Lying via Capacity Guarantees

It seems there’s no shortage of creative marketing ways to fool customers these days.

I’ve written before about how to protect against some of the shadier storage business “guarantee” schemes, but a relatively new development has prompted this post.

You see, a rather large vendor-who-shall-not-be-named is now offering a capacity guarantee across their entire storage portfolio, shown right on their website. The interesting part is that their guarantee mentions not just thin provisioning but also snapshots.

Many vendors use thin provisioning in capacity guarantees – it doesn’t make the practice acceptable, merely common like influenza or selfies, only more annoying.

However, no other vendor I’m aware of has the sheer audacity to also count snapshots in their capacity guarantee.

Executive Summary

If you can read no further, if someone is promising you a data reduction guarantee, ask for a legally binding document that explains exactly how the data reduction ratio is calculated. What is included? Compression and dedupe – fine. What about snaps and/or thin provisioning? If those are included, then ask for a different guarantee without those features. What is the number then?

Why You Should Care

Just like with thin provisioning, including snaps will artificially (and far more dramatically) raise the data reduction ratio, so that this vendor will likely not have to pay out on their guarantee. That’s really the objective – to offer a guarantee that they will never have to satisfy. Vendor wins, customer loses.

You see, with thin provisioning, most of the time customers won’t have volumes more than 2:1 overprovisioned. This will inflate data reduction ratios by 2x. Which one would think is a lot, right? Even a modest 2:1 true data reduction becomes 4:1 with 2x overprovisioning (see the second row in the table below).

But wait, there’s more!

Snapshots are a feature that practically every storage system has (some do it better than others, but I digress).

Most customers will take several snaps a day, some doing it once an hour or even more frequently. This can inflate data reduction ratios by orders of magnitude…

Allow me to explain what this means.

Adding snaps to the equation looks like the system has a lot more data than it really does. 100TB of data becomes really 200TB of logical data with just 1 snap (original plus 100TB copy) so that’s a 2:1 “data reduction” right there 🙂

To make the math easy, let’s assume that between snaps, there’s zero change rate (usually it will be in the low single digits percentage).

User-friendly table with some examples (applicable to any storage vendor that supports these features):


Notice that in all cases, the true data reduction in this example system is on the low side (just 2:1). The moment even 1 snap is done, the overall number is inflated due to much more logical data being there.

I added on purpose a really pessimistic estimate in the last row. Even with 1.5:1 data reduction, 1.5:1 thin provisioning and a single snap, the result is 4.5:1!

Even worse, these numbers mean the total number of snaps, not the per day numbers. 1 snap in this table means that for every single LUN in the system you’ve taken 1 snap. No more. Only 1 snap per volume/LUN exists in the system.

Most people take more than 1 snap per volume per system – I’d think at least 4 a day, and 24 snaps a day is extremely common (merely once per hour).

The snap “savings” add up, and counting them in data reduction can easily lead to crazy data reduction numbers (hundreds or thousands:1).

This is why this vendor is so comfortable offering this “guarantee”. They will almost never miss, which means they will almost never have to pay up! It’s utterly meaningless to a customer, though.

Read The Small Print!

If you’re looking at any guarantees, read the terms carefully. For example, terms like “logical capacity” mean that there’s a good likelihood snaps count against the final number.

If you must purchase from that vendor, at least keep yourself safe and make them work for it: demand a legally binding contract that expressly stipulates snapshots (and thin provisioning) will not be counted against the savings ratio.

Even better, make them prove it with data from their entire install base, showing numbers with and without thin provisioning and snapshots.

And, last but not least, a message to that vendor: You know who you are and you should be ashamed of yourselves.

Dunce hat


3 Replies to “Lying via Capacity Guarantees”

  1. so I dont know who this is etc. Dont really care. But if you dedupe across snapshots whats wrong with including snapshots in space guarantees?

    Having said that I have zero doubt its a tad too “creative”.

    1. It’s not about deduping across snapshots. Deduping across the small snapshot deltas – so what. Those are small.

      No, what I mean is that even a SINGLE snap means an inflation of logical capacity by 2x. Even without dedupe.

      That’s a bit too creative.

      Especially since almost every single vendor can do snaps, and could claim the exact same thing.

  2. Old(ish) article I know, but about 5 years ago we signed up for two all-flash storage arrays with a storage guarantee. Once we started to move data it was quickly discovered that they arrays were only providing about half of the capacity savings that were promised. The vendor did come good and provided the additional disk shelves and drives at their expense. However, when it came to adding capacity for growth 3 years later, we couldn’t justify the ridiculous price. In the end we bought two smaller basic arrays with nearline disks and moved some workloads there that didn’t require much in terms of performance.

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