Discover Dynamic Disks

One of Windows 2000'S Data-Storage Enhancements

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:

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.

Spanned volumes. Microsoft renamed NT's volume sets spanned volumes in Win2K. Despite the name change, the underlying concept remains the same—spanned volumes contain disk space from two or more physical disks. You can use Win2K's Logical Disk Manager (LDM) to extend an existing simple volume into a spanned volume by adding disk space from other disks on the system. The LDM, which Screen 1 shows, is Win2K's Microsoft Management Console (MMC)-based disk-management snap-in and the successor to NT 4.0's Disk Administrator utility.

Win2K and NT partitions, volumes, and sets can be confusing. To clarify, in NT parlance, disks contain partitions, which are basic disks' primary unit of division. Win2K refers to partitions as volumes. In both OSs, each partition or volume must be a particular type (e.g., primary, extended, simple, spanned, volume set, mirror set). Table 1 compares Win2K and NT disk terminology.

Which file system (e.g., NTFS, FAT, FAT32) you use to format a volume or partition is an entirely separate matter. For the most part, Win2K and NT offer more than one file system choice for a given volume type. Three notable exceptions are volume sets, spanned volumes, and simple volumes that you want to extend using additional free space on the same drive, all of which you must format as NTFS. However, simple volumes can be FAT or FAT32 if they use only one area of free space on one drive.

Dynamic Disk Enhancements
Dynamic disks support new features and technology that provide enhanced fault tolerance and online-management capabilities for disk volumes. These enhancements result in higher availability for Win2K servers than basic disks provide for NT servers.

Unlike basic disks, dynamic disks maintain information about critical disk data structures in multiple locations. In addition, dynamic disks don't rely on one partition table for volume type and location information. Instead, all dynamic disks maintain a 1MB database at the end of the drive that contains not only crucial information about the volumes on that dynamic disk but also information about all dynamic disks on the system. This database contains information similar to information that a basic disk's partition contains, including the starting and ending sectors, total number of sectors, and the volume or RAID type of each dynamic disk volume. (This database doesn't include the FAT data for FAT and FAT32 volumes or the Master File Table—MFT—data for NTFS volumes; the system maintains this information separately.) The database contains information for all volumes, and the system duplicates the database on all dynamic disks. This functionality greatly increases the possibility of recovering the system in the event of database damage.

Win2K automatically establishes and replicates the database without user intervention. In addition, the system can automatically use the database's information to recover a corrupted or destroyed volume database. Corruption or destruction of a basic disk's partition table under NT 4.0 is a catastrophic event, but Win2K can automatically repair this type of condition on a dynamic disk without the user even knowing what happened. In the event of database corruption, Win2K simply identifies a dynamic member disk with a complete undamaged copy of the database, then copies the undamaged database version to the corrupted disk.

Win2K introduces a new concept called disk groups, which are what the name implies: groups of dynamic disks. Win2K automatically creates a disk group as you add dynamic disks to your system. Dynamic disks replicate the volume database with the members of their disk group. By default, Win2K permits only one disk group. The system derives the disk group's name from the computer name and adds a Dg0 suffix. If you're curious about the name of your system's disk group, check the HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesdmioBoot InfoPrimary Disk GroupName Registry subkey. Win2K's LDM is a light version of VERITAS Software's Logical Disk Manager Pro, and if you use the Pro version of VERITAS' product, you can create and manage multiple disk groups in Win2K.

Another benefit of Win2K's dynamic disks is that you can manage disks online without rebooting the system. Unlike basic disks' reconfiguration options (e.g., creating, extending, and mirroring a volume), which require a system reboot to take effect, dynamic disks let you change volume configurations on the fly. Win2K also lets you add new dynamic disks to the system (e.g., with buses that support hot-pluggable drives) without rebooting. Unlike NT 4.0's Disk Administrator, almost all disk-configuration changes you make in Win2K's LDM take effect immediately—you don't need to select a separate menu command to save or commit changes you make during a session. These new storage-management features are a quantum improvement over those in NT 4.0.

