High Performance NT4 Optimization and Tuning:Implementing Redundant Systems

Creating A Mirror Set

Creating a mirror set can only be accomplished if you have at least one unformatted partition of equal or greater size than the partition you want to mirror. The partitions must be on two separate physical drives. The physical characteristics (number of cylinders, number of heads, and so forth) of the disk drives do not matter. It doesn’t even matter if the disk systems are the same type. You can use EIDE or SCSI drives. You can even mix the two and mirror a partition on an EIDE drive to a partition on a SCSI drive.

Mirroring The System Partition
While it is not a requirement to use identical disk types and configurations to mirror a partition, it is a requirement if you are mirroring your system partition. This becomes even more important if you mirror an entire disk and plan on using the mirror (slave) disk to replace the original (master) disk. The reason for this is fairly simple. If you want to replace a failed master disk with your slave disk and boot the system without a startup disk, you will need to restore the Windows NT master boot record and partition-related information contained in the disk’s boot sector. The master boot record contains partition information and code to jump to the operating system partition, while the boot sector contains code to load the operating system. The boot sector is not saved (or copied) when you create a mirror set. You can use the tools provided in the Windows NT Server Resource Kit (DISKPROBE.EXE or DISKSAVE.EXE) to save the boot sector of the master disk. But, you can only restore the saved boot sector to a disk that has the exact same configuration as the original.

This means that for any disk type, the slave disk must use the same geometry (logical to physical translation) as the master disk It must also have the exact same partition layout (should you have more than one partition on the disk). If you are using multiple disk controllers, both controllers must be the same model and also be configured to use the same translation methodology. Some SCSI controllers, for instance, support an extended translation method for disks with more that 1,024 cylinders. Should this be the case on your SCSI controller, you must be sure that both SCSI controllers are configured identically. The controllers can both use the extended translation methodology or both be configured to not use the extended translation methodology, but you cannot mix and match.

You must also disable the BIOS on the second SCSI controller. To ensure that both drives are properly configured, you should low-level format both drives using the BIOS on the first SCSI controller. When you format the partitions, make sure the cluster size is identical, as well.

What matters is that only a primary partition can be mirrored. You cannot mirror an extended partition, nor can you mirror a volume set, stripe set, or stripe set with parity. Furthermore, if the primary partition to be mirrored is also the system partition, the master boot record on the primary partition will not be copied, nor will the partition-related information, nor any third-party information contained in the first track (0) of the disk. This means that if the master partition fails and you reboot the computer, you will be unable to start Windows NT Server. Therefore, you need to create a startup disk and boot from your floppy drive. Creating a startup disk is discussed in more detail later in this chapter in the section titled “Creating A Startup Disk.”

One restriction not mentioned in any of the documentation is that you cannot use any removable media as a slave partition. This means you cannot use a Jaz disk or similar removable hard disk for your slave partition (or disk)—even if you never plan on removing the media.

So, now that you understand the restrictions for a mirror set, you can follow the steps presented here to create your own mirror set

1. Launch the Disk Administrator located in your Administrative Tools program group.

2. Choose Disk Configuration from the View menu, press Ctrl+D, or click the second icon on the toolbar.

3. Select the master partition (or disk) to mirror.

4. While holding down the Ctrl key, click an unused partition of equal or greater size on another physical disk.

5. Choose Establish Mirror from the Fault Tolerance menu.

6. Choose Commit Changes Now from the Partition menu, or close the Disk Administrator and save your changes when prompted.

You will then be reminded to update your configuration information on your emergency repair disk or create a new emergency repair disk. This is a very good idea, and one you should follow up on immediately. At this time, the mirror set will then be established, but the master partition will not be copied to the slave partition until after you reboot the server. Once you have rebooted, you should see the status change (using Disk Administrator in Volumes view) from New to Regenerating. When the data has been fully copied, the status should change to Healthy and remain that way until you break the mirror or a failure occurs.

Now that you know how to create a redundant disk system, or mirror set, the next step is to learn how to create a stripe set with parity. The next section will walk you through this process.

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