Overview of Mirroring

DiskSuite provides the ability to replicate data stored on a particular metadevice onto as many as two additional (for a total of three) metadevices, referred to as submirrors. By setting up a minimum configuration of a two-way mirror you can recover from a single-component failure and perform online backups of file systems.

Mirroring components improves data reliability. An error on a component does not cause the entire mirror to fail. For continued reliability after failure, DiskSuite provides a facility for creating hot spare components. For further information on this utility, see Chapter 7, "Hot Spares."

Although mirroring helps ensure data reliability, it can cause the I/O performance to suffer on some systems. However, the performance penalty can be minimized by the correct configuration of the mirrors.

To get maximum protection and performance, place mirrored metadevices on different physical components (disks) and on different disk controllers. Since the primary purpose of mirroring is to maintain availability of data, defining mirrored metadevices on the same disk is not recommended. If the disk were to fail, both metadevices would fail.

Operation of Mirrors

Mirroring is accomplished by the creation of mirrors. A mirror is a special type of metadevice that is made up of one or more other metadevices. Each metadevice within a mirror is called a submirror.

Mirrors use the same naming convention as other metadevices (concatenations and stripes). After a mirror is defined, additional submirrors can be added at any time without disruption of writes or reads to the existing mirror.

Mirrors should be created with a single submirror at first. Then additional submirrors can be added. When you are mirroring existing file systems or data, be sure the existing data is contained on the submirror initially defined with the mirror. When a second submirror is subsequently attached, data from the initial submirror is copied onto the attached submirror automatically

If you are creating mirrors from slices that contain existing data, you must create a one-way mirror of the slices where the data exists, then attach the second submirror.

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Caution - In cases where you are mirroring existing data, be sure that the initial submirror contains the data you want mirrored.

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Creating Mirrors

DiskSuite Tool provides two methods for creating mirrors:

• Define a one-way mirror using the templates. This is accomplished by dragging and dropping slices into a Concat/Stripe template to create either a concatenation or stripe. You then drag the newly created Concat/Stripe object into a mirror template and select Commit.
• Define a one-way mirror using the Mirror Information window, which is explained in "Mirror Information Window" on page 137. This is done by first filling in the names of the devices for one submirror, then entering the names of the other devices.

The first method is used throughout this section.

Mirroring Unused Slices

The following example shows the creation of metadevices (concatenations) and one-way mirrors using the drag-and-drop functionality of DiskSuite Tool.

The example is a mirror that is made up of two submirrors, each containing two physical components that make up the metadevices. The physical components are named c4t0d1s0, c5t0d1s0, c6t1d2s0, and c7t0d3s0. The two submirrors are named d6 and d7. The Mirror is named d8.

To set up this mirror of unused slices using the DiskSuite Tool, you would perform the following steps:

1. Start DiskSuite Tool.

# metatool

2. Open two Concat/Stripe templates on the Metadevice Editor canvas.

   Click on the Concat/Stripe template twice. The Concat/Stripe templates opened are d6 and d7. Each of the two metadevices created in the Concat/Stripe templates will be a submirror in the mirror you are creating.

3. Open the Slice Browser.

   Click on Slices on the button panel.

4. Select slices for the Concat/Stripe templates.

   Scroll through the list of the devices in the Slice Browser and locate the slices that will be placed in the Concat/Stripe templates. The slices you select for one concatenation or stripe should be from disks that are attached to different controllers than those selected for the other concatenation or stripe. This will help ensure availability of the data if a controller fails. Also, approximately the same size slices should be selected for each of the Concat/Stripe templates. Any mismatch in the submirror sizes will lessen capacity, as the mirror will be only as large as the smallest submirror.

   Click on the first slice that will be placed in the Concat/Stripe object, then hold down the Control key and click on the second. (Multiple slices can also be selected by holding down the left button and dragging the mouse cursor.)

5. Drag the slices into the Concat/Stripe templates.

   Point to one of the selected slices, press and hold down the middle button, and drag the slice to the Concat/Stripe template. Repeat the procedure for the second Concat/Stripe template.

   In this example, each of the Concat/Stripe templates have been populated with two 989.30-Mbyte slices. The slices were concatenated to create two 1.93-Gbyte metadevices. (Refer to Chapter 4, "Concatenating and Striping," for additional information on creating concatenations and stripes.)