Dynamic Upgrades
You can use three methods to create a dynamic disk: convert an existing basic disk, import an existing dynamic disk set from another Win2K system, or configure a new, uninitialized drive as a dynamic disk. You use the conversion method when you're upgrading an existing NT machine to Win2K or migrating legacy disks to a Win2K system. You create and convert dynamic disks from the LDM.

Converting basic disks to dynamic disks isn't mandatory. Win2K can work with basic disks as well as dynamic disks, and a Win2K system can contain combinations of basic and dynamic disks. The first time you run the LDM on a system that contains one or more basic disks, Win2K will ask whether you want to convert the basic disks to dynamic disks. If you forgo conversion, you can convert later by right-clicking a basic disk's drive in the left pane of the LDM console and selecting Upgrade to Dynamic Disk, as Screen 2 shows. This option isn't available if you're using a laptop computer because Microsoft doesn't support dynamic disks on laptops.

Dynamic Disk Dilemmas
Unfortunately, several situations exist in which Microsoft doesn't recommend using dynamic disks because they're not beneficial or they just don't work. One limitation is that Win2K is the only OS that can use and read dynamic disks and their volumes, even if you format the volumes with a file system that other OSs support (e.g., FAT or FAT32). If you're running a multiboot system, you need to be aware of this restraint before you convert your basic disks. In addition, removable drives such as Zip, Jaz, SyQuest, and magneto-optic drives and hard disks that use USB or IEEE 1394 connections don't support dynamic disks. In the article "IEEE 1394 Hard Disk Support in Windows 2000" (http://support.microsoft.com/support/kb/articles/q244/9/19.asp), Microsoft states that the reason for this lack of support is the hot-removable nature of removable drives. Removing a drive that contains a dynamic disk from one of these systems invalidates the contents of the dynamic disks' replicated volume database.

The fact that you can't use dynamic disks on laptop computers can also be a problem. I suspect that this lack of support relates to the fact that you can remove and swap many laptops' hard disks. As with removable drive types, removing dynamic disks causes problems with other dynamic disks on the system. However, dynamic disks don't provide any major advantages to laptop users. A dynamic disk's replicated volume database is useful only on systems that contain multiple physical drives, which is rarely the case for laptops. In addition, many Win2K laptops are multiboot systems that also run Win9x, DOS, Linux, BeOS, or other OSs, and dynamic disks and their volumes are unreadable to any OS other than Win2K.

Dynamic disk volumes support the same RAID levels as basic disk partitions (i.e., 0, 1, and 5). Although you can import and maintain basic disks' existing RAID volumes in Win2K, you can't create new RAID volumes on basic disks—Win2K supports the creation of new RAID volumes only on dynamic disks. If you need to add RAID volumes to your Win2K system, you must either add a new disk and define it as dynamic or convert an existing basic disk that contains enough free space for the new RAID volume.

Be aware that you can't easily convert a dynamic disk and its volumes back to a basic disk. The only way to accomplish this task is to remove each volume from the dynamic volume, at which point you can convert the disk back to a basic disk. To preserve the data on the dynamic disk's volumes, you must back up and restore the files on those volumes.

Finally, Win2K doesn't let you extend a simple volume on a converted dynamic disk by adding free space to the disk or convert a simple volume on a converted dynamic disk to a spanned volume by adding free space on a different disk. The reason for this limitation is that basic disk partitions that you convert to simple volumes on dynamic disks must retain their partition table entries in the MBR so that Win2K can boot from the volume or install to it. (Win2K Setup lets you perform installations only to dynamic disks that include the system or boot volume.) Expanding the simple volume would disable the ability to boot from or install to the volume.

Deliberating Disk Decisions
Dynamic disks are a core technology in Win2K and represent a significant step in the evolution of OS storage management. However, in certain environments dynamic disks present limitations, including backward-compatibility problems with legacy OSs and lack of support from some hardware configurations and devices. Understanding how dynamic disks differ from their basic disk predecessors and knowing dynamic disks' strengths and weaknesses will help you make the best disk-configuration choices for your Win2K systems.