroot command and before attaching the second submirror.

After a reboot, the submirror d2 is attached to the mirror, causing a mirror resynchronization. The system confirms that the concatenations and the mirror are set up, and that submirror d2 is attached.

The ls -l command is run on the root raw device to determine the path to the alternate root device in case the system might later need to be booted from it.

x86: How to Create a RAID-1 Volume From the root (`/`) File System

The process for mirroring the root (/) file system on an x86 based system is similar to mirroring root on a SPARC system. However, on x86 based systems, the BIOS and fdisk partitioning add an additional layer of complexity.

In the example used in the procedures, the existing slice is c1t0d0s0. The second slice, c1t1d0s0, is available for the second half of the mirror. The submirrors are d1 and d2, respectively, and the mirror is d0.

Before You Begin

Before implementing any of the procedures, check Prerequisites for Creating Solaris Volume Manager Components and Creating and Maintaining RAID-1 Volumes.

Verify that the ordering for the BIOS boot device can be configured to allow the system to boot off of the second disk in the mirror.
Before the kernel is started, the system is controlled by the read-only-memory (ROM) Basic Input/Output System (BIOS), which is the firmware interface on an x86 based system. The BIOS is analogous to the boot PROM on a SPARC based system. Some of the BIOS's tasks are as follows:
- Perform startup functions.
- Detect the correct device from which to boot the system.
- Load the master boot record from that device to allow the system to self-boot.
You can usually configure the BIOS to select the order of devices to probe for the boot record. Additionally, most modern BIOS implementations allow you to configure your devices so that the failover to the secondary submirror is automatic. If your system's BIOS does not have this feature and the primary submirror fails, you need to access the BIOS during system boot to reconfigure the system to boot from the secondary root slice. Consult the user's guide for your BIOS for instructions on how to configure settings in your BIOS
Before setting up a root mirror, check the BIOS on your system to verify that you can boot off of more than one disk. Some device drivers are configured to only see one disk on the system.
Verify that the fdisk partitions are configured to support root mirroring.
The existence of a separate x86 boot partition presents a problem when mirroring the root (/) file system. Because it exists outside of the Solaris fdisk partition, the x86 boot partition cannot be mirrored by Solaris Volume Manager. Additionally, because only one copy of the x86 boot partition exists, it represents a single point of failure.
The GRUB-based installation program of the Solaris 10 1/06 software and subsequent releases no longer automatically creates an x86 boot partition. However, if the x86 already exists in the system, the installation program preserves that partition by default.
To determine if your system has a separate x86 boot partition, check the /etc/vfstab file. The x86 boot partition exists if the file contains an entry similar to the following:
```
/dev/dsk/c2t1d0p0:boot  -       /boot   pcfs    -       no      -
```
To use the Solaris Volume Manager to mirror the root (/) file system, the file system must use the single Solaris fdisk partition. Therefore, if the x86 boot partition already exists in the system, delete this partition with the fdisk command and then reinstall the Solaris software. When you reinstall, the boot partition is no longer recreated.

Note - Solaris Volume Manager can only mirror slices within the Solaris fdisk partition If you have multiple fdisk partitions, you need to use another approach to protect the data outside of the Solaris fdisk partition.

Make the secondary submirror bootable with a master boot program.

Specify the master boot program.

# fdisk -b /usr/lib/fs/ufs/mboot /dev/rdsk/c1t1d0p0

The following screen appears:

             Total disk size is 31035 cylinders
             Cylinder size is 1146 (512 byte) blocks

                                               Cylinders
      Partition   Status    Type          Start   End   Length    %
      =========   ======    ============  =====   ===   ======   ===
          1        Active   Solaris           1  31034    31034    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:

Choose number 5 from the menu, and press return.

Make the secondary disk bootable.
```
# /sbin/installgrub /boot/grub/stage1 /boot/grub/stage2 /dev/rdsk/c1t1d0s0
```
For more information about installgrub, refer to the installgrub(1M) man page.
Identify the slice that contains the existing root (/) file system to be mirrored.
This example uses the slice c1t0d0s0.
Create a new RAID-0 volume on the slice from the previous step.
Only the single slice can be included in the RAID-0 volume. Use one of the following methods:
- From the Enhanced Storage tool within the Solaris Management Console, open the Volumes node, then choose Action⇒Create Volume. Follow the onscreen instructions. For more information, see the online help.
- Use the following form of the metainit command:
```
# metainit -f volume-name number-of-stripes components-per-stripe component-name
```
  -f
  