6. Commit the two concatenations.

   Click on the top rectangle of one of the Concat/Stripe objects. Hold down the Control key and click on the top rectangle of the second. Then click on the Commit button.

7. Open a Mirror template on the Metadevice Editor Canvas.

   Click on the Mirror template. An unassigned Mirror template is opened. When you point to the template, the message line displays the metadevice name, size, use, and status.

Click here to open the Mirror template.

8. Drag the Concat/Stripe templates into the Mirror template.

   Click on the top rectangle of one of the Concat/Stripe objects. Hold down the Control key and click on the top rectangle of the second. Point inside one of the Concat/Stripe templates and press and hold down the middle button. Drag the objects into the Mirror template.

9. Commit the Mirror.

   Click on the top rectangle of the Mirror. Then click on the Commit button.

Mirroring Existing File Systems

DiskSuite Tool enables you to mirror existing file systems without backing up data or reconfiguring devices.

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Note - Because the root (/), swap, and some other file systems contain binary files that are necessary when the system is booting, special instructions must be used to mirror these file systems. Refer to "Mirroring File Systems You Cannot Unmount" in this chapter for detailed instructions.

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To mirror an existing file system, you must use an additional component of equal or greater size. It is possible to use a concatenation of two or more components that have adequate space available to contain the submirror. For example, if /var is mounted on a 1-Gbyte component named c4t0d2s0, at least one Gbyte of space must be available on the metadevice being defined for the new submirror.

When you mirror an existing file system, first configure a one-way mirror with a submirror containing the existing file system. A submirror that you add subsequently should not contain any data that is needed, because it will overwritten by the resync that follows the attachment.

The following example shows the step-by-step creation of a metadevice (Concat/Stripe), which is then added to an existing one-way mirror to create a two-way mirror. This is done using the drag and drop functionality of DiskSuite Tool.

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Caution - The instructions for mirroring an existing file system are very similar to the instructions for mirroring unused slices. However, it is important to follow the procedure in the correct order or the mirroring will not function properly and existing data will be lost.

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The example is a mirror that is made up of two physical components and two submirrors (metadevices). The physical components are named c4t0d2s0 and c5t0d2s0. The two submirrors are named d9 and d17. The Mirror is named d18.

To set up this mirror of an existing file system using DiskSuite Tool, you would perform the following steps:

1. Unmount the file system.

   In this example, the existing file system is named /var and is on slice c4t0d2s0. If the file system is busy, an error message is displayed.

# /sbin/umount /var

2. Start DiskSuite Tool.

# metatool

3. Open a Concat/Stripe template on the Metadevice Editor canvas.

   Click on the Concat/Stripe template. An unassigned and uncommitted Concat/Stripe template is opened. When you point to the template, the message line displays the metadevice name, size, use, and status.

4. Open the Slice Browser.

   Click on Slices on the button panel.

5. Locate the slice where /var resides.

   Click on slice c4t0d2s0 in the Slice Browser. Note that the Use field does not reflect that the slice is where the /var resides because the file system has been unmounted.

6. Drop the slice into the Concat/Stripe template.

   Point to slice c4t0d2s0, press the middle button, and drag the slice to the Concat/Stripe d9 template.

7. Open another Concat/Stripe template.

   Click on the Concat/Stripe template.

8. Select another slice.

   Scroll through the list of slices in the Slice Browser and locate a slice that is the same size or larger than c4t0d2s0. To help ensure availability, select a slice that is attached to a different controller than c4.

9. Drop the slice in the Concat/Stripe template.

   Point to slice c5t0d2s0 and hold down the middle button. Drag the slice to the Concat/Stripe d17 template.

10. Commit the two Concat/Stripe templates.

    Click on the top rectangle of one of the Concat/Stripe objects. Hold down the Control key and click on the top rectangle of the second. Then click on the Commit button.

11. Open a Mirror template on the Metadevice Editor Canvas.

    Click on the Mirror template. An unassigned Mirror template is opened. When you point to the template, the message line displays the metadevice name, size, use, and status.

