Before we get into disk fault tolerance, there are some key terms to know. RAID stands for Redundant Arrays of Inexpensive Disks. Initially put forth in 1987 by Patterson, Gibson, and Kratz of the University of California at Berkeley, their research attempted to draw together various options for building virtually large drives from smaller inexpensive drives. This is counter to the SLED (Single Large Expensive Disk) concept commonly used on mainframe and mini computers. The paper establishes five different schemes for deploying disks. It is a common mistake to think one builds on top of another. Each has their advantages and disadvantages. RAID 0 has since been added by the industry to represent striping without parity, known in Microsoft Windows NT as striped sets. RAID 1 represents disk mirroring. RAID 5 represents striping with parity. Microsoft Windows NT supports RAID 0, 1, and 5 with an additional variant of RAID 1 in disk duplexing.
We also want to make the differentiation between a disk, a drive, and a partition. A disk is a physical component of hardware. It may be divided into one or more partitions that are logical subsections of the disk. A drive is one or more partitions logically associated to a single system identifier (drive letter). A drive may also be the logical association of a single system identifier (drive letter) to a network share point. This is a redirected drive. A fair number of computers have one hard disk with one partition and a single drive C:. Microsoft Windows NT can still do this, but it can do so much more.
There are two special types of partitions in Microsoft Windows NT: the boot partition and the system partition. The system partition is the partition that the ROM BIOS selected to begin loading the computer's operating environment from. This is also referred to as the active partition in programs such as Fdisk. The boot partition is the partition containing the Microsoft Windows NT system being booted. This is the partition BOOT.INI file points to. This file is sometimes called the SYSROOT in reference to the internal environment variable, which points to this path. In Microsoft Windows NT, the boot partition and the system partition can be the same, on different disks, and on different controllers. They may be FAT, HPFS, or NTFS. However, they may be mirrored or duplexed. They may not be striped, parity striped, or volume sets.
Disks are handled differently in Microsoft Windows NT than in other system. In Microsoft Windows NT, each disk is given a unique signature. This signature is stored in the Master Boot Record area on the disk (physical sector 0) and used by Microsoft Windows NT to identify disks. It allows disks to be moved from controller to controller or within a SCSI chain without problems. The signature is used to look up the disk and the partitions on those disks in the Registry. In the Registry there is an entry called DISK, which informs Microsoft Windows NT of the participation of each partition on each disk. This eliminates reliance on hidden information on the disk. It does, however, present a minor problem.
The DISK information is stored in the Registry. But the Registry is overwritten when a new version of Microsoft Windows NT is installed. The Disk Administrator has provided a simple solution to this problem. Under the Partition menu option is an entry called Configuration with three sub-options: Save, Restore, and Migrate. Save allows you to save the DISK information to a floppy disk. It does this by saving a current copy of the system component of the Registry. Restore allows the DISK information to be restored from this disk. Migrate is used if the SYSTEM file from the previous version of Microsoft Windows NT is available on disk. The current disks are scanned for previous version(s) of Microsoft Windows NT, and the user is allowed to select from which director to load the DISK information. Only the DISK information is loaded from the old SYSTEM file.
A quick comment on Disk Administrator. Activities done in the Disk Administrator such as creating or deleting partitions, breaking mirrors, and building sets, are not committed to the disks until you attempt to exit Disk Administrator. The user is given one last chance at this point to confirm that the modifications are correct.