Basic disks maintain the structure with which you are probably most familiar. Each basic disk is partitioned, and each partition functions as a physically separate unit of storage. The information about the location and size of each partition is stored in the partition table of the Master Boot Record (MBR) on the drive. A basic disk can contain as many as four partitions, consisting of either four primary partitions or three primary partitions and one extended partition. The logical volumes on a basic disk are primary partitions and logical drives. The logical volume, as mentioned, can be represented by zero or more drive letters and can be mounted to folders on an existing NTFS volume.
Primary partition : Each primary partition maintains one logical volume on a basic disk. If a basic disk is used to start the operating system, one and only one primary partition on the disk must also be marked as active.
Tip The computer’s basic input/output system (BIOS) looks to the active partition to locate the hardware-specific files required to load the operating system. That partition is technically referred to as the system partition and is usually assigned drive letter “C”. Once the boot process has begun, the operating system is loaded.The partition on which the operating system is stored is called the boot partition. Yes, it can get confusing, particularly because the same volume is referred to by the variable %Sysvol%. Fortunately, it’s not a distinction you’re likely to need to know, since most installations are completely on drive C, making the C drive the system partition, the boot partition, and %Sysvol%.
Extended partition A basic disk may also contain an extended partition. Unlike primary partitions, extended partitions are not formatted or assigned drive letters. Instead, extended partitions are further divided into logical drives. Logical drives are logical volumes on a basic disk.
In earlier versions of Microsoft operating systems, including Windows 95, Windows 98, and MS-DOS, the operating system could only “see” the primary partition on which it was installed, plus the extended partition on the drive, if one existed. If you wanted additional storage segments on the drive, you had to configure an extended partition and apportion it into one or more logical drives. Because Windows NT, Windows 2000, Windows XP, and Windows Server 2003 can access all partitions on a disk, you only need an extended partition if you want more than four logical drives on a single disk.
Dynamic Disks and Volumes
Microsoft Windows 2000, Windows XP, and the Windows Server 2003 family also support dynamic storage. The storage units on dynamic disks are called volumes, and the first distinctions between basic and dynamic storage are that dynamic disks support an unlimited number of volumes, and that the configuration information about the volumes is stored in a database controlled by the Logical Disk Manager (LDM) service.
The logical volume of dynamic disks is the volume. Dynamic disks support simple volumes on a single disk. When a computer has more than one dynamic disk, you are provided more storage options from which to choose. Spanned, mirrored (RAID-1), striped (RAID-0), and striped with parity (RAID-5) volumes are logical volumes that utilize space on more than one physical disk. Each volume type uses disk space differently, and is characterized by a different level of fault tolerance. The list below summarizes the volume types
The equivalent to a basic disk partition is a dynamic disk simple volume. Simple volumes utilize space on a single physical disk, and correspond to a single logical volume. Simple volumes can be extended by appending unallocated space on other regions of the same disk, allowing you to adjust a volume’s capacity with the growth of data stored in that volume. Because simple volumes exist on only one physical disk, they are not fault-tolerant.
A spanned volume includes space on more than one physical disk. Up to 32 physical disks can participate in a spanned volume, and the amount of space used on each disk can be different. Data is written to the volume beginning with the space on the first disk in the volume. When the space on the first disk fills, the second disk is written to, and so on. Spanned volumes provide an option for increasing drive capacity. If a simple or spanned volume is filling up, you can extend the volume onto additional new storage capacity.
But spanned volumes are not fault-tolerant, and cannot participate in any fault-tolerant configurations. Because their size tends to be greater, and because multiple physical disks are involved, the risk for failure increases. If any one disk in a spanned volume is corrupted or lost, data on the entire volume is lost as well. For these reasons, Windows Server 2003 will not allow the installation of the operating system on a spanned volume, nor can you extend or span the system volume. Spanned volumes are recommended only as a stop-gap measure when an existing volume fills to capacity, or else in situations where tolerance for failure is high— for example, a large library of read-only data that can easily be restored from tape backup in the event of failure.
A striped volume (RAID-0) combines areas of free space from multiple hard disks into one logical volume. Unlike a spanned volume, however, data is written to all physical disks in the volume at the same rate. Because multiple spindles are in use, read and write performance is increased almost geometrically as additional physical disks are added to the stripe. But like extended simple volumes and spanned volumes, if a disk in a striped volume fails, the data in the entire volume is lost.
A mirrored volume (also known as RAID Level 1, or RAID-1) consists of two identical copies of a simple volume, each on a separate hard disk. Mirrored volumes provide fault tolerance in the event that one physical disk fails.
A RAID-5 volume is a fault-tolerant striped volume. Space on three or more physical disks is unified as a single volume. Data is written to all physical disks at the same rate, but unlike a striped volume, the data is interlaced with checksum information, called parity. Should a single disk in the volume fail, the data on that disk can be regenerated through calculations involving the remaining data and the checksum information. It is an interesting technical note that parity is distributed among all volumes in the RAID-5 set.
Basic Disks vs. Dynamic Disks
So now that you know about basic and dynamic storage, and the types of partitions, logical drives, and volumes they support, which is better? The answer, as is frequently the case, is: “It depends.” Dynamic disks that store data are easily transferred between servers, allowing you to move a disk from a failed server to a functioning server with little downtime. Dynamic disks flex their muscle when there is more than one dynamic disk in a computer. Each Windows 2000, Windows XP, and Windows Server 2003 computer can support one disk group, which itself can contain multiple dynamic disks. The LDM database is replicated among all disks in the disk group, which increases the resiliency of disk configuration information for all the group’s disks. In addition, disks can be configured to work together to create a variety of flexible and powerful volume types including spanned volumes, striped volumes (RAID-0), mirrored volumes (RAID-1), and striped-with-parity volumes (RAID-5).
Basic disks will continue to be used, however, for several reasons:
Basic storage is the default in Windows Server 2003 and windows XP, so all new disks are basic disks until you convert them to dynamic.
Dynamic disks do not offer advantages over basic disks in a computer that will have only one disk drive.
The behavior of the LDM database also makes it difficult to transfer a dynamic disk used for starting the operating system to another computer when the original computer fails.
Dynamic disks are not supported for removable media, and are not supported on laptops.
Basic storage is the industry standard, so basic drives are accessible from many operating systems, including MS-DOS, all versions of Microsoft Windows, and most non-Microsoft operating systems (there are a few). Therefore, dynamic disks cannot be used if you need to dual-boot an earlier operating system that requires access to the disks. Keep in mind that we are talking about local access only. When a client of any platform accesses files over the network, the underlying storage and volume type are transparent to the client.