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ND 10855 Disk Mirroring Module

Introduction

Norsk Data's Disk Mirroring Module is part of the FTX systems, ND's fault tolerant systems. The module simultaneously copies, or mirrors, data on two or three disks, providing greater system security by protecting against disk failures. It also allows on-line backup while the system (e.g., the database system) is running. This is crucial to those who are heavily dependent upon a system being "up" at all times, such as in transaction processing applications like banking and finance, manufacturing, transportation and retail markets.

Features

  • Extra security against disk failures; parallel writing on two or three disks
  • On-line backup while the system is running
  • Disk mirroring operations are invisible to application programs and most parts of the operating system
  • The system throughput is maintained even though duplicated disk units are used

Product Description

The system employs two or three equal-sized disk drives that are updated simultaneously by system software. There is a "primary" disk, the one that the applications see; and "secondary" disk(s), invisible to the applications and most of the operating system. The disks are configured in a "disk cluster." If one disk drive of a cluster fails, an error message is recorded and the failed disk drive is automatically "disconnected." The system keeps running without interruption, and the failing drive may be serviced at a convenient time.

All write operations issued to a primary disk in a cluster are also automatically performed on the secondary disk(s) in order to keep them equal. Read operations are performed on only one unit. An internal "read distribution flag" may be set to direct reads to various units. If this flag is not set, all reads will go to the primary. This minimizes disk head movements in cases where reads are more frequent than writes.

The disk drives are kept as equal as possible. However, they may become unequal due to, for example, a failure of one of them or after a backup operation. In this case, the "revive" facility of the disk mirroring module is used to equalize them again, by copying the primary disk to the secondary disk(s). User applications may keep running while the reviving process is taking place.

Requirements

  • Two or three disks of the same physical size (fixed or removeable), connected to the same or two SMD type disk controllers. Connecting them to the same controller is not recommended due to low disk I/O performance.
  • The disk controller to which the secondary disk(s) are connected cannot control any other non-secondary type disk units.
  • Sintran III operating system, version J or later.
  MAXIMUM HARDWARE CONFIGURATION

     +--------+    +--------+    +--------+
     |        |    |        |    |        |
     |  A  (1)|    |  A1 (1)|    |  A2 ( )|
     +--------+    +--------+    +--------+

     +--------+    +--------+    +--------+
     |        |    |        |    |        |
     |  B  (1)|    |  B1 (1)|    |  B2 ( )|
     +--------+    +--------+    +--------+

     +--------+    +--------+    +--------+
     |        |    |        |    |        |
     |  C  (1)|    |  C1 (1)|    |  C2 ( )|
     +--------+    +--------+    +--------+

     +--------+    +--------+    +--------+
     |        |    |        |    |        |
     |  D  (1)|    |  D1 (1)|    |  D2 ( )|
     +--------+    +--------+    +--------+

  PRIMARY      SECONDARY   SECONDARY 2