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You can download an evaluation copy from here : Download Page
Introduction to Double-Take
Nightmare Scenario: - A gas explosion in the mid-afternoon destroys the part of the building where your server is located - your server (which had mirrored disks) is destroyed too. What do you do?
Your disk-mirrored system can't help you. Normally you would have to go to your over-night back-up tape in an off-site location and load the data from it onto another server and bring the server back up and then get your users back on-line.
It would take at best hours and possibly days to restore a large server and you would have lost the whole days work to boot !!
There is an alternative - Double-Take.
If Double-Take had been installed on the source server it would have replicated all file changes (as they occurred) to a target server in another building. This would mean that the target server would have a second by second up-to-date copy of what was on the source machine.
Failover
The target server also hosts the failover module and continually
monitors the source server. In the event of a server failure the
target server could automatically assume the names and IP addresses
of the failed server (in addition to its original name), it would
also invoke scripts to restart applications. Users would be back
on line in seconds or minutes with little or no data loss.
Double-Take configurations
Double-Take can be used in one-to-one mode as above or one-to-many,
or many-to-one, chained or with two servers each acting as source
and target for the other. In the example above the target server
would have taken over as "source" server BUT could still be acting
as "target" server for other source servers in a many-to-one configuration.
Double-Take augments your existing tape back-up strategy.
Double-Take can be used with your existing tape back-up system.
The advantages are that if the tape system backed up the target
server it could back up files that were locked open on the source
server. This means that your source server need never be shut down
to back up these files. Also if you were in a many-to-one configuration,
you could do all back-ups from the single target server thus simplifying
management and improving the performance of back-ups.
Setting up Double-Take
You choose the files, directories and server volumes to be replicated.
A simple to use client system then copies this replication set to
the targeted server (this mirroring operation can be done whilst
both servers are active). These operations are both automatic and
transparent. After that the replication, monitoring and failover
functions are automatic.
Platforms
Multi-platform consistency provides identical functionality on NT/W2000/W2003.
Cross platform management provides an integrated control point for
all Double-Take machines. The Double-Take Management Console and
Text clients displays a comprehensive picture of allNT/W2000/W2003
Double-Take machines on your network.
Resource Utilisation Features
Network - Double-Take provides features to minimise network resource
utilisation and provides options that allow network administrators
to optimise bandwidth usage.
Asynch. Operation - Loosely coupled communication allows Double-Take to operate asynchronously enabling source machines to continue processing even when the network link is congested. By contrast, in synchronous systems, WAN connection bottlenecks can severely impair the performance of source machines.
File Updates - The system only transmits changes made to files, not whole disk blocks or whole files. This dramatically reduces the amount of bandwidth required and means that Double-Take runs efficiently even over low bandwidth links.
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