
COMPATIBILITY BULLETIN
Fault Tolerance Subsystems: Unitrol Immunity 
IMMUNITY
 01.31.92

Compatibility Statement 
Unitrol's Immunity Disk Mirroring Software v1.x is compatible with LANtastic 
NOS version 4.xx running with MS-DOS 3.3 or higher.

Product Overview
Disk Mirroring, a process of operating 2 hard disk drives as one, can be 
achieved on a LANtastic network through the use of Unitrol's Immunity 
system.  Any activity that occurs on one disk (the primary drive) is 
duplicated or "mirrored" on the second disk (the secondary drive).   In 
case of hard disk failure on one of the drives, processing will automatically 
be continued on the remaining error-free disk, thus avoiding data loss and 
interrupts in system operation.  Disk mirroring can also be used as a means 
of providing on-line backup.

Product Installation Notes
To start with, you'll need 2 physical hard disks within one PC.  They may 
be of different capacity, brand and type, and can be connected to one or 
two disk controllers.  The two disks must reside within the same PC as 
mirroring cannot be performed over the network.  

Immunity disk mirroring can reside on servers or workstations or both.  
Before installing the software, both drives must be partitioned using 
the FDISK command (from MS-PC/DOS 3.3 or higher) and formatted using 
FORMAT (also a DOS utility).   Run INSTALL from the floppy disk that 
comes with Immunity, which will modify your config.sys and install 
the device driver IMMUNITY.SYS.  It will also add 1 line in your 
autoexec.bat: IMMUNITY -b to verify that contents of the mirrored 
drives are synchronized every time you start your computer.  

INSTALL also performs a "Resynchronize Primary Disk To Secondary Disk", 
which mirrors all the selected data on the primary disk to the secondary 
disk.  Upon completion of the "Resynchronization", both disks will contain 
identical data.  From this point on, disk mirroring will be transparent to 
the operating system.

Product Execution Notes
Under normal operation, disk mirroring requires no attention.  It sits 
invisibly in the background duplicating all writes to the secondary disk 
and monitoring the status of both disks.  Just read the manual in advance 
to make sure that you follow the proper operation procedures in the event 
of a hard disk failure.  When a disk failure does occur, a message is 
flashed on the top of the PC's screen indicating that a disk has failed.  

Operation then continues on the error-free disk, but you will no longer 
have disk-mirroring protection.  It is strongly recommended that you 
repair or replace the defective disk, re-synch the data and then put 
disk-mirroring back on again.

If drive c:(primary) is mirrored to drive d:(secondary), and the user 
attempts to access drive d:(secondary), s/he will get a general failure 
message. 

Performance Evaluations
Since Immunity writes to 2 disks instead of 1, a small performance 
tradeoff should be expected.  However, the system will try to compensate 
this by saving time during the read operations through a method known as 
Fastseek.  Fastseek allows the system to read from the hard disk whose 
head is closest to the data.  This also decreases the wear and tear on a 
hard disk due to head movement.  Unitrol's lab tests have shown a 5-8% 
overall decrease in performance, since most systems do much more reading 
(about 80%) than writing (about 20%).  Performance testing results of 
course depend on the benchmark tests being performed.

Vendor Information
Unitrol Data Protection Systems, Inc.
604-815 Hornby Street, Vancouver BC V6Z 2E6 Canada. (Head Office)
1480 Gulf Road, Point Roberts WA 98281 (US Office)
Phone (604)681-3611   Fax (604)687-0814
Tech. Support (604)681-3613   U.S. Sales (800)665-2212




ARTISOFT, Inc. makes no warranties as to the completeness or accuracy 
of this document.  LANtastic is a trademark of ARTISOFT, Inc.  Brand 
names, company names, and product names are trademarks or registered 
trademarks of their respective companies.

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