Solaris Volume Manager Administration Guide

Creating and Maintaining RAID-1 Volumes

This section provides guidelines can assist you in creating mirrors. This section also provides performance guidelines for the mirrors that you create.

Configuration Guidelines for RAID-1 Volumes

Performance Guidelines for RAID-1 Volumes

About RAID-1 Volume Options

The following options are available to optimize mirror performance:

You can define mirror options when you initially create the mirror. You can also change mirror options after a mirror has been set up and is running. For tasks related to changing these options, see How to Change RAID-1 Volume Options.

RAID-1 Volume Read-and-Write Policies

Solaris Volume Manager enables different read-and-write policies to be configured for a RAID-1 volume. Properly set read-and-write policies can improve performance for a given configuration.

Table 10–1 RAID-1 Volume Read Policies

Read Policy 


Round-Robin (Default) 

Attempts to balance the load across the submirrors. All reads are made in a round-robin order (one after another) from all submirrors in a mirror. 


Enables reads to be divided among submirrors on the basis of a logical disk block address. For example, with a two-way submirror, the disk space on the mirror is divided into two equally-sized logical address ranges. Reads from one submirror are restricted to one half of the logical range. Reads from the other submirror are restricted to the other half. The geometric read policy effectively reduces the seek time that is necessary for reads. The performance gained by this read policy depends on the system I/O load and the access patterns of the applications. 


Directs all reads to the first submirror. This policy should be used only when the device or devices that comprise the first submirror are substantially faster than the devices of the second submirror. 

Table 10–2 RAID-1 Volume Write Policies

Write Policy 


Parallel (Default) 

Performs writes to a mirror that are replicated and dispatched to all of the submirrors simultaneously. 


Performs writes to submirrors serially (that is, the first submirror write completes before the second submirror write is started). This policy specifies that writes to one submirror must be completed before the next submirror write is initiated. This policy is provided in case a submirror becomes unreadable, for example, due to a power failure. 

Pass Number

The pass number, a number in the range 0–9, determines the order in which a particular mirror is resynchronized during a system reboot. The default pass number is 1. The lower pass numbers are resynchronized first. If zero is used, the mirror resynchronization is skipped. A pass number of zero should be used only for mirrors that are mounted as read-only. Mirrors with the same pass number are resynchronized at the same time.

Understanding Submirror Status to Determine Maintenance Actions

The metastat command of Solaris Volume Manager reports status information on RAID 1 volumes and submirrors. The status information helps you to determine if maintenance action is required on a RAID-1 volume. The following table explains submirror states shown when you run the metastat command on a RAID-1 volume.

Table 10–3 Submirror States




The submirror has no errors and is functioning correctly. 


The submirror is actively being resynchronized. An error has occurred and has been corrected, the submirror has just been brought back online, or a new submirror has been added. 

Resync canceled 

The resynchronization process on the submirror has been canceled using the metasync command.

Needs Maintenance 

A slice (or slices) in the submirror has encountered an I/O error or an open error. All reads and writes to and from this slice in the submirror have been discontinued. 

Additionally, for each slice in a submirror, the metastat command shows the following:


Indicates the device name of the slice in the stripe

Start Block

Indicates the block on which the slice begins


Indicates if the slice contains a state database replica


Indicates the state of the slice

Hot Spare

Indicates that a slice is being used as a hot spare for a failed slice

The submirror state only provides general information on the status of the submirror. The slice state is perhaps the most important information to review when you are troubleshooting mirror errors. If the submirror reports a “Needs Maintenance” state, you must refer to the slice state for more information.

You take a different recovery action depending on if the slice is in the “Maintenance” state or in the “Last Erred” state. If you only have slices in the “Maintenance” state, they can be repaired in any order. If you have slices both in the “Maintenance” state and in the “Last Erred” state, you must fix the slices in the “Maintenance” state first. Once the slices in the “Maintenance” state have been fixed, then fix the slices in the “Last Erred” state. For more information, see Overview of Replacing and Enabling Components in RAID-1 and RAID-5 Volumes.

The following table explains the slice states for submirrors and possible actions to take.

Table 10–4 Submirror Slice States





The slice has no errors and is functioning correctly. 



The slice is actively being resynchronized. An error has occurred and been corrected, the submirror has just been brought back online, or a new submirror has been added. 

If desired, monitor the submirror status until the resynchronization is done. 


The slice has encountered an I/O error or an open error. All reads and writes to and from this component have been discontinued. 

Enable or replace the failed slice. See How to Enable a Slice in a Submirror, or How to Replace a Slice in a Submirror. The metastat command will show an invoke recovery message with the appropriate action to take with the metareplace command. You can also use the metareplace -e command.

Last Erred 

The slice has encountered an I/O error or an open error. However, the data is not replicated elsewhere due to another slice failure. I/O is still performed on the slice. If I/O errors result, the mirror I/O fails. 

First, enable or replace slices in the “Maintenance” state. See How to Enable a Slice in a Submirror, or How to Replace a Slice in a Submirror. Usually, this error results in some data loss, so validate the mirror after it is fixed. For a file system, use the fsck command, then check the data. An application or database must have its own method of validating the device.