Best practices for using dynamic disks on Windows 2000-based computers

This article describes the best practices for using dynamic disks on Windows 2000-based computers.

Dynamic disks offer advantages over basic disks, which use the original MS-DOS-style master boot record (MBR) partition tables to store primary and logical disk partitioning information. Dynamic disks use a private region of the disk to maintain a Logical Disk Manager (LDM) database, which contains volume types, offsets, memberships, and drive letters of each volume. The LDM database is also replicated, so each dynamic disk knows about every other dynamic disk configuration. This feature makes dynamic disks more reliable and recoverable than basic disks.

The following articles contain information that you need to understand the information in this article. For more information about dynamic disks, hardware limitations, and the terminology that is used in this article, click the following article numbers to view the articles in the Microsoft Knowledge Base:

Best practices and limitations of using dynamic disks

Before you use dynamic disks, consider the following recommended best practices and limitations of using dynamic disks.

The Latest Fixes

Microsoft highly recommends that you install Windows 2000 Service Pack 3 (SP3) or later if you run Windows 2000 and you currently have dynamic disks or plan to upgrade any of your basic disks to dynamic disks. SP3 contains several important fixes for basic-disk-to-dynamic-disk conversions and for extending hardware RAID volumes if the disk is already dynamic.

For additional information about the fixes that SP3 contains, click the article number below to view the article in the Microsoft Knowledge Base: Microsoft also recommends that you install a supported fix that is now available that corrects a problem that occurs when you start the Disk Management snap-in. This hotfix is not included in Windows 2000 SP3. For more information about this hotfix, click the following article number to view the article in the Microsoft Knowledge Base:

Dynamic disks vs. basic disks

Before you convert basic disks to dynamic disks, determine whether you require features that only dynamic disks provide. If you do not need spanned volumes, striped volumes, mirrored volumes, or RAID-5 sets, it is best to use basic disks.

Note Windows 2000 supports physical disk sizes up to 2TB in size. If your hardware vendor allows you to increase the size of an existing hardware RAID-5 disk logical unit number (LUN) up to the 2TB limit, you do not have to span the NTFS file system volume across multiple physical disks LUNs. Instead, we recommend that you continue to use basic disks, and use the DiskPart.exe utility to extend the NTFS volume after you add new physical storage capacity to the hardware RAID LUN. For more information, click the following article number to view the article in the Microsoft Knowledge Base: If you must extend an NTFS volume across multiple hard disks (LUNs) to create a spanned volume, use dynamic disks. For Windows 2000 only, the NTFS volume must have been originally created on a dynamic disk before it can be spanned. You cannot extend or span a volume that was created on a basic disk and then converted to a dynamic disk. This restriction does not apply to Windows XP and later. For more information about this Windows 2000 restriction, click the following article number to view the article in the Microsoft Knowledge Base:

Storage devices

If you decide to use dynamic disks and you have both locally attached storage (IDE-based storage or Small Computer System Interface [SCSI]-based storage) and storage that is located on a storage area network (SAN, which is typically connected by means of fiber channel), do one of the following, depending on your situation:

• Use dynamic disks on only the SAN storage drives and keep the locally attached storage as basic disks.
  
  -or-
• Use basic disks on the SAN storage drives and configure the locally attached storage as dynamic disks.

This recommendation is based on the way that Logical Disk Manager (LDM) keeps track of dynamic disks and synchronizes the databases. If you follow this recommendation, and you experience an unplanned outage (for example, fabric problems or a power outage) and lose access to the SAN storage that houses the dynamic disks, all dynamic disks will drop offline from the Windows 2000 system at the same time. Because you have no dynamic disks attached locally, there are no LDM database synchronization issues to contend with when the SAN disks eventually come back online. If you have even one dynamic disk on the locally attached storage, you run the risk of the LDM databases being mismatched, and you may have trouble getting one or more SAN-attached dynamic disks back online.

To combat that problem, if you must have dynamic disks in a mixed configuration of both locally attached storage and SAN-attached storage, it is a good idea to protect all fiber hubs, routers, switches, SAN cabinets, and the server from power outages by using uninterruptible power supplies (UPSs) on all connecting devices.

Notes

• In a mixed dynamic disk configuration, if you must take the SAN storage offline for maintenance, it is best to shut down the server before you take the SAN storage unit offline, and then make sure that all the SAN devices are available again when you bring the server back online.
• Windows does not support mounting a disk volume to multiple hosts at the same time. This restriction applies to volumes that are located on a BASIC disk or a dynamic disk. Volume corruption may occur if changes are made to the volume by both hosts. Windows also does not support exposing and then importing dynamic disks on multiple hosts (nodes) simultaneously. This practice can also lead to data loss or to LDM database corruption.

