What's the relationship between storage capacity, storage utilization and storage performance?

"We have often been warned about exceeding 80% of a disk's capacity, yet storage virtualization promises to allow up to 100% storage utilization. How do you reconcile these two storage considerations?"

There's a lot of focus on storage capacity rather than storage performance. One of the marketing pitches behind

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storage virtualization is to increase storage utilization so that you don't need to buy so many drives.

Unfortunately, "how" the storage is being utilized remains a significant consideration. I've heard "rules of thumb" cautioning against running hard drives at more than 50% capacity. Some concerns are myths; some are based on real issues, like application requirements or workload characteristics.

Still, there's an increasing "storage to server I/O performance gap" where the disk drives are offering more capacity, but the performance isn't keeping up. If you're going to drive utilization up, you also need to consider the performance impact on your service level. You may need faster storage controllers/arrays, caching devices or other technologies to accelerate your effective storage I/O.

Ultimately, the top utilization level is the highest utilization that you can drive to while still meeting an acceptable response time or service delivery time. Don't focus just on capacity utilization. Look at storage performance, availability and reliability. Also keep in mind that more disk drives do not always equate to more performance depending on the type of storage system and controller configuration.

Go back to the beginning of the Disk Hardware FAQ Guide.

This was first published in May 2007

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