  Forces the command to continue. You must use this option when the slice contains a mounted file system.
  
  volume-name
  
  Specifies the name of the volume to create. For information on naming volumes, see Volume Names.
  
  number-of-stripes
  
  Specifies the number of stripes to create.
  
  components-per-stripe
  
  Specifies the number of components each stripe should have.
  
  component-names
  
  Specifies the names of the components that are used. This example uses the root slice, c0t0d0s0.
Create a second RAID-0 volume (c1t1d0s0 in this example) on an unused slice to act as the second submirror.
Note - The secondary submirror must be the same size as the original submirror, or larger. Also, the slice you use as the second submirror must have a slice tag of “root” and the root slice must be slice 0.
For information on configuring the slice tag field, see the format(1M) man page.

Use either of the following methods:
- From the Enhanced Storage tool within the Solaris Management Console, open the Volumes node, then choose Action->Create Volume and follow the instructions on screen. For more information, see the online help.
- Use the following form of the metainit command.
```
# metainit volume-name number-of-stripes components-per-stripes component-names
```
  Note - See Step 6 for an explanation of the options.
Create a one-way mirror by using one of the following methods:
- From the Enhanced Storage tool within the Solaris Management Console, open the Volumes node, then choose Action⇒Create Volume. Follow the onscreen instructions. For more information, see the online help.
- Use the following form of the metainit command.
```
# metainit volume-name -m submirror-name
```
  volume-name
  
  Specifies the name of the volume to create.
  
  -m
  
  Specifies to create a mirror.
  
  submirror-name
  
  Specifies the name of the component that will be the first submirror in the mirror. In this example, it is the RAID-0 volume that contains the root slice.
Remount your newly mirrored file system, then reboot the system.
```
# metaroot volume-name
# reboot
```
For more information, see the metaroot(1M) man page.
Attach the second submirror.
```
# metattach volume-name submirror-name
```
volume-name

Specifies the name of the RAID-1 volume on which to add the submirror.

submirror-name

Specifies the name of the component that will be the second submirror attached to the mirror.

See the metattach(1M) man page for more information.
Define the alternative boot path in the menu.lst file.
To enable the system to boot off of the disk that holds the secondary submirror, configure the system to see the disk as the alternate boot device. In the current example, c1t1d0s0, the alternative path is on the first slice of the first fdisk partition on the second disk. Thus, you would edit the menu.lst with the following entry:
```
title alternate boot
 root (hd1,0,a)
 kernel /boot/multiboot
 module /boot/x86.miniroot-safe
```
Note - To properly edit entries in menu.lst, you must be familiar with disk-naming conventions in GRUB. For details, see Chapter 11, GRUB Based Booting (Tasks), in System Administration Guide: Basic Administration

After you have completed editing the menu.lst file, the system is set to failover to the second disk. If the primary disk fails, disk numbering changes so that the system boots from the secondary disk.

Caution - On certain cases, the automatic disk-renumbering feature of the BIOS might affect recovery from an unavailable primary disk. When disk renumbering forces the system to boot from the secondary disk, the primary disk's boot archive becomes stale. If the same primary disk becomes available later and you boot the system, the disk numbering switches again to use the default primary disk for the system boot. However, at this stage, the primary disk's boot archive remains stale. Consequently, the system might not boot at all. Therefore make sure that in such cases, you select the correct entry from the GRUB menu to boot the system from the valid boot archive. After the system completes the boot up process, perform the normal metadevice maintenance which synchronizes both primary and secondary disks and restores the valid boot archive to the primary disk.

Understanding Boot Time Warnings When Mirroring the root (`/`) File System

After you mirror your root (/) file system, error messages will be displayed in the console and logged in the system log that is defined in the /etc/syslog.conf file. These error messages do not indicate a problem. These messages are displayed for each device type that you are not currently using, because an unused module cannot be force loaded. The error messages are similar to the following:

Jul 13 10:17:42 ifr genunix: [ID 370176 kern.warning] WARNING: forceload of 
    misc/md_trans failed
Jul 13 10:17:42 ifr genunix: [ID 370176 kern.warning] WARNING: forceload of 
    misc/md_raid failed
Jul 13 10:17:42 ifr genunix: [ID 370176 kern.warning] WARNING: forceload of 
    misc/md_hotspares failed

You can safely disregard these error messages.