Server clusters

Dynamic disks are not supported for use with Windows Clustering. Windows 2000 Advanced Server and Windows Datacenter Server that are non-OEM versions do not support dynamic disks in a server cluster infrastructure.

This restriction does not prevent you from extending an NTFS volume that is contained on a cluster shared disk (a disk that is shared between the computers in the cluster) that is basic. For more information about how to extend an NTFS volume on a cluster shared disk, click the following article number to view the article in the Microsoft Knowledge Base: You can use the third-party add-on software Veritas Volume Manager for Windows 2000 to add the dynamic disk features to a server cluster infrastructure. If you install Veritas Volume Manager and configure Volume Manager Disk Group resources, contact Veritas Support for server cluster issues that are related to those resources. For more information about Veritas Volume Manager, click the following article number to view the article in the Microsoft Knowledge Base: The third-party products that are discussed in this article are manufactured by companies that are independent of Microsoft. Microsoft makes no warranty, implied or otherwise, regarding the performance or reliability of these products.

Moving dynamic disks

If you move dynamic disks between systems, you may not be able to move the dynamic disks back to the original host. If you must move the dynamic disks, move all the dynamic disks from a computer at one time, and make sure that they are all online and running on the destination system before you try to import them to the new host. This order is important so that the disk group name and ID of the primary disk group of the host system is always retained (if a dynamic disk is present). What makes the difference is whether there is at least one dynamic disk on the destination system or not. One problem scenario occurs when there is no dynamic disk on the destination system (so that that computer ends up with the same disk group name as the source computer when the disk are moved to it) and then you want to move the disks back to the source computer. You may encounter a problem if foreign disks that you are re-imported have the same disk group name as the local computer.

For more information about moving dynamic disks, click the following article numbers to view the articles in the Microsoft Knowledge Base:

The write signature and upgrade disk wizard

When you start the Disk Management snap-in, it will enumerate all disks in the system to see if any disks have changed or if any new disks were attached to the system. If Disk Management finds any disks that are unknown, that are not initialized, or that do not have a disk signature in the MBR, Disk Management starts the Write Signature and Upgrade Disk Wizard.

The wizard first prompts you to select which disks that you want to write a disk signature to; by default, no disks are selected. To select the disks, click the check boxes next to the disk numbers.

The wizard then prompts you to select which disks that you want to upgrade to dynamic disks. Each disk that you selected on the first page is automatically selected here. If you continue without clearing the disk check boxes, LDM writes a disk signature and upgrades all the disks that you selected to dynamic disks automatically.

Note This wizard was changed in Windows XP and later. It no longer automatically selects the disks to be upgraded to dynamic.

If the MBR of a dynamic disk is zeroed out (for example, because of a hardware problem), when you start Disk Management, the Write Signature and Upgrade Disk Wizard starts. If you permit the disk to be reconverted to dynamic, the original LDM database is overwritten by the newly initialized LDM database. Disk Management shows that disk as healthy and only shows unallocated free space. If you have another healthy dynamic disk in the system at the time of conversion, its LDM database is then replicated to the newly converted dynamic disk and a "missing" disk (representing the original dynamic disk) is also shown in Disk Management.

Because of this, it is best that you disable the Write Signature and Upgrade Disk Wizard in Windows 2000, or that you caution users about the default behavior of the wizard, and be careful not to accidentally reconvert a disk that was previously dynamic.

To manually disable the wizard, follow these steps.

Note If you previously selected the Do not show this wizard again check box in the wizard, you do not have to use this procedure because the key and value already exist and are set to 0x1 (disabled).

Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:

1. Start Registry Editor.
2. Locate and then click the following registry key:
   HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft
3. On the Edit menu, click Add Key, type Logical Disk Manager for the key name, and then click OK.
4. Click the newly created Logical Disk Manager key.
5. On the Edit menu, click Add Value, type Dont Show InitWizard for the value name, click REG_DWORD for the data type, and then click OK. Type 1 for the data value.
   
   To reactivate the wizard in the future, change the data value back to 0.

Note After you disable the wizard, if you have to write a disk signature on a new disk, right-click the physical disk number in Disk Management, and then click Write Signature.

