Disks, Partitions and Volumes

In order to understand the structures and concepts in this chapter, some basic terminology must be defined and understood .

The file system is part of a hierarchical storage system, which means that the system has successive nested levels or layers . Physically it begins with a disk drive, such as a hard disk. A physical hard disk drive is divided into one or more partitions , which are physical areas of the disk. Windows normally uses two types of partitions, called primary and extended . Primary partitions are assigned drive letters directly by the operating system and are also called volumes , and volumes are also called logical drives . Think of the partition as the raw physical space on the disk used by the volume, and think of a volume as the space after it is formatted with a file system, after which it appears to the OS as a drive letter. Extended partitions can contain one or more logical drives or volumes. Each volume must be formatted before it can be used, a process which writes the file system to the volume. The file system then manages the directories (also called folders) and files stored on the volume.

Basic Disks and Volumes

A disk set up with the standard system of using primary and extended partitions is referred to as a basic disk , and the volumes on a basic disk are also called basic volumes . Basic disks and volumes are accessible by virtually all operating systems, and represent the default setup used on most drives.

Basic disks can have up to four primary partitions, or up to three primary and one extended partitions. Each primary partition is a basic volume, and the extended partition can contain an unlimited number of logical drives, which are also basic volumes. Each basic volume must use contiguous physical space on the disk, and can only be extended in size if there is unallocated ( non-partitioned ) space immediately following the end of the volume. Aftermarket utilities can be used to extend a volume if there is available space before the volume as well.

Dynamic Disks and Volumes

Windows 2000 and later versions can create and use a special partitioning scheme called a dynamic disk , which contains dynamic volumes . Dynamic disks use a special hidden database in the last megabyte of the disk to manage information about the dynamic volumes on the disk, as well as any other dynamic volumes on other dynamic disks in the system. Once a new dynamic disk is created or imported into a system, the hidden database for the new disk is added to the database on all dynamic disks in the system. Because each dynamic disk contains the same database, if the database on one is corrupted Windows can rebuild it from the database on any of the other dynamic disks.

Dynamic volumes support several features not possible with basic volumes, such as:

A dynamic volume can be extended to include non-contiguous unallocated space on a single drive.
A dynamic volume can be extended to span multiple (up to 32) drives.
A dynamic volume can be striped (written distributively across multiple drives) for greater performance.

Windows 2000 and later Server editions also support these additional features:

A dynamic volume can be mirrored (identical data written to two drives) for fault tolerance.
A dynamic volume can be RAID-5, which divides the volume among three or more physical drives and stripes data and error correcting (parity) information for performance and fault tolerance.

There are unfortunately many limitations to the use of dynamic disks and volumes:

Dynamic disks and volumes are only supported by Windows 2000 and later versions.
All volumes on a physical disk must be either basic or dynamic.
Spanned dynamic volumes cannot be striped.
Striped dynamic volumes cannot be extended or spanned.
Removable media disks (Floppy, Optical, SuperDisk, Jazz, Zip) cannot be dynamic.
USB or FireWire disks cannot be dynamic.
Portable (that is, laptop) systems do not support dynamic disks, even on internal drives.
Mirrored or RAID-5 dynamic volumes are supported only by Windows 2000 or later Server Editions.

Because of the features, differences, and limitations of working with dynamic disks and volumes, they are mostly suited for server systems and not standard desktop or laptop PCs. As such, all further discussion of disks and volumes in this chapter will be about standard basic disks and volumes.