Chapter 9 RAID 1 (Mirror) Volumes (Overview)

This chapter explains essential Solaris Volume Manager concepts related to mirrors and submirrors. For information about performing related tasks, see Chapter 10, RAID 1 (Mirror) Volumes (Tasks).

This chapter contains the following information:

• Overview of RAID 1 (Mirror) Volumes

• RAID 1 Volume (Mirror) Resynchronization

• Background Information for RAID 1 Volumes

• How Booting Into Single-User Mode Affects RAID 1 Volumes

Overview of RAID 1 (Mirror) Volumes

A RAID 1 volume, or mirror, is a volume that maintains identical copies of the data in RAID 0 (stripe or concatenation) volumes. Mirroring requires an investment in disks. You need at least twice as much disk space as the amount of data you have to mirror. Because Solaris Volume Manager must write to all submirrors, mirroring can also increase the amount of time it takes for write requests to be written to disk.

After you configure a mirror, it can be used just as if it were a physical slice.

You can mirror any file system, including existing file systems. You can also use a mirror for any application, such as a database.

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Tip –

Use Solaris Volume Manager's hot spare feature with mirrors to keep data safe and available. For information on hot spares, see Chapter 15, Hot Spare Pools (Overview) and Chapter 16, Hot Spare Pools (Tasks).

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Overview of Submirrors

The RAID 0 volumes that are mirrored are called submirrors. A mirror is made of one or more RAID 0 volumes (stripes or concatenations).

A mirror can consist of up to three submirrors. Practically, a two-way mirror is usually sufficient. A third submirror enables you to make online backups without losing data redundancy while one submirror is offline for the backup.

If you take a submirror “offline,” the mirror stops reading and writing to the submirror. At this point, you could access the submirror itself, for example, to perform a backup. However, the submirror is in a read-only state. While a submirror is offline, Solaris Volume Manager keeps track of all writes to the mirror. When the submirror is brought back online, only the portions of the mirror that were written while the submirror was offline (resynchronization regions) are resynchronized. Submirrors can also be taken offline to troubleshoot or repair physical devices which have errors.

Submirrors can be attached or detached from a mirror at any time, though at least one submirror must remain attached at all times.

Normally, you create a mirror with only a single submirror. Then, you attach a second submirror after you create the mirror.

Scenario—RAID 1 (Mirror) Volume

Figure 9–1 illustrates a mirror, d2, that is made of two volumes (submirrors) d20 and d21.

Solaris Volume Manager software makes duplicate copies of the data on multiple physical disks, and presents one virtual disk to the application. All disk writes are duplicated; disk reads come from one of the underlying submirrors. The total capacity of mirror d2 is the size of the smallest of the submirrors (if they are not of equal size).

Figure 9–1 Mirror Example

Providing RAID 1+0 and RAID 0+1

Solaris Volume Manager supports both RAID 1+0 (which is like having mirrors that are then striped) and RAID 0+1 (stripes that are then mirrored) redundancy, depending on the context. The Solaris Volume Manager interface makes it appear that all RAID 1 devices are strictly RAID 0+1, but Solaris Volume Manager recognizes the underlying components and mirrors each individually, when possible.

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Note –

Solaris Volume Manager cannot always provide RAID 1+0 functionality. However, in a best practices environment, where both submirrors are identical to each other and are made up of disk slices (and not soft partitions), RAID 1+0 will be possible.

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For example, with a pure RAID 0+1 implementation and a two-way mirror that consists of three striped slices, a single slice failure could fail one side of the mirror. And, assuming that no hot spares were in use, a second slice failure would fail the mirror. Using Solaris Volume Manager, up to three slices could potentially fail without failing the mirror, because each of the three striped slices are individually mirrored to their counterparts on the other half of the mirror.

Consider this example:

Figure 9–2 RAID 1+ 0 Example

Mirror d1 consists of two submirrors, each of which consists of three identical physical disks and the same interlace value. A failure of three disks, A, B, and F can be tolerated because the entire logical block range of the mirror is still contained on at least one good disk.

If, however, disks A and D fail, a portion of the mirror's data is no longer available on any disk and access to these logical blocks will fail.

When a portion of a mirror's data is unavailable due to multiple slice errors, access to portions of the mirror where data is still available will succeed. Under this situation, the mirror acts like a single disk that has developed bad blocks. The damaged portions are unavailable, but the rest is available.

Frequently Asked Questions About RAID 1 (Mirror) Volumes

Why would I use a mirror?

For maximum data availability. The trade-off is that a mirror requires twice the number of slices (disks) as the amount of data to be mirrored.

How many submirrors can a mirror contain?

