Storage for DBAs: Do you want to sell your car? You have one, which you weren’t especially planning on selling, but I’m making you an offer you can’t refuse. I’m offering you one million dollars so how can you say no?
The only thing is, when we come to make the trade I turn up not with a suitcase full of cash but a single Mega Millions lottery ticket. How would you feel about that? You may feel aggrieved that I’m offering you something which cost me just $1 but my response is this: it has an effective value of well over $1m. Does that work for you?
Blurred Lines
This happens all the time in product marketing and we just put up with it. Oracle’s Exadata Database Machine X4-2 has 44.8TB of raw flash in a full rack configuration, yet the datasheet states it has an effective flash capacity of 448TB. Let’s read the small print: apparently this is “the size of the data files that can often be stored in Exadata and accessed at the speed of flash memory”. No guarantees then – you just might get that, if you’re lucky.
Meanwhile, a look at some flash array vendor datasheets throws up a similar practice. One vendor shows the following flash capacity figures:
- 2.75 – 11 TBs raw capacity
- 5 – 50 TBs effective capacity
In my last two posts I covered deduplication and data compression as part of an overall data reduction strategy. To recap: dedupe has no place with databases, while data compression has benefits but is not necessarily best implemented at the storage level.
Here’s the thing. Your database vendor’s software has options that allow you to perform data reduction. You can also buy host-level software to do this, or storage products. So which is best? It probably depends on which vendor you ask – and in some storage vendor cases the data reduction is “always on”, meaning you get it whether you want it or not. What you should know is this: your friendly flash storage vendor has the most to gain or lose when it comes to data reduction software.
Lies, Damned Lies and Capacities
When you purchase storage, you invariably buy it based on price per usable capacity, most commonly dollars per GB. If a storage array costs $X and gives you Y GB of usable capacity, the price in $/GB is X/Y.
Now this practice originally developed when buying disk arrays – and there are arguments to be made that $/GB carries less significance with flash… but everyone does it. Even if you aren’t doing it, chances are somebody in your purchasing department is. You can bet that the vendor with the lowest $/GB price will be the one looking most comfortable on decision day.
But what if there was a way to massage those figures? Each vendor wants to beat the competition, so they start saying things like: “Hey, what about our storage compression features? On average our customers see a 10x reduction in data. This means usable capacity is actually 10Y!” The price per gig (now X/10Y) just came down by 90%!
The First Rule of Compression
You all know this, but I’m going to say it anyway. Different sets of data result in different levels of compression (and deduplication). It’s obvious. Yet in the sterile environment of datasheets and TCO calculations it often gets overlooked:
The first rule of compression is that the compression ratio is entirely dependant on the data being compressed.
Thus if you are buying or selling a product that uses compression, deduplication and data reduction, you cannot make any guarantees. Sure you can talk about “average compression ratios”, but what does that mean? Is there really such a thing as the average dataset?
Conclusion: Know What You Are Paying For
When you buy a flash array (or any storage array) be sure to understand the capacity values you are buying and paying for. Dollar per GB values are only relevant with usable capacities, not so-called effective or logical capacities. Also, don’t get too hung up on raw capacity values, since they won’t help you when you run out of usable space.
Definitions are important. Without them, nothing we talk about is … well, definite.
This article is part of the Storage for DBAs series. If you found this series useful, you might also be interested in Databases in the Age of AI, which explores how AI agents are changing the assumptions at the heart of enterprise data systems.
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