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What's the difference between 'usable' and 'effective' flash capacity?

Vendors often publish numbers for 'usable' capacity versus 'effective' capacity. Can you explain this and how can you plan flash capacity needs with these numbers in mind?

So, you have usable flash capacity, which is the capacity before deduplication and compression is applied. The...

effective flash capacity is the array's capacity with dedupe and compression turned on.

Many vendors claim a 4:1 or 5:1 data reduction ratio for their systems -- based on typical use. I think that's primarily because of the type of environments they've gone in to. For example, databases have a lot of redundant data, and as such, are very conducive to data reduction.

If you are using an array for general purpose storage for a variety of types of data, not just databases, you're likely to see somewhere in the neighborhood of 3:1 data reduction. So, if I was planning, I would plan for 3:1 even if the vendor says they're able to get 5:1.

That 3:1 ratio is a very conservative number, and typically the people who use the systems are very conservative. And so, that's a practical number work with. Many vendors offer an absolute guarantee of at least 2:1. So that's great, take advantage of it.

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It's like gas mileage. They might estimate 35 mpg, but you better not count on that, or you might find yourself walking down the highway with a gas can. :)
For some reason when I read this I thought of the recent flop over the available space on the iPhone after iOS.  Or the capacity of a hard drive, which is effectively lest after formatting (and even more after the OS and a few basic programs are installed.)