Solaris Volume Manager allows you to create up to a three-way mirror (a mirror of three submirrors). However, two-way mirrors usually provide sufficient data redundancy for most applications, and are less expensive in terms of disk drive costs.

Why should I create a one-way mirror then attach additional submirrors?

If you have existing data that you are mirroring, you must create the primary submirror, then attach additional submirrors so they can be updated with the data that is contained in the primary submirror.

When should I create a two-way or three-way mirror with a single command?

If you have no existing data that you are mirroring and you are comfortable destroying all data on all submirrors, you can speed the creation process by creating all submirrors with a single command.

RAID 1 Volume Configuration Guidelines

• Keep the slices of different submirrors on different disks and controllers. Data protection is diminished considerably if slices of two or more submirrors of the same mirror are on the same disk. Likewise, organize submirrors across separate controllers, because controllers and associated cables tends to fail more often than disks. This practice also improves mirror performance.

• Use the same type of disks and controllers in a single mirror. Particularly in old SCSI storage devices, different models or brands of disk or controller can have widely varying performance. Mixing the different performance levels in a single mirror can cause performance to degrade significantly.

• Use the same size submirrors. Submirrors of different sizes result in unused disk space.

• Only mount the mirror device directly. Do not try to mount a submirror directly, unless it is offline and mounted read-only. Do not mount a slice that is part of a submirror. This process could destroy data and crash the system.

• Mirroring might improve read performance, but write performance is always degraded. Mirroring improves read performance only in threaded or asynchronous I/O situations. No performance gain results if there is only a single thread reading from the volume.

• Experimenting with the mirror read policies can improve performance. For example, the default read mode is to alternate reads in a round-robin fashion among the disks. This policy is the default because it tends to work best for UFS multiuser, multiprocess activity.

  In some cases, the geometric read option improves performance by minimizing head motion and access time. This option is most effective when there is only one slice per disk, when only one process at a time is using the slice/file system, and when I/O patterns are highly sequential or when all accesses are read.

  To change mirror options, see How to Change RAID 1 Volume Options.

• Use the swap -l command to check for all swap devices. Each slice that is specified as swap must be mirrored independently from the remaining swap slices.

• Use only similarly configured submirrors within a mirror. In particular, if you create a mirror with an unlabeled submirror, you will be unable to attach any submirrors that contain disk labels.

RAID 1 Volume Options

The following options are available to optimize mirror performance:

• Mirror read policy

• Mirror write policy

• The order in which mirrors are resynchronized (pass number)

You can define mirror options when you initially create the mirror, or after a mirror has been set up. 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 mirror. Properly set read and write policies can improve performance for a given configuration.

Table 9–1 Mirror Read Policies

Read Policy	Description
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.
Geometric	Enables reads to be divided among submirrors on the basis of a logical disk block address. For instance, 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, and reads from the other submirror are restricted to the other half. The geometric read policy effectively reduces the seek time necessary for reads. The performance gained by this mode depends on the system I/O load and the access patterns of the applications.
First	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 those of the second submirror.

Table 9–2 Mirror Write Policies

Write Policy	Description
Parallel (Default)	A write to a mirror is replicated and dispatched to all of the submirrors simultaneously.
Serial	Performs writes to submirrors serially (that is, the first submirror write completes before the second is started). The serial option specifies that writes to one submirror must complete before the next submirror write is initiated. The serial option is provided in case a submirror becomes unreadable, for example, due to a power failure.

RAID 1 Volume (Mirror) Resynchronization

RAID 1 volume (mirror) resynchronization is the process of copying data from one submirror to another after submirror failures, system crashes, when a submirror has been taken offline and brought back online, or after the addition of a new submirror.

While the resynchronization takes place, the mirror remains readable and writable by users.

A mirror resynchronization ensures proper mirror operation by maintaining all submirrors with identical data, with the exception of writes in progress.

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Note –

A mirror resynchronization is mandatory, and cannot be omitted. You do not need to manually initiate a mirror resynchronization. This process occurs automatically.

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Full Resynchronization

When a new submirror is attached (added) to a mirror, all the data from another submirror in the mirror is automatically written to the newly attached submirror. Once the mirror resynchronization is done, the new submirror is readable. A submirror remains attached to a mirror until it is explicitly detached.

If the system crashes while a resynchronization is in progress, the resynchronization is restarted when the system finishes rebooting.

Optimized Resynchronization

During a reboot following a system failure, or when a submirror that was offline is brought back online, Solaris Volume Manager performs an optimized mirror resynchronization. The metadisk driver tracks submirror regions and knows which submirror regions might be out-of-sync after a failure. An optimized mirror resynchronization is performed only on the out-of-sync regions. You can specify the order in which mirrors are resynchronized during reboot, and you can omit a mirror resynchronization by setting submirror pass numbers to 0 (zero). (See Pass Number for information.)

