High availability means much more than five nines or 24 x 7 operation. It's about getting your hardware, networks, software, policies, and people all working together smoothly.
What does high availability mean to you? How do you calculate how much it is worth spending to achieve it? In this feature, we look at strategies for hardware, software, and network configuration, as well as policy and people considerations, for keeping your systems running. The key concept behind high availability is that a system should be available when it is needed.
Evan Marcus, data availability maven at Veritas, draws an analogy with the rocket motor on the Apollo lunar excursion module. The rocket was only needed for about five minutes during each mission, but during those five minutes it had to be available.
But what does "available" mean? Exact definitions will vary between organisations, but in most contexts the only realistic measure is that people can use their applications and receive results with an acceptable delay. If users can't get their work done on time, the system is down.
As Niall Gallagher, VP, Intelligent Internet, Nortel Networks Asia, puts it, "A technical equation for calculating network availability exists, but availability should really be viewed from the user's perspective. The user does not know about the individual components in a network, but instead is limited to his/her experience of its performance. In this context, the maximum availability of the end-to-end network is paramount."
We have all heard about "five nines", but what does that actually mean? There are 525,600 minutes in a year, so 99.999 percent availability equates to a little over five minutes downtime per year. So for five nines, if the system comes down just once a year--planned or unplanned--you need to get it back up in less than six minutes. In their book Blueprints for High Availability, Evan Marcus and Hal Stern wrote, "Downtimes of less than 10 minutes per year (about 99.998 percent) are probably achievable, but it would be difficult to get much less than that."
In reality, it is only in rare cases that systems need to be available 24 hours a day. If the operational window for the system is 18 hours per day rather than 24, that gives you a handy window for planned downtime. Down is down, whether it is deliberate or unexpected, but it only counts against you when users need access to the systems.
For most organisations, one day isn't exactly like another. If you were an e-tailer, would you rather your system was down for a couple of hours on an evening in mid-December, or in the small hours of a February morning? If you're running an inbound call centre, would you rather have an outage on the day your biggest client starts a major advertising campaign, or the day before?
A system therefore can be described as highly available if it meets or exceeds the availability requirements. "You buy a computer to do something [and] you expect value back," says Marcus.
"A technical definition does not directly translate into the business impact of high availability. Mission critical networks, upon which vital revenue streams depend, must be highly robust and able to sustain an availability that meets the highest standards," says Gallagher.
It's also important to remember that downtime isn't over as soon as the failed subsystem is restored. If your desktop PC suffers a disk crash, an in-house technician might be able to replace it within 10 or 15 minutes if a spare is available and you have sufficient clout to demand that level of service. It's going to take a lot longer to reinstall the operating system, applications, and your data.
When you're talking about high availability, time to repair can be a significant part of the equation. If you need to reload a multi-terabyte database from removable media, that time is measured in hours, not minutes. And even when it is reloaded, you'll need more time to apply the changes that have accumulated in the journal files since that backup was made. Clearly, high availability calls for different approaches.