12. Drag the d9 Concat/Stripe into the Mirror template. Drag the d9 Concat/Stripe, which contains the slice where the /var file system resides, into the Mirror template. The d9 Concat/Stripe object must be the first one placed in the Mirror template.

13. Commit the Mirror.

    Click on the top rectangle of the Mirror. Then click on the Commit button.

14. Edit the /etc/vfstab file.

    After you complete the previous steps, you must change the entry for /var in the /etc/vfstab file. For example, the following line:

/dev/dsk/c4t0d2s0 /dev/rdsk/c4t0d2s0 /var ufs 4 yes --

should be changed to read:

/dev/md/dsk/d18 /dev/md/rdsk/d18 /var ufs 4 yes --

15. Mount the /var file system.

    Enter the following command to mount the /var file system.

# /sbin/mount /var

16. Drag the other Concat/Stripe into the Mirror template.

    Point to the d17 Concat/Stripe template and hold down the middle button. Drag the object into the Mirror template. A resync of the Mirror (copying all the information from d9 to d17) automatically occurs. The Use field of the Mirror will change to /var after you press the Update Status button in the button panel.

17. Commit the Mirror again.

    Click on the top rectangle of the d18 Mirror. Then click on the Commit button.

Unmirroring File Systems

This subsection contains the procedure for unmirroring file systems that can be unmounted while the system is running. The following is an example of unmirroring the /var file system. In this example, /var is made up of a two-way Mirror with the names d9 and d17 in a Mirror named, d18. The names of the components are c4t0d2s0 and c5t0d2s0.

The steps to follow when unmirroring the file system are:

1. Unmount the /var file system.

# /sbin/umount /var

2. Start DiskSuite Tool.

# metatool

3. Open the d18 Mirror.

   Scroll through the device list in the Metadevice Editor window and locate the d18 Mirror. Point to the Mirror and double-click.

4. Delete the Mirror.

   Point inside the top rectangle of the Mirror and press the right button to bring up the menu. Select the Delete option.

5. Confirm the deletion of the Mirror.

   A Confirmation dialog box asks if you are sure you want to delete the Mirror. Click on the Really Delete button. This dialog box appears even if all the steps are followed correctly.

6. Edit the /etc/vfstab file.

   After you complete the previous steps, you must change the entry for /var in the /etc/vfstab file. For example, the following line:

/dev/md/dsk/d18 /dev/md/rdsk/d18 /var ufs 4 yes --

should be changed to read:

/dev/md/dsk/d9 /dev/md/rdsk/d9 /var ufs 4 yes --

7. Remount the /var file system.

   Enter the following command to mount the /var file system.

# /sbin/mount /var

When /var is remounted, the Concat/Stripe d9 contains the file system.

To have the file system mounted on the slice rather than in the Concat/Stripe, you must first delete the Concat/Stripe. Then edit /etc/vfstab file to read:

/dev/dsk/c4t0d2s0 /dev/rdsk/c4t0d2s0 /var ufs 4 yes --

Mirroring File Systems You Cannot Unmount

DiskSuite Tool supports mirroring the root (/) and swap file systems, along with any other file systems (for example, /opt and /usr) that cannot be unmounted while the system is running.

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Note - If problems occur when mirroring root (/), swap, and /usr, refer to Appendix C of the Solstice DiskSuite 4.0 Administration Guide. This appendix explains how to recover from a failed boot.

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Caution - When you are mirroring the root file system, you must record the secondary root slice name so you can reboot the system if the primary submirror fails. This information should be written down, not recorded on the system which may not be available. Read the section, "Recording and Booting From Alternate Root File Systems" on page 120 for details.

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Note - DiskSuite Tool does not support mirroring root (/) with submirrors that are made up of more than one slice, because the system cannot boot.

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This subsection contains an example that places the root (/) file system on a two-way mirror. The same instructions apply to mirroring swap and other file systems that cannot be unmounted (for example, /opt or /usr).

In this example, root (/) resides on c0t2d0s0. It will be mirrored on c0t3d0s0. The names of the Concat/Stripe objects will be d6 and d8. The name of the Mirror will be d7. The steps to follow are:

1. Start DiskSuite Tool.

# metatool

2. Open a Concat/Stripe template on the Metadevice Editor canvas.

   Click on the Concat/Stripe template. An unassigned and uncommitted Concat/Stripe template is opened. When you point to the template, the message line displays the metadevice name, size, use, and status.

