Preparing to Create or Modify a Fault Tolerant Set

• Microsoft Windows NT Workstation versions 3.5, 3.51, and 4.0
• Microsoft Windows NT Server versions 3.5, 3.51, and 4.0

SUMMARY

This article provides some basic procedures and a checklist for creating and modifying a fault tolerant set. For this article, a fault tolerant set includes a stripe set, stripe set with parity, volume set, and disk mirror/duplex.

Basic Procedures

1. To create a fault tolerant set, perform a full backup of all data. Back up anything that cannot be reinstalled and back up any data that you cannot regenerate on physical media, other than the drives you are going to use.

2. Because information about fault tolerant sets is kept in the registry, it is preferable that you back up the disk key in the system hive separately. You can easily do this by using the Save option of the Configuration entry under the Partition menu in Disk Administrator. This option will copy the disk key in an uncompressed format to a floppy disk. The disk key keeps the Disk Signature and any fault tolerant set information of the drives in your system.

   NOTE: It does not save the master boot record, boot sector, or any partition information. You may also want to use RDISK to back up the rest of the registry including the entire SYSTEM hive.

3. All third-party services and drivers need to be stopped through Services tool and Devices tool in Control Panel. These services may not only be limited to virus scanners, backup agents, and network services, but may extend to utilities that may maintain open files, or access system services directly (low-level hardware access such as Compaq Insight Manager). Also, applications such as Microsoft Exchange Server and SQL Server, among others that perform low-level disk I/O, need to be stopped.

   The drive needs to be checked for file structure and physical corruption with a utility such as CHKDSK. For CHKDSK use the /r to search for bad sectors and /f to fix any errors it finds. If possible, use the SCSI controller low-level media test to run a nondestructive test (consult your hardware vendor for details and availability).

   ARTICLE-ID: Q100110
   TITLE     : Overview of Redundant Arrays of Inexpensive Disks (RAID)
   ARTICLE-ID: Q114872
   TITLE     : Restoring Disk Configuration Information

Stripe Sets

Stripe sets offer no data redundancy, unlike striping with parity. All data is lost in the set if one drive fails in a stripe set. Better performance usually results because of the ability to read and write to several drives simultaneously. To do this, the drives and controller must support the read and write request simultaneously. In general, most SCSI controllers and drives allow this, while IDE controllers and drives only allow one device per channel at a time to accept read and write requests. Making a stripe set out of IDE drives will not improve performance.

Stripe sets with parity offer data redundancy. They maintain a fairly high level of performance. Read performance matches a stripe set without parity. When writing data, however, everything is written twice -- once to the disk and once to the parity strip. The pagefile should not be placed on a stripe set with parity because redundant data is written and performance degrades. After a stripe set is created, it cannot be enlarged or extended without backing up the data and reformatting the set. You cannot incorporate the Windows NT system or boot files into a stripe set.

For additional information, please see the following articles in the Microsoft Knowledge Base:

   ARTICLE-ID: Q113933
   TITLE     : Disk Striping and Disk Striping with Parity in Windows NT
   ARTICLE-ID: Q108082
   TITLE     : Windows NT Cannot Regenerate Stripe Set with Parity

Volume Set

Volume sets offer no data redundancy but allow you to span multiple drives, and to assign them one drive letter. When you use NTFS on a volume set, it is possible to add another drive to volume set without formatting the drive. FAT does not allow extending the drive without reformatting and restoring from a backup. When extending a volume set, take the same precautions as when you create any other fault tolerant set. For additional information, please see the following article in the Microsoft Knowledge Base Article related to Volume Sets:

   ARTICLE-ID: Q113503
   TITLE     : Overview of Disk Volume Sets in Windows NT

Disk Mirror/Duplex

Disk mirroring and disk duplexing offer full data redundancy under Windows NT Server. The difference between the two is that mirroring assumes you have two hard drives and one controller, while duplexing has a controller for each hard drive. Duplexing provides maximum protection if a controller or a hard drive fails. The type and size of your hard drive does not matter, but there must be enough free space available on the shadow drive. When duplexing, drive translation can be an issue on SCSI drives. When the BIOS is enabled on a SCSI controller, the drive translation is done by the BIOS. For controllers not under BIOS control, Windows NT defaults to 64 heads and 32 sectors per track. This happens because Windows NT cannot read the translation scheme from the disabled BIOS. An issue arises when there are two identical controllers and two identical drives, with two different sizes under Disk Administrator. In this situation, even if the mirror can be established, recovery in the event of failure may be extremely difficult.

For additional information, please see the following articles in the Microsoft Knowledge Base:

   ARTICLE-ID: Q114779
   TITLE     : Overview of Disk Mirroring (RAID Level 1) in Windows NT
   ARTICLE-ID: Q113977
   TITLE     : Booting From Mirror After Primary Partition Is Lost
   ARTICLE-ID: Q141702
   TITLE     : How to Recover Mirroring Windows NT Using IDE devices
   ARTICLE-ID: Q161563
   TITLE     : How Windows NT Handles Drive Translation
   ARTICLE-ID: Q148425
   TITLE     : Duplexed Mirror Fails with Insufficient Space

MORE INFORMATION

Windows NT Server support for RAID is software based; information about the configuration is kept in the registry. Hardware-based RAID has many advantages over software-based RAID, such as support for hot swapping of hard drives. Hardware-based RAID provides better performance because it runs independently of the operating system and does not need to scan a series of drivers. The operating system can be placed on a hardware-based RAID because the array is built in the controller BIOS, and not by a driver that the operating system supplies.

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