"Missing" dynamic disks

If Disk Management shows a "Missing" dynamic disk, this means that a dynamic disk that was attached to the system cannot be located. Because every dynamic disk in the system knows about every other dynamic disk, this "missing" disk is shown in Disk Management. Do NOT delete the missing disk's volumes or select the Remove Disk option in Disk Management unless you intentionally removed the physical disk from the system and you do not intend to ever reattach it. This recommendation is important, because after you delete the disk and volume records from the remaining dynamic disk's LDM database, you may not be able to import the missing disk and bring it back online on the same system after you reattach it.

Text-mode setup and recovery console

Never delete or create a partition on a dynamic disk during Windows 2000 or Windows XP text-mode setup or when you start the computer by using the Recovery Console. Doing so may result in permanent data loss. For more information about this recommendation, click the following article numbers to view the articles in the Microsoft Knowledge Base:

The Mirrored Drive

Never break a healthy system disk or boot dynamic mirrored volume and expect the mirrored drive to replace the original primary drive if it fails. The drive letter that is assigned to the manually broken mirrored drive is assigned the next available drive letter and is a permanent record in the LDM database. This means that no matter what position that drive takes in the boot process, it is assigned the new (and incorrect) drive letter, so the operating system cannot function correctly.

For more information about common symptoms that are related to the incorrect boot partition drive letter, click the following article numbers to view the articles in the Microsoft Knowledge Base:
Note Windows software mirroring is a fault-tolerant solution that ensures that you can maintain access to data if you have a hardware disk failure. Software mirroring is not intended to be used as an offline backup mechanism.

Hardware mirroring

If you use dynamic disks with hardware mirroring, it is very important that you do not expose BOTH halves of the hardware-mirrored drives to the same operating system at the same time. To do this, break the hardware mirror by using the OEM RAID configuration utility, and then configure both disks as standalone drives that are both accessible to the operating system. The LDM databases will be exactly the same (because you hardware-mirrored them), whereas each dynamic disk on a system should contain a unique DiskID in the LDM header so that LDM can distinguish one dynamic disk from another.

The same holds true if you use software disk imaging products that duplicate hard disks sector-by-sector. Unpredictable results and, more likely, undesirable results may occur if two dynamic disks that are exactly the same are exposed to the operating system at the same time.

Maximum sizes on an NTFS file system volume

Before you format an NTFS volume, evaluate the types of files to be stored on the volume so that you can determine whether to use the default cluster size. When formatting NTFS volumes, you can specify a cluster size of up to 64 kilobytes (KB) by using the Disk Management snap-in. If you format a volume, but you do not specify a cluster size, default values are used. If you want to change the cluster size after the volume is formatted, you must reformat the volume. Before you select a cluster size other than the default, note the following important limitations:

• For Microsoft Windows NT, Windows 2000, Windows XP, and Windows Server 2003, the cluster size of FAT16 volumes ranging from 2 gigabytes (GB) through 4 GB is 64 KB. This can create compatibility issues with some applications. For example, Setup programs do not compute free space correctly on a volume with 64-KB clusters and cannot run because it is perceived that there is insufficient free space. Therefore, you can use either NTFS or FAT32 to format volumes larger than 2 GB.
• Because file compression is not supported on cluster sizes larger than 4 KB, the default NTFS cluster size for Windows Server 2003 never exceeds 4 KB. In theory, the maximum NTFS volume size is 2^64 clusters minus 1 cluster. However, the maximum NTFS volume size as implemented in Windows Server 2003 is 2^32 clusters minus 1 cluster. For example, using 64-KB clusters, the maximum NTFS volume size is 256 terabytes minus 64 KB. By using the default cluster size of 4 KB, the maximum NTFS volume size is 16 terabytes minus 4 KB. For more information about this topic, visit the following Web site:
  http://technet2.microsoft.com/windowsserver/en/library/8cc5891d-bf8e-4164-862d-dac5418c59481033.mspx (http://technet2.microsoft.com/windowsserver/en/library/8cc5891d-bf8e-4164-862d-dac5418c59481033.mspx)

Conclusion

By using dynamic disks, you can create fault-tolerant volumes (mirrored volumes and RAID-5 sets) and very large multiple-disk (LUN) volumes by using striped and spanned volumes. These features are available only on dynamic disks. Dynamic disks are more robust and fault-tolerant in the way they store and replicate disk and volume configuration information. Dynamic disks are primarily designed to be "online" all the time, and this is why they are not available on removable media. If you use the recommendations in this article, your data should remain online and accessible under various controllable and uncontrollable circumstances.