3. Open the Slice Browser.

   Click on Slices on the button panel.

4. Locate the slice to be mirrored.

   Slice c0t2d0s0, which contains the root file system, is selected in this example.

5. Drag c0t2d0s0 into the Concat/Stripe template. Click on slice c0t2d0s0. Hold down the middle button. Then drag the slice to the Concat/Stripe d6 template. When the slice is dropped, a Warning dialog box is displayed. Click on the Continue button.

6. Commit the Concat/Stripe template.

   Click inside the top rectangle of the Concat/Stripe object. Then click on Commit on the button panel.

When the Commit action is invoked, the following warning dialog box is displayed. Click on the Really Commit button.

7. Open a Mirror template.

   Click on the Mirror template. An unassigned and uncommitted Mirror template is opened.

8. Drag the d6 Concat/Strip into the Mirror template. Click on the d6 Concat/Stripe object. Hold down the middle button. Drag the object into the Mirror template. The following warning dialog box is displayed. Click on the Continue button.

9. Commit the Mirror.

   Click on the top rectangle in the Mirror template. Then click on Commit on the button panel.

When the Commit action is invoked, the following dialog box appears. Click on the Really Commit button.

10. Reboot the system.

    You must reboot the system.

# reboot

11. After the system reboots, start DiskSuite Tool.

# metatool

12. Open the d7 Mirror object.

    Scroll through the list of metadevices in the device list and double-click on the Mirror that contains the root file system (d7). The status remains Urgent because another submirror needs to be assigned.

13. Open a Concat/Stripe template on the Metadevice Editor canvas.

    Click on the Concat/Stripe template.

14. Populate the d8 Concat/Stripe with a slice.

    Select a slice in the Slice Browser that is the same size or larger than the root file system. The slice must be on a different disk and attached to a different controller. Point to the slice and press and hold down the middle button. Drag the slice into the d8 Concat/Stripe template.

15. Commit the d8 Concat/Stripe.

    Click on the top rectangle of the Concat/Stripe template. Then click on the Commit button.

16. Drag the d8 Concat/Stripe into the d7 Mirror. Point to the d8 Concat/Stripe object. Press and hold down the middle button. Drag the cursor into the Mirror template.

17. Commit the Mirror.

    Click on the top rectangle of the Mirror object. Then click on Commit on the button panel.

If problems arise, refer to Chapter 5 of the Solstice DiskSuite 4.0 Administration Guide.

Unmirroring File Systems You Cannot Unmount

DiskSuite Tool does not support unmirroring the root (/) and swap file systems, or any other file systems (for example, /opt and /usr) that cannot be unmounted while the system is running.

Instead, use DiskSuite commands as documented in Chapter 5 of the Solstice DiskSuite 4.0 Administration Guide.

Recording and Booting From Alternate Root File Systems

If you mirrored root on either a SPARC or x86 system and the primary boot disk is lost, you must perform special steps to boot the system. A discussion of these procedures is given in Chapter 5 of the Solstice DiskSuite 4.0 Administration Guide.

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Note - You must have recorded the path to the alternate root device when you are creating the mirror.

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The method to use to obtain this path on a SPARC system is shown in the following example. This example assumes the root file system is being mirrored on slice c1t3d0s0.

# ls -l /dev/rdsk/c1t3d0s0 lrwxrwxrwx 1 root root 55 Mar 5 12:54 /dev/rdsk/c1t3d0s0 -> ../../devices/sbus@1,f8000000/esp@1,200000/sd@3,0:a

In the above SPARC example, you would record:

/sbus@1,f8000000/esp@1,200000/sd@3,0:a

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Note - DiskSuite Tool users who are using system with open boot prom can use the OpenBoot nvalias command to define a "backup root" devalias for the secondary root mirror. For example:

ok   nvalias backup_root /sbus@1,f8000000/esp@1,200000/sd@3,0:a



In the event of primary root disk failure, you then would only enter: ok



     
boot backup_root

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The method to use to obtain this path on an x86 system is shown in the following example. This example assumes the root file system is being mirrored on slice c1t0d0s0.

