Overview of Disk Mirroring (RAID Level 1) in Windows NT (114779)

This article was previously published under Q114779

This article provides an overview of disk mirroring (RAID Level 1) under Windows NT.

In disk mirroring, partitions on two drives store identical information so that one is the mirror of the other. All data written to the partition on the primary disk is also written to the mirror, or secondary, partition. If one disk fails, the system is able to use the data from the other disk.

The following facts apply to disk mirroring under Windows NT:

• Only Windows NT Server can create and break mirror sets.
• Mirrors are file system independent. Any partition using a file system that Windows NT recognizes or that is blank can be used to create a mirror.
• Mirrors are not dependent on disk geometry. The only requirement is that free disk space used to place the mirror on be equal to or greater than the size of the primary partition. Mirroring is not restricted to a partition of identical geometry (size, number of heads, cylinders, tracks, sectors, etc.) nor is it restricted to a drive of the same type (IDE, ESDI, SCSI, etc.).
• Primary and mirror partitions must be on separate hard disk drives. They cannot be on the same physical hard disk drive.
• Whether the two hard disks containing the primary and mirrored partitions are on the same or different disk controllers, Windows NT still defines this as mirroring and makes no distinction. (Placing the disks on separate controllers is sometimes referred to as disk duplexing.)
• A single mirror set is limited to two hard disks only. Use disk striping with parity if fault tolerance over more than two disks is needed.
• Mirror sets are invisible to the user. When a mirror set is created, both partitions are assigned the same drive letter.
• Mirroring is the only Windows NT fault tolerant option available for use on boot and system partitions.
• If the boot or system partition is mirrored and the primary partition is damaged, the computer can boot off the secondary or mirror partition by using a fault tolerant boot floppy disk. See the Concepts and Planning Guide or online documentation for information on how to create a fault tolerant boot floppy disk.
• Only the Windows NT Server installation that created the mirror set will normally recognize it. Other operating systems will not recognize the mirrored partition. MS-DOS will identify the partitions of the mirror as "Non-DOS" partitions. Windows NT and other installations of Windows NT Server will identify the primary and mirror partitions as having an "Unknown" file system type in Disk Administrator.
  
  NOTE: Windows NT and other installations of Windows NT Server can recognize a mirror set created by Windows NT Server by restoring disk configuration information.
• A new installation of Windows NT cannot be installed on an existing mirror set. During setup, when selecting the partition to install Windows NT on, setup identifies the mirror set as "Windows NT Fault Tolerance." If you attempt to select this partition for installation, a message appears, stating that Windows NT does not recognize this partition, and it must be deleted for setup to use it.
• The fault tolerance driver makes the loss of one partition in a mirror set invisible; you will be able to read from and write to the remaining partition as if the mirror set was healthy. However, if only one partition of a mirror set is functioning, then it is no longer fault tolerant. Loss of the remaining partition will result in an unrecoverable loss of all data in the mirror set.
• A key to determining the condition of a mirror set is the status bar in Disk Administrator. When you select one of the partitions of a mirror set, Disk Administrator displays information about the mirror in the lower left corner of the window. For example, "Mirror set #0 [HEALTHY]" indicates the status of mirror set #0 is healthy. Other status indicators include:
  

  [NEW] appears immediately after the mirror set has been created in Disk Administrator, but before shutting down the system and actual generation of the mirror begins.
  
  [REGENERATING] is displayed when generation of the mirror set by the system has been started but is not yet complete.
  
  [RECOVERABLE] appears when either one of the partitions in the set has been lost but the other partition is undamaged. This message also appears when one partition loses synchronization with the other.

• No loss in performance occurs when a member of a mirror set fails.
• Disk mirroring provides better overall write performance than striping with parity and better read performance in the event of a drive failure.

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