In modern storage devices (especially All Flash Arrays), extensive data reduction techniques are commonplace and expected by customers.
This has, unavoidably, led to various marketing schemes that aim to make certain systems seem more appealing than the rest. Or at least not less appealing…
I will attempt to explain what customers should be looking for when trying to decipher capacity claims from a manufacturer.
In a nutshell – and for the ADD-afflicted – the most important number you should be looking for is the Effective Capacity Ratio, which is simply: (Effective Capacity)/(Raw Capacity). Ignore the commonly quoted but far less useful Data Reduction Ratio, which is: (Effective Capacity)/(Usable Capacity).
Ultimately, as a customer you shouldn’t even care about ratios. Only about the true effective capacity you can safely use.
Important: Any time you see any vendor quoting a ratio, they always quote the data reduction ratio. So, it is crucial that you don’t calculate effective capacity by multiplying that ratio times raw, but rather times usable. I had to edit the article to add this since I recently spoke to a customer that was making this grave mistake (various vendors only gave them the raw capacity and the data reduction ratio!)