# ls -l /dev/rdsk/c1t0d0s0 lrwxrwxrwx 1 root root 55 Mar 5 12:54 /dev/rdsk/c1t0d0s0 -> ../../devices/eisa/eha@1000,0/cmdk@1,0:a

In the above x86 example, you would record:

eisa/eha@1000,0/cmdk@1,0:a

Reconfiguring Submirrors

DiskSuite Tool provides several methods for reconfiguring submirrors with little or no disruption of service. The reconfiguring procedures are performed using the "Mirror Information Window" on page 137. The following subsections explain the steps used to attach or detach submirrors, bring submirrors online and offline, and to replace components within submirrors.

Replacement of a failed submirror can be performed in DiskSuite Tool by dropping a new metadevice on the failed component. The replacement can be for a single slice or the entire submirror (multiple slices).

To replace a failed slice, drag a new slice of equal or greater size onto the failed slice and release. Then commit the object. The instructions for replacing a slice in a mirror can be found in "Replacing and Enabling Submirror Components" on page 134.

Replacing Submirrors

To replace an entire submirror, you must first construct a new Concat/Stripe object that has a size equal to or greater than the size of the submirror that is being replaced. The new object is dropped onto the failed submirror. The mirror must be committed following the replacement.

The procedure for replacement of an entire submirror includes the following steps:

1. Start DiskSuite Tool.

# metatool

2. Open the Mirror object on the Metadevice Editor canvas.

   Point to the Mirror (in this example d5) in the device list and double-click.

3. Open a Concat/Stripe template on the Metadevice Editor canvas.

   Click on the Concat/Stripe template.

4. Open the Slice Browser.

   Click on Slices on the button panel.

5. Select slices for the Concat/Stripe template.

   Locate slices in the Slice Browser that when combined are equal to or greater than the size of the slices being replaced.

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Note - The slices should be on different controllers than the slices in the other submirror. In this example, the Concat/Stripe objects in the d5 Mirror have each been populated with two 989.30-Mbyte slices to create two 1.93-Gbyte concatenations.

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6. Drag the slices into the Concat/Stripe template.

   Point to one of the two selected slices and press and hold down the middle button. Drag the slices to the Concat/Stripe d9 template.

7. Commit the Concat/Stripe object.

   Click inside the top rectangle of the Concat/Stripe. Then on the Commit button.

8. Replace the submirror.

   Point to the d9 Concat/Stripe object and press and hold down the middle button. Move the cursor to the top of the submirror that is being replaced. In this example, the d3 Concat/Stripe object is replaced with the d9 Concat/Stripe.

9. Commit the Mirror.

   Click inside the top rectangle of the Mirror. Then click on the Commit button.

A resync will be initiated when you click on the Commit button.

Attaching and Detaching Submirrors

Concat/Stripe objects (metadevices) are attached to mirrors by dragging the object into the mirror template and committing the new mirror. Submirrors are detached by dragging the Concat/Stripe object from the mirror and then committing the mirror.

DiskSuite Tool automatically starts a resync operation on the new Concat/Stripe object after it has been attached to an existing mirror and the mirror is committed. The addition or replacement of a submirror occurs without an interruption of service to the component.

When Concat/Stripe objects are detached from the mirror and committed, they are no longer part of the mirror. Reads from and writes to the mirror no longer go to the detached submirror. The detached metadevice can be used for other purposes. To help protect your data, DiskSuite Tool does not allow the last remaining submirror of a mirror that is in use to be detached.

For example, a Concat/Stripe object could be detached (removed) when errors are being reported. To perform this procedure, use the following steps:

1. Start DiskSuite Tool.

# metatool

2. Open the Mirror on the Metadevice Editor canvas.

   Locate the desired Mirror in the device list of the Metadevice Editor window. Point to the Mirror and double-click.

3. Bring up the Mirror Information window.

   Point to the top of the Mirror object and press the right button. Select the Info option.

4. Detach the submirror.

   When the Mirror Information window opens, select the submirror that will be detached by pointing to it and clicking the left button. Click on the Remove button. (Refer to "Mirror Information Window" on page 137 for additional information about this window.)

