There are software and hardware constraints to consider, as well as the cost and reliability of the system. Each potential vendor should have design guidelines for their system, whether you are looking for a RAID array or want to use the fault-tolerant features in Windows NT Server.
There are configurations and situations in which fault tolerance might not work as you would expect. The first thing you should check is whether all of the disk hardware is on the Windows NT Hardware Compatibility List (HCL). If any equipment is not included on the HCL, it might not work well. Chapter 8, "General Troubleshooting," contains more information about the HCL.
If you can, use identical disks. Although this is not a requirement, there are several advantages in using identical hardware for fault-tolerant configurations:
Have a backup disk available and ready for use. When purchasing the disks, it is a good idea to purchase an extra disk for use as a backup. An identical disk guarantees that in the event of a hardware failure, the backup disk is compatible, and will not degrade the system performance. All you need to do is install the new disk. (For SCSI disks, you have to configure the SCSI ID.)
Have a backup controller available, or configure your disks with duplexed controllers. The system can continue operating when a disk fails, but not when a controller fails. While there is no way to avoid system downtime when a controller fails, downtime can be minimized by having a pre-configured controller available. If the replacement controller is not the same as the original, you have to install a new driver, and your configuration becomes more complex.
Note
If you have configured your system partition on a mirror set, you should have the same make and model of controller for your backup, and be sure to use the same translation for both the original and shadow partition.
To a large extent, how your configure your mirror sets depend on the number of disks and controllers that you want to have on the computer running Windows NT Server. Having your boot partition on a disk (and controller) that does not contain data sets gives better performance. There are also other cost considerations that are related to overall reliability, such as your UPS strategy.
The decision to mirror the system or boot partition depends on how fast you want to be able to get your computer back up following a failure to the disk that contains these files. To some extent, it depends on whether you are using a x86-based computer or a RISC-based computer — x86-based computers experience most of the problems with sectors corrupted by viruses. These problems mean that you cannot start your computer from disk, although you can usually start it by using the Windows NT startup floppy disk. See Chapter 5, "Preparing for and Performing Recovery," for information about the Windows NT startup floppy disk.
When you use a mirror set for your system or boot partitions, these are some configurations that might cause problems:
For a stripe set with parity, you should buy disks on the basis of cost per megabyte and performance. Since the space used for storing parity information is 1/n, storage efficiency increase with a larger number of disks. If you use high performance SCSI controllers, an array of six 2 GB disks provides more efficient storage than four 3 GB disks. If you have to replace a disk, you can rebuild a smaller disk faster, so the system returns to maximum performance is less time.
Keep the stripe set with parity on a different controller and disk than your system and boot partition. Using separate controllers improves performance, and can make it faster and easier to recover from a hardware failure. Remember that you cannot configure your system partition or your boot partition on a stripe set with parity.
If the computer itself or the disk containing the boot partition fails (and the boot partition is not mirrored), it might be faster to move the stripe set with parity to a different computer. Chapter 7, "Disk, File System, and Backup Utilities" contains information about moving disks in the section "Using FT Edit to Update the Registry."