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Caution –

A pass number of 0 (zero) should only be used on mirrors that are mounted as read-only.

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Partial Resynchronization

Following a replacement of a slice within a submirror, Solaris Volume Manager performs a partial mirror resynchronization of data. Solaris Volume Manager copies the data from the remaining good slices of another submirror to the replaced slice.

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. Smaller pass numbers are resynchronized first. If 0 is used, the mirror resynchronization is skipped. A pass number of 0 should be used only for mirrors that are mounted as read-only. Mirrors with the same pass number are resynchronized at the same time.

Background Information for RAID 1 Volumes

• Unmirroring – The Enhanced Storage tool within the Solaris Management Console does not support unmirroring root (/), /opt, /usr, or swap, or any other file system that cannot be unmounted while the system is running. Instead, use the command-line procedure for these file systems.

• Attaching – You can attach a submirror to a mirror without interrupting service. You attach submirrors to mirrors to create two-way and three-way mirrors.

• Detach vs. Offline – When you place a submirror offline, you prevent the mirror from reading from and writing to the submirror, but you preserve the submirror's logical association to the mirror. While the submirror is offline, Solaris Volume Manager keeps track of all writes to the mirror and they are written to the submirror when it is brought back online. By performing an optimized resynchronization, Solaris Volume Manager only has to resynchronize data that has changed, not the entire submirror. When you detach a submirror, you sever its logical association to the mirror. Typically, you place a submirror offline to perform maintenance. You detach a submirror to remove it.

Background Information for Creating RAID 1 Volumes

• Before you create a mirror, create the RAID 0 (stripe or concatenation) volumes that will make up the mirror.

• Any file system including root (/), swap, and /usr, or any application such as a database, can use a mirror.

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  Caution –

  When you create a mirror for an existing file system, be sure that the initial submirror contains the existing file system.

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• When creating a mirror, first create a one-way mirror, then attach a second submirror. This strategy starts a resynchronization operation and ensures that data is not corrupted.

• You can create a one-way mirror for a future two-way or three-way mirror.

• You can create up to a three-way mirror. However, two-way mirrors usually provide sufficient data redundancy for most applications, and are less expensive in terms of disk drive costs. A three-way mirror enables you to take a submirror offline and perform a backup while maintaining a two-way mirror for continued data redundancy.

• Use components of identical size when creating submirrors. Using components of different sizes leaves wasted space in the mirror.

• Adding additional state database replicas before you create a mirror can improve the mirror's performance. As a general rule, add two additional replicas for each mirror you add to the system. Solaris Volume Manager uses these additional replicas to store the dirty region log (DRL), used to provide optimized resynchronization. By providing adequate numbers of replicas to prevent contention or using replicas on the same disks or controllers as the mirror they log, you will improve overall performance.

Background Information for Changing RAID 1 Volume Options

• You can change a mirror's pass number, and its read and write policies.

• Mirror options can be changed while the mirror is running.

How Booting Into Single-User Mode Affects RAID 1 Volumes

If a system with mirrors for root (/), /usr, and swap, the so-called “boot” file systems, is booted into single-user mode (by using the boot -s command), these mirrors and possibly all mirrors on the system will appear in the “Needing Maintenance” state when viewed with the metastat command. Furthermore, if writes occur to these slices, the metastat command shows an increase in dirty regions on the mirrors.

Though this situation appears to be potentially dangerous, there is no need for concern. The metasync -r command, which normally occurs during boot to resynchronize mirrors, is interrupted when the system is booted into single-user mode. Once the system is rebooted, the metasync -r command will run and resynchronize all mirrors.

If this is a concern, run the metasync -r command manually.

Scenario—RAID 1 Volumes (Mirrors)

RAID 1 volumes provide a means of constructing redundant volumes, in which a partial or complete failure of one of the underlying RAID 0 volumes does not cause data loss or interruption of access to the file systems. The following example, drawing on the sample system explained in Chapter 4, Configuring and Using Solaris Volume Manager (Scenario), describes how RAID 1 volumes can provide redundant storage.

As described in Interlace Values for Stripes, the sample system has two RAID 0 volumes, each of which is approximately 27Gbytes in size and spans three disks. By creating a RAID 1 volume to mirror these two RAID 0 volumes, a fully redundant storage space can provide resilient data storage.

Within this RAID 1 volume, the failure of either of the disk controllers will not interrupt access to the volume. Similarly, failure of up to three individual disks might be tolerated without access interruption.

To provide additional protection against problems that could interrupt access, use hot spares, as described in Chapter 15, Hot Spare Pools (Overview) and specifically in How Hot Spares Work.