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Note - To attach a new submirror, you would open the Mirror Information window for the desired Mirror, enter the name of the device in the Device field, and click on the Attach button.

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5. Commit the Mirror.

   Click inside the top rectangle of the Mirror. Then click on the Commit button.

Placing Submirrors Online and Offline

DiskSuite Tool enables you to take submirrors offline and bring them back online. This functionality is useful when, for instance, one component in a physical SCSI chain fails. In this case, all other components on the chain could be taken offline while the broken component is replaced. After replacing the component, the other components in the SCSI chain can be brought back online.

When a submirror is offline, the mirror does not read from or write to that submirror. While the submirror is offline, DiskSuite Tool keeps track of all writes to the mirror. When the submirror is brought back online, the mirror and submirror are resynced. Writes are also directed to the submirror during the resync. Reads, however, come from a different submirror during the resync operation. Once the resync is complete, reads and writes are once again performed on the submirror previously taken offline.

Use the following steps to take a submirror offline.

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Note - You would follow the same procedure to bring a submirror back online. The difference is selecting the "Online" button on the Mirror Information window in Step 4.

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1. Start DiskSuite Tool.

# metatool

2. Open the Mirror on the Metadevice Editor canvas.

   Locate the desired Mirror in the device list of the Metadevice Editor window. Point to the Mirror and double-click.

3. Bring up the Mirror Information window.

   Point inside the top rectangle of the Mirror object and press the right button. Select the Info option.

4. Take the submirror offline.

   When the Mirror Information window opens, click on the submirror that will be taken offline. Then click on the Offline button. (Refer to "Mirror Information Window" on page 137 for additional information about using this window.)

The status of the Concat/Stripe object becomes Offline (scheduled).

5. Commit the Mirror.

   Click inside the top rectangle of the Mirror. Then click on the Commit button.

Replacing and Enabling Submirror Components

DiskSuite Tool can be used to replace failed components within a submirror with a new component. When the component is replaced, a resync automatically starts, bringing the new component in sync with the rest of the mirror.

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Note - Before using DiskSuite Tool to replace or enable submirror components, make sure the component is partitioned correctly. Refer to Chapter 5 of the Solstice DiskSuite 4.0 Administration Guide for additional information.

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A component being used as a replacement in a submirror must be at least as large as the component it is replacing.

Use the following instructions to replace a failed or errored component in a mirror:

1. Start DiskSuite Tool.

# metatool

2. Open the Mirror on the Metadevice Editor canvas.

   Locate the desired Mirror in the device list of the Metadevice Editor window. Point to the Mirror and double-click.

3. Open the Slice Browser.

   Click on Slices on the button panel.

4. Select a slice to replace the failed or errored component.

   Scroll through the list of devices in the Slice Browser and locate a slice that is the same size or larger than the failed or errored component.

5. Replace the failed or errored component with the new slice.

   Point to the new slice in the Slice Browser. Press down the middle button while moving the cursor to the rectangle that contains the slice you are replacing.

6. Commit the Mirror.

   Click inside the top rectangle of the Mirror. Then click on the Commit button.

Using Mirrors for Online Backup

Mirroring enables you to perform online backups. Because each submirror is an exact copy of the file system, it can be taken offline and backed up to tape. This eliminates taking the system down to make a backup of the system.

You cannot perform online backups using DiskSuite Tool. Instead, use the steps in Chapter 5 of the Solstice DiskSuite 4.0 Administration Guide.

Mirror Information Window

DiskSuite Tool provides several options to optimize mirror performance. These options deal with the read and write policy for mirrors, and the order in which mirrors are resynced during reboot. You set these and other options using choices on the Mirror Information window, as shown in Figure 5-1. There are three ways to display the Mirror Information window:

• Point to the mirror in the Metadevice Browser and double-click. The mirror will open on the Metadevice Editor's canvas and the Mirror Information window is displayed.
• If the mirror object is on the Metadevice Editor's canvas, point inside the mirror and bring up the mirror menu. Select the Info choice.
• Double-click on a mirror object on the Metadevice Editor's canvas.

Figure 5-1

The Mirror object must be committed before the policy changes take effect.

