Doing a Server Double-Take
Despite its lackluster support, Double-Take may be the safety net you need.
If you’re looking for failover protection, NSI’s Double-Take may be worth
a look. Double-Take replicates data from one or mores sources (the servers
you want protected) to a target server, which can stand in for the source
in case of failure. Double-Take has a clean set-up and is easy to configure,
but NSI could do a better job of support and documentation. Here’s my
Value—A. If you wanted to set up failover in Windows 2000 without Double-Take
(or other third-party software), you need to buy Windows Advanced Server,
along with an external RAID array or other external storage unit. This
can get pretty expensive; in comparison, the $2,495 price tag for Double-Take
is a real bargain. One caveat: You must purchase licenses for each server
you install Double-Take on, even if a server will be used strictly for
Installation and set-up—B+. The installation process is clean and simple—just
run autorun.exe and follow the defaults. The connection wizard guides
you through setting up replication and failover (See Figure 1). For simple
data file protection, specify the data directory on the source you want
protected, as well as a directory on the target to which it should be
mirrored. Double Take takes care of the rest.
Setting up failover for SQL Server is just a tad more complicated. You
have to set the SQL services startup to “manual” and set up Double-Take
to run pre- and post-failover batch files.
During installation, you’re given the option of starting the Double-Take
service automatically or manually. If you specify manually and try to
start Double-Take later without starting the service, you’ll get a completely
unhelpful error message. This is certainly easy to troubleshoot, but a
message like, “The Double-Take service is not currently started. Would
you like to start it now?” would be nice.
Functionality—B. Security for Double-Take is based on NT security, rather
than a separate application login and password. Great! One less password
Failover works wonderfully for basic data file protection. The target
server creates the shares and assumes the machine name and IP address
of the failed source server. To test this, I opened a Word document on
a source server from my workstation. I then shut down the source server
while editing the document. The failover process was invisible, and I
was able to continue to edit and save the document. The only minor issue
is that, by default, there’s a 25-second window after the source server
fails and before failover occurs (you can change this setting). I tried
to save during that time, and there was a short delay while the system
searched for the server, after which I was prompted to save to my default
local data directory.
To test SQL Server failover, I connected to a SQL database from linked
tables in Access on my workstation. I opened a table and started editing,
then shut down the source server. I was able to continue to read and edit
the table, but couldn’t save changes or open another table. I had to shut
down and restart Access to continue.
Documentation—D. Double-Take’s documentation is occasionally inaccurate
and generally very frustrating. The User’s Guide emphasizes that it assumes
the administrator doing the installation has a certain baseline of knowledge.
Fair enough, but that shouldn’t be an excuse for poorly organized or incomplete
You have to download additional documentation from NSI’s Web site if
you’re setting up anything outside of a basic data file failover. This
is in addition to a 538-page User’s Guide and 40-page Installation Guide.
While I’m on my documentation soapbox, one last point: The documentation
is copied to Double-Take’s installation directory. It bothers me that
the installation process doesn’t put shortcuts to the PDF files in the
Service—F. My experience communicating with NSI was not positive. I was
disconnected, dumped into voicemail and never got a return call. I want
to point out that I know what it’s like to work on the phones, and I was
polite and courteous to the people I spoke with at NSI. I just got the
impression they didn’t really care whether or not I was helped.
|The connection wizard in Double-Take makes setting up
failover simple and straightforward.
So what’s the final evaluation? In my case, Double-Take was being evaluated
for potential use on our corporate SQL Servers. The so-so SQL failover
performance, frustrating documentation and bad support experience prompted
me to buy a different product. On the other hand, I was talking to a very
knowledgeable colleague about a Microsoft Exchange issue; without being
prompted he started raving, “…and we use this great product on our Exchange
servers. It’s called Double-Take.” He loved it, without reservation. So,
if you don’t mind the documentation and support issues, Double-Take might
be the product for you.
Suzanne Pacheco is the IT Manager for Micro Information Products in Austin, Texas. In her spare time, she and her husband John play golf and try to keep their three labradors entertained.