Michael J. Swart

The devil you know vs. the devil you don’t.
A good friend of my told me about an issue she recently came across. It seems that a particular group of admins implemented SQL Server database mirroring in order to implement a kind of relatively cheap high availability solution. It’s a useful strategy that’s discussed in a number of places. Here and here for example. 

Well, something went wrong or I wouldn’t be writing right now. I’m not familiar with all of the details, but there was some wrinkles with the performance of the network and the database server became extremely slow. (despite what you might think, this really isn’t an article about database mirroring) It’s ironic that the very thing that was implemented to increase up-time was the cause of down-time.

From another perspective, this is a case of “The devil you know vs. The devil you don’t.” Here, they traded the devil they knew – that is, not having a warm standby server – with a devil they didn’t – in this case, production uptime now depended on network performance in an unexpected way.

Other examples of known vs. unknown devils:

• Known bugs in version 1.0 vs. Unknown introduced bugs in version 2.0
• Raid 0 vs. Misconfigured SANs
• Reliability of municipal power vs. Possibly fawlty UPS systems
• Add your own examples to the comments! (best two comments get a No-Prize – extra points for irony.)

I’ve actually witnessed the unknown devils above. And as a lover of irony, I smiled (tempered slightly by sympathy for those whose necks were on the line).

The devil is in the details
In the examples I’ve mentioned above, the unknown devils reared their heads because of exposure to an unforeseen risk. What can you do about it though? Well, what else is there to do? Get to know that unknown devil. Test, test, test with all the time and resources available to you. There really is no short-cut. And sometimes, with the right testing, if you’re thorough enough you can get it right…

God is in the details*
Getting it right means you get the high availability you were hoping for. The uninterupted power. Secure systems, the robust bug free software. Maybe even five nines.

I’ll leave you with one last devil you know vs. devil you don’t example. It’s a picture of me and my brother. Presumably you know me and not him. But which is the evil one? Increased difficulty: neither of us have a goatee. Unfair: the photos have been scrambled.

* — BTW, “God is in the details” is a quote by architect Mies van der Rohe. I’m in good company when quoting architects for this industry; you’ve heard of design patterns right?