Inside Microsoft Windows 2000, Third Edition (Microsoft Programming Series)

The evolution of storage management in Windows 2000 begins with MS-DOS, Microsoft's first operating system. As hard disks became larger, MS-DOS needed to accommodate them. To do so, one of the first steps Microsoft took was to let MS-DOS create multiple partitions, or logical disks, on a physical disk. MSDOS could format each partition with a different file system type (FAT12 or FAT16) and assign each partition a different drive letter. MS-DOS versions 3 and 4 were severely limited in the size and number of partitions they could create, but in MS-DOS 5 the partitioning scheme fully matured. MS-DOS 5 was able to divide a disk into any number of partitions of any size.

Windows NT borrowed the partitioning scheme that evolved in MS-DOS both to provide disk compatibility with MS-DOS and Windows 3.x and to let the Windows NT development team rely on proven tools for disk management. Microsoft extended the basic concepts of MS-DOS disk partitioning in Windows NT to support storage-management features that an enterprise-class operating system requires: disk spanning and fault tolerance. Starting with the first version of Windows NT, version 3.1, systems administrators have been able to create volumes that comprise multiple partitions, which allows large volumes to consist of partitions from multiple physical disks and to implement fault tolerance through software-based data redundancy.

Although this MS-DOS-style partitioning support in versions of Windows NT prior to Windows 2000 is flexible enough to support most storage-management tasks, it suffers from several drawbacks. One drawback is that most disk-configuration changes require a reboot before taking effect. In today's world of servers that must remain online for months or even years at a time, any reboot—even a planned reboot—is a major inconvenience. Another drawback is that the Windows NT 4 registry stores multipartition disk-configuration information for MS-DOS-style partitions. This arrangement means that moving configuration information is onerous when you move disks between systems, and you can easily lose configuration information when you need to reinstall the operating system. Finally, a requirement that each volume have a unique drive letter in the A through Z range plagues users of all Microsoft operating systems prior to Windows 2000 with an upper limit on the number of possible local and remote volumes they can create.

To fully understand the rest of this chapter, you need to be familiar with some basic terminology:

• Disks are a physical storage device such as a hard disk, a 3.5-inch floppy disk, or a CD-ROM.
• A disk is divided into sectors, addressable blocks of fixed size. Sector sizes are determined by hardware. All current x86-processor hard disk sectors are 512 bytes, and CD-ROM sectors are typically 2048 bytes. (Future x86 systems might support larger hard disk sector sizes.)
• Partitions are collections of contiguous sectors on a disk. A partition table or other disk-management database stores a partition's starting sector, size, and other characteristics.
• Simple volumes are objects that represent sectors from a single partition that file system drivers manage as a single unit.
• Multipartition volumes are objects that represent sectors from multiple partitions and that file system drivers manage as a single unit. Multipartition volumes offer performance, reliability, and sizing features that simple volumes do not.