Understanding Disk Terminology

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Before going into the details of managing disks and storage, let's review some definitions.

• Physical drive The actual hard disk itself, including case, electronics, platters, and all that stuff. Not terribly important to the disk administrator.
• Partition A portion of the hard disk. In many cases, this will be the entire hard disk space, but it needn't be.
• Allocation unit The smallest unit of managed disk space on a hard disk or logical volume. Also called a cluster.
• Primary partition A portion of the hard disk that's been marked as a potentially bootable logical drive by an operating system. MS-DOS can support only a single primary partition, but Microsoft Windows NT and Windows 2000 can support multiple ones. There can be only four primary partitions on any hard disk.
• Extended partition A nonbootable portion of the hard disk that can be subdivided into logical drives. There can be only a single extended partition per hard disk, but it can be divided into multiple logical drives.
• Extended volume Similar to, and sometimes synonymous with, a spanned volume, this is any dynamic volume that has been extended to make it larger than its original size. When it uses portions of more than one physical disk, it is more properly referred to as a spanned volume.
• Logical drive A section or partition of a hard disk that acts as a single unit. An extended partition can be divided, for example, into multiple logical drives.
• Logical volume Another name for a logical drive.
• Basic disk A traditional disk drive that is divided into one or more partitions, with a logical drive in the primary partition, if present, and one or more logical drives in any extended partitions. Basic disks do not support the more advanced functions of Disk Management, but they can be converted to dynamic disks in many cases.
• Dynamic disk A Disk Management-managed hard disk that can be used to create various volumes.
• Volume A unit of disk space composed of one or more sections of one or more dynamic disks.
• Simple volume The Disk Management equivalent of a partition. A portion of a single dynamic disk, it can be assigned either a single drive letter or no drive letter and can be attached (mounted) on zero or more mount points.
• RAID (redundant array of independent [formerly "inexpensive"] disks) The use of multiple hard disks in an array to provide for larger volume size, fault tolerance, and increased performance. RAID comes in different levels, such as RAID-0, RAID-1, RAID-5, and so forth. Higher numbers don't indicate greater performance or fault tolerance, just different methods of doing the job.
• Spanned volume A collection of portions of hard disks combined into a single addressable unit. A spanned volume is formatted like a single drive and can have a drive letter assigned to it, but it will span multiple physical drives. A spanned volume—occasionally referred to as an extended volume—provides no fault tolerance and increases your exposure to failure but does permit you to make more efficient use of the available hard disk space.
• Striped volume Like a spanned volume, a striped volume combines multiple hard disk portions into a single entity. A striped volume uses special formatting, however, to write to each of the portions equally in a stripe to increase performance. A striped volume provides no fault tolerance and actually increases your exposure to failure, but it is faster than either a spanned volume or a single drive. A stripe set is often referred to as RAID 0, although this is a misnomer because plain striping includes no redundancy.
• Mirror volume A pair of dynamic volumes that contain identical data and appear to the world as a single entity. Disk mirroring can use two drives on the same hard disk controller or use separate controllers, in which case it is sometimes referred to as duplexing. In case of failure on the part of either drive, the other hard disk can be split off so that it continues to provide complete access to the data stored on the drive, providing a high degree of fault tolerance. This technique is called RAID 1.
• RAID-5 volume Like a striped volume, this combines portions of multiple hard disks into a single entity with data written across all portions equally. However, it also writes parity information for each stripe onto a different portion, providing the ability to recover in the case of a single drive failure. A RAID-5 volume provides excellent throughput for read operations but is substantially slower than all other available options for write operations.
• SLED (single large expensive disk) Now rarely used, this strategy is the opposite of the RAID strategy. Rather than using several inexpensive hard disks and providing fault tolerance through redundancy, you buy the best hard disk you can and bet your entire network on it. If this doesn't sound like a good idea to you, you're right. It's not.