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Configuring Boot Disks

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Reference Configuration Features

Four disks are used for the boot device and its entourage. The section "Reference Configuration Variations" addresses the relative merits of several variations on this design with greater number of or fewer disks.

For VxVM installations, these four disks are the only items to be included in the root disk group (rootdg). Any data volumes or file system spaces to be created outside of the core operating system (OS) should reside in other disk groups. Because Solstice DiskSuite software does not partition disks into administrative groups (except in multihost environments), these four disks are not in any sort of separate group in Solstice DiskSuite software configurations.

The disk locations shown in the following table refer to the disk device names used in the examples throughout the book.

TABLE 1 Disks in the Reference Configuration

Disk Location

Disk Name









Notice that the disk media name for each disk reflects its function. By providing clear and obvious naming, you can prevent confusion later. If you standardize these names throughout the enterprise, the potential for confusion is even further reduced. Note that rootdisk and rootmirror are on different controllers. These are the two SCSI host adapters that service each side of the Sun StorEdge D1000 array discussed in Chapter ;1 "Partitioning Boot Disks." Recall that all of the examples in this book use a Sun StorEdge D1000 array in a split configuration. The following paragraphs outline the purpose of each disk.

The root disk provides the basis of the BE. It includes the root volume and the swap volume. As described in Chapter ;1, unless a more secure configuration is required, only one partition should be used to store the root volume (root, usr, var, and so forth). In addition to the root volume, an LU volume can be introduced on or off the boot disk to enable easier patch management and upgrades for the OS.

The root mirror disk provides redundancy for the root disk by duplicating all of the boot disk contents. This increases the availability of the system because the BE can still be reached through the root mirror if the boot disk is unavailable. It is important to have the root disk and root mirror on independent paths so the failure of a controller or an array will not adversely affect both of them. The goal of this configuration is to produce a root mirror that is physically identical to the root disk, thereby simplifying serviceability.

The hot spare or additional root mirror enables an even higher level of availability by acting as a spare for the root disk or root mirror if either fails. This can provide an additional level of redundancy and also reduces the effect of service delays on the redundancy of the system. Because there are only three mirrors in this scenario, there is still a chance that a controller failure will leave the root disk unmirrored. This can be dealt with by using additional mirrors. An additional mirror is preferred to a hot spare in this situation because there is only one mirrored volume in rootdg. The time it would take a hot spare to resync to this mirror would reduce availability when compared to the time it would take to access the additional root mirror. Using a second mirror also allows flexibility because it can be broken off and used as an easy point-in-time backup during a complex service event.

The contingency disk allows a final level of protection. The contingency disk is a known-good BE. If certain boot files are accidentally modified or deleted, the boot disk may not boot properly. Since the boot mirror or hot spare mirrors these irregularities, the result is an inability to boot. Because some of these files are checked only at boot time, the problem could be months, or even years old before it is detected. The contingency disk provides a bootable environment, with any necessary volume manager and diagnostic utilities, that is frozen in time and not affected by changes to the boot disk. This enables you to quickly gain normal access to the machine in order to track down and repair the problems with the BE. Contingency disks are not as necessary in Solstice DiskSuite environments because their utilities are available on the bootable Solaris OE CDs.

LU volumes can be configured on one or more of these disks to provide additional options for bootability. If the BE on an LU volume is similar enough to the BE on the boot disk, this could allow the services hosted on the server to be brought up through the BE on the LU disk. Thus, the LU disk allows bootability, and possibly even service availability, even if the boot disk has been accidentally modified so that it cannot boot. If all of the data and applications are stored outside the root disk group, it is much more likely that a non-current disk will support both of these goals. LU volumes on the root disk or contingency disk can be used for this purpose. If these volumes exist on the contingency disk, they should be in addition to the known-good BE, which should be kept static.

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