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Network Disc Mirror

An easy solution to lost productivity

It used to be complicated and expensive to protect organizations from production-loss due to disk failures or complete system crashes.
Twincom changed that radically with the Dual Disc Mirror concept and goes even further towards fault tolerance with the introduction of the Network Disc Mirror .
This revolutionary software minimizes data loss and system downtime by duplicating all disk data on two separate hosts. This results in full redundancy for minimum interruption and maximum data accessibility.

Revolutionary operations

Every write command to disk is simultaneously written to the second disk of a pair (DUAL WRITE). This disk can reside on another host.

Situation 1 -- disk failure

If one disk becomes inoperative due to a head crash or breakdown, the remote disk of the pair automatically assumes operations without any production loss (HOT STANDBY). The Network Disc Mirror (NDM) Program is linked to the UNIX operating system. The NDM Programs are fully transparent to users and application programs (FULL TRANSPARENCY).

Obvious advantages

The idea behind the Network Disc Mirror: Maximum accessibility by using two identical disks on two different hosts in a network.

Situation 2 -- CPU failure

By redirecting the users to the alternate CPU (as simple as a keystroke or flipping a switch), data accessibility is maintained with minimum interruption should the primary computer fail. Network Disc Mirror eliminates time-consuming system restoration from tape backup and the intervening data loss. Because information is redundant in real time, the data is always current, even in the event of a disk or CPU failure (OPTIMIZED FAULT TOLERANCE).

Organizations are susceptible to failure or corruption of critical data by mechanical errors. This creates the requirement for fault tolerant network solutions. During normal operations, the HOT SPARE HOST can be used for less critical applications, thus optimizing your investment in hardware and software.

Network Disc Mirror operates in a standard UNIX system. NDM requires little memory or disk space. Existing systems can upgrade to NDM software without rebuilding from scratch.

Remarkable options

In the event of a disk failure, the system immediately notifies the console and continues data processing automatically. The status can be printed, displayed and/or reported via datacom (MIRROR STATUS REPORTS).

After a failure is corrected, the "MIRROR" automatically synchronizes the second disk in the background. All processing continues normally (ONLINE SYNCHRONIZATION).

Fast, easy installation

Network Disc Mirror (NDM) of Twincom can run on many popular UNIX systems. Sites who already benefit from Dual Disk Mirror can easily upgrade to the Network Disk Mirror.

Requirements: the NDM software, pair(s) of hard disks on two separate UNIX systems communicating via TCP/IP, an optional backup device, and the Twincom installation support manual (included).
Note: each server needs its own internal boot drive.

The Operating system must be loaded on a separate disk for each server, not on the mirrored disks. These "system-disks" can be small and can be mirrored locally too, in order to guarantee the highest level of availability. The Operating System must be the same on both servers.


The heartbeat programs monitor any failure of the primary system and also verifies if both servers are still there. It detects when one of the 2 servers become unavailable.


When a failure occurs on the primary system, the configuration will go into the "fail-over" mode. The Twincom software sends a message to the 2 systems and also to the system administrator, notifying him that the main host is down. The stand-by system will automatically mount the filesystems on the mirrored volumes and also executes a "fsck". Users on the primary system can then access the secondary system and carry on with the data from the point the primary system failed. A typical switch-over takes only a few minutes.

In this way, the costs of interruptions of mission critical applications, due to server problems are eliminated. The easy of use because standard off-the-shelf parts that can be used for the Twincom solution, make it a time and cost efficient alternative.

Impressions of users

Moorkens Projects, Belgium

Largest professional office equipment company in Belgium secures daily order processing data on two separate host CPUs. "Systems designed around the Twincom Disc Mirror compete with Tandem and Stratus ® at a much lower cost."

Trendar Corporation, USA

Every hour of every day of the year, trucks are on the move, tracing and retracing the continent. Tracking fuel transactions is a major issue for the trucking industry.

To resolve its point of sale dilemma, Trendar developed a system designed around Twincom's Disc Mirror software for greater efficiency in truckstop refueling.

"This software works with standard configurations and can be handled by laymen!"

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Dual Disc Mirror and Network Disc Mirror are registered trademarks of Twincom USA and Twincom International BV.
All other names indicated by or TM are registered trademarks or trademarks of their respective manufacturers.

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