Table 5-1 lists the functionality associated with the regions of the Mirror Information window.

Table 5-1

Field	Functions
Device name	The metadevice name of the mirror.
Status	Description of the mirror's status. See "Checking the Status of Mirrors" on page 141 for a description of the values that can appear here.
Size	The size of the mirror.
Use	Shows how the mirror is currently used, for example, file system, swap, or shared log. If the use is shared log, a button labeled Show Trans is displayed. The Show Trans button opens a Sharing Information window that shows the Trans devices that share the Mirror.
Show slices	This toggle button enables you to turn on and off the slice manipulation region. The number of slices in the stripe are shown in parentheses on the button.
Pass	A pass number in the range 0-9 can be assigned to a mirror using the Pass button menu. The pass (resync) number determines the order in which that mirror is resynced during a system reboot. The default is 1. Smaller pass numbers are resynced first. If 0 is chosen, the resync is skipped. A 0 should only be used for mirrors mounted as read-only. If different mirrors have the same pass number, they are resynced concurrently.

Table 5-1

Field	Functions
Read Option	There are three kinds of read options associated with mirrors: Round Robin, Geometric, and First. The default read option is Round Robin, also called balanced load. When set to Round Robin, all reads are made in a round robin order from all the submirrors in the mirrors. That is, the first read comes from the first submirror, the next read comes from the second submirror, and so forth. The Geometric option provides faster performance on sequential reads or when you are using disks with track buffering. Geometric reads allow read operations to be divided among submirrors on the basis of a logical disk block address. For instance, with a three-way submirror the disk space on the mirror is divided into three (equally sized) logical address ranges. Reads from the three regions are then performed by separate submirrors (for example, reads to the first region are performed by the first submirror). The First option specifies reading from only the first submirror. This would be specified only if you have a second submirror that has poor read I/O characteristics.
Write Option	A button that enables you to set parallel or serial writes to the submirror. Parallel writes are the default action of the metadisk driver, meaning the writes are dispatched to all submirrors simultaneously.

Table 5-1

Field	Functions
Submirror manipulation region	The following functionality is available in this region: Show Submirrors - This toggle button enables showing or hiding the list of submirrors. Scrolling List - Shows submirrors included in the mirror. The information in this region includes the name, type, size, and status. Click on the submirror to select it. When submirrors are selected, actions can be performed on them. Online - Enables you to bring selected submirrors back online. This button is active only when the selected submirror is offline. Offline - Enables you to take selected submirrors offline. This button is active only when the selected submirror is online. Remove - Enables you to detach the selected submirrors. Info - Opens the Concat Information window for the selected submirror. Device - Enables you to specify a new submirror in the field to attach or replace. The field is cleared when you click on the Attach or Replace buttons. Attach - Adds the specified submirror. This button is active only when a submirror or device is entered in the Device field. Replace - Replaces the selected submirror with the submirror entered in the field. This button is active only when a submirror or device is entered in the field and one in the list is selected.

Checking the Status of Mirrors

There are three ways to check the status of submirrors using DiskSuite Tool. The methods are:

• Move the mirror from the device list and check the Status field on the submirrors.
• Bring up the Mirror Information window and check the Status field for each of the submirrors.
• Bring up the Problem List window, which is available using the Browse menu on the menu bar of the Metadevice Editor window. If a problem exists, a detailed description of the problem is given.

The Status fields of Mirror objects use the status keywords explained in Table 5-2.

Table 5-2

Keyword	Meaning
OK	The submirror has no errors and is functioning correctly.
Component Error	The submirror has a slice that has errors.
Resyncing	The submirror is actively being resynced. An error has occurred and been corrected, the submirror has just been brought back online, or a new submirror has been added.
Attaching	The submirror is being attached.
Attached (resyncing)	The submirror is being resynced after the attach occurred.
Offlined	The submirror is offline.
Errored	The submirror has an error.
Online (scheduled)	The submirror will be brought online the next time you click on the Commit button.
Attention	There is a problem with the submirror, but there is no immediate danger of losing data. There are still two copies of the data, but a problem exists somewhere in the configuration.
Urgent	A submirror has failed and only one copy of the data exists. You are only one failure away from losing data.
Critical	Data has potentially been corrupted.