Discover Dynamic Disks

Get ready, because everything you know about disks is about to change. By now, you're probably well aware that Windows 2000 (Win2K) is rife with tantalizing new features. Although a great deal of the attention focuses on Active Directory (AD) and its array of services, some of Win2K's most enticing features relate to storage management. In "NTFS5 vs. FAT32," April 2000, I discussed Win2K's NTFS-centric storage-management improvements. However, the improvements don't end with the new NTFS5 features. Win2K also contains a broad array of new storage-related terminology, tools, and technology, most of which Microsoft designed to meet the needs of enterprise customers.

Win2K's new buzzwords and disk-management features include dynamic disks, which are a new breed of disk volume that provide more flexibility and reliability than Windows NT 4.0 provides. To get the most from your Win2K systems, you must understand the features, advantages, and constraints of this new category of disk volume.

The Basics of Basic Disks
Before diving into an analysis of dynamic disks, let's discuss their predecessors—basic disks. Although the term is new, basics disks are the legacy drives that you already know (i.e., drives you created with NT 4.0, Windows 9x, and MS-DOS). Basic disks comprise one or more of the following elements:

• Primary partition—Primary partitions are the only type of partition that can act as the system partition (i.e., the partition that contains a boot sector and the OS-specific startup files such as NT Loader—NTLDR—and hal.dll). During the boot process, the system assigns a logical drive letter (e.g., C, E) to a primary partition. Win2K and NT can recognize more than one primary partition per physical drive; however, other OSs, such as MS-DOS and Win9x, can create and recognize only one primary partition per drive.
• Extended partition—Extended partitions can house one or more logical drives. A basic disk can contain only one extended partition; however, many logical drives can exist within that partition.
• Logical drives—Logical drives are individual logical space allocations within an extended partition. You can't use logical drives for the system partition, but you can use logical drives within extended partitions for many other purposes, including data volumes and the Win2K or NT boot volume (i.e., the partition that contains the \winnt installation folder). The system assigns drive letters to logical drives.

Win2K and NT can create and recognize as many as four partitions on a basic disk: one extended partition and as many as three primary partitions. Win9x and MS-DOS can recognize only two partitions on a basic disk: one primary partition and one extended partition.

In addition to primary and extended partitions, NT 4.0 basic disks can include several other partition types, including volume sets, stripe sets, mirror sets, and stripe sets with parity.

Volume sets are volumes composed of disk space on more than one physical disk. In stripe sets (aka RAID 0 volumes), the system stripes the volumes' data evenly across two or more physical disks to improve data access performance. Mirror sets (aka RAID 1 volumes) are fault-tolerant volumes that duplicate data on two physical disks: a primary disk and a shadow disk. Stripe sets with parity (aka RAID 5 volumes) are fault-tolerant volumes that contain data and parity information striped across three or more physical disks. If a physical disk member of a RAID 5 volume fails, you can use the data and parity information on the remaining drives to recreate the data on the failed disk.

Basic Limitations
Basic disks have been the only game in town for quite some time, but they suffer from inherent design flaws that make them less than ideal for use in mission-critical machines (e.g., enterprise servers). A basic disk houses several critical data structures in one sector on the disk, thus creating a single point of failure for the disk's contents. One of these critical structures is the partition table, which defines the types and locations of each partition on the disk. The partition table is in the disk's first physical sector, which the partition table shares with another critical disk structure, the Master Boot Record (MBR). The MBR provides the on-disk code that the system uses to boot.

Basic disks provide no redundancy for the partition table or MBR, so each structure presents a single point of failure should one structure or the other become damaged or corrupted. You might ask, "Don't RAID volumes solve this problem?" The answer is that they can—in some circumstances. Although software-based RAID 1 mirrored volumes created under Win2K and NT provide redundancy by duplicating the contents of a primary disk to a shadow disk, these mirrored volumes don't duplicate the MBR. However, most hardware-based RAID solutions (e.g., RAID con-troller cards) mirror a basic disk's entire contents, including the MBR. This fact is one of many reasons why hardware RAID solutions are preferable to Win2K and NT's software-based RAID offerings.

Another problem with basic disks is that certain changes you make to a basic disk's partition configuration (e.g., creating a mirror volume on the system volume) require a reboot to take effect. This requirement can pose logistical inconveniences for systems administrators because it forces them to schedule disk-configuration changes for off-peak or nonusage periods.

The Dynamics of Dynamic Disks
Microsoft introduced dynamic disks in Win2K as a complement to, and in some cases a successor for, basic disks. Dynamic disks primarily benefit systems that use multiple physical disks and disk volumes (e.g., network servers). Dynamic disks offer several advantages over basic disks, including support for online management, disk reconfiguration, and duplication of critical data structures across multiple disks.

Win2K's dynamic disks support volumes (aka dynamic volumes), which replace the concept of partitions that NT 4.0 and other OSs use. However, the types of volumes that dynamic disks can use are largely the same as the partition types that NT 4.0 supports, with two differences: simple volumes and spanned volumes.

Simple volumes. Dynamic disks don't use primary and extended partitions; instead, they use a simple volume. This volume provides no fault-tolerance and contains one or more regions of disk space on one physical disk.