When is a Storage System Full?
When designing the underlying storage infrastructure for a set of applications, several metrics are important.
First, there’s capacity. How much storage do you need? This is a metric that’s well understood by most people. People see GBs and TBs on their own devices and subscription plans on a daily basis, so they’re well aware of it.
There’s also performance, which is a bit more difficult. People tend to think in terms of “slow vs. fast," but these are subjective metrics. For storage, the most customer-centric metric is response time. How long does it take to process a transaction? Response time is, however, the product of a few other metrics, including I/O operations per second, the size of an I/O, and the queue depth of other I/O in front of you.
SIZING A STORAGE SYSTEM
If you size a storage system to meet both capacity and peak performance requirements, you will generally have low response times. Capacity is easy; I need X Terabytes. Ideally, you’d also have some performance numbers to base the size of your system on, including expected IOps, I/O size, and read:write ratio to name a few. If you don’t have these performance requirements, a guesstimate is often the closest you can get.
With this information, and an idea of which response time you’re aiming for, it’s possible to configure a system that should be in the sweet spot. Small enough to make it cost effective, yet large enough that you can absorb some growth and/or unexpected peaks in performance and capacity. Depending on your organization and budget, you might undersize it to only cover the 95th percentile peak performance, or you might oversize it to facilitate growth in the immediate future.
LET IT GROW, LET IT GROW… AND MONITOR IT!
Over time though, your environment will start to grow. Data sets increase and more users connect to it. Performance demands grow in step with capacity. This places additional demands on the system; demands that it wasn’t sized for initially.
Monitoring is crucial in this phase of the storage system lifecycle. You need to accurately measure the capacity growth over time. Automated forecasts will help immensely. Keep an eye on the forecasting algorithms and the statistics history. If the algorithm doesn’t use enough historical data, it might result in extremely optimistic or pessimistic predictions!
Similarly, performance needs to be guaranteed throughout the life of the array. The challenge with performance monitoring is that it’s usually a chain of components that influence each other. Disks connect to busses, which connect to processors, which connect to front-end ports, and you need to monitor them all. Depending on the component that’s overloaded, you might be able to upgrade it. For example, connect additional front-end ports to the SAN or upgrade the storage processors. At some point though, you’re going to hit a limit. Then what?
Fewer, larger systems have several advantages over multiple smaller arrays. There are fewer systems to manage, which saves you time in monitoring and day-to-day maintenance. Plus, there’s fewer losses, as silos tend to not be fully utilized.
One important aspect to consider, though, is the failure domain. What's the impact if a system or component fails? Sure, you could grow your storage system to the largest possible size. But if it fails, how long would you need to restore all that data? In a multi-tenancy situation, how many customers would be impacted by a system failure? Licenses for larger systems are sometimes disproportionally more expensive than their smaller cousins; does this offset the additional hassle of managing multiple systems? There’s multiple approaches possible.