My field of enterprise storage has its share of generally-accepted theories:

• Availability, backup, and archive form a Data Protection Trinity: They are unique requirements calling for focused solutions.
• The Rule of RAID: Combining multiple disk drives in creative ways allows us to change the inherent reliability and performance of the system.
• When it comes to storage management, Homogeneity is Paramount: A single storage administrator can manage thousands of identical systems but would be hard-pressed to support a half-dozen unique ones.
• The entire history of computing demonstrates that Connectivity Trumps Capacity when sizing systems: Performance bottlenecks always limit the scalability of storage systems.

Each of these theories underpins the our industy’s daily routine of storing and retrieving the data that drives modern society. These storage theories are also targets for innovation, with the best minds constantly trying to bend or break them.

This album of storage theories also has a B-side, however. These are the no-longer-true theories that have been transcended, as well as the dubious beliefs that were never really true.

• Commutability of Management and Cost is highly suspect: Unless one is considering only identical and homogenous systems, the total cost of ownership (TCO) or number of administrators associated with a given system (TB/admin) cannot be compared between environments.
• The Price of Parity: The impact of parity calculations and multi-disk commits used to kill write performance, giving RAID-5 a bad name. But write-back caches and array intelligence have all but eliminated this “write penalty” for modern enterprise systems.
• Whenever the high cost of enterprise storage is to be refuted, someone is bound to trot out The Dumb Disk Fallacy, claiming that per-GB array costs ought to be comparable to the price of a bare disk drive. But the value of enterprise storage has always been greater than the sum of its parts.

Over the next few weeks, I will be sharing focused articles about these “holy cows” of the enterprise storage world. I encourage everyone in the industry to join me in taking a step back and shining some light on these and other truisms. Which do you agree with or dispute? Are there other theories that I have overlooked?

© sfoskett for Stephen Foskett, Pack Rat, 2009. | We Hold These (Storage) Truths…
This post was categorized as Computer History, Enterprise storage, Everything, Gestalt IT. Each of my categories has its own feed if you'd like to filter out or focus on posts like this.

Now EMC’s Chuck Hollis has (bravely) stuck his neck out to try to actually compare the capacity efficiency three storage arrays in a realistic way. Good luck, Chuck! I can hear the knives sharpening over at NetApp and HP already!

Why This is a Good Idea

In all seriousness, this is exactly the sort of analysis that customers ought to be taking on. Buying a new storage device? Spec out how you want to use it and ask for proposals from vendors that are configured according to recommended practice. That’s the only way to really compare two devices: Real usable configurations.

It’s not realistic to expect that an EMC array with the same capacity and number of disks as a supposedly similar device is going to give you the same usable space, performance, energy efficiency, manageability, or really anything else. Despite the basic architectural similarities of, say, a CLARiiON and an EVA, there are just too many critical differences to think of them as a pair of apples, even if you strive for the same specs.

In fact, there are no apples or oranges anymore. No one uses straight textbook RAID. No one makes a pure NAS filer or Fibre Channel array or anything. They’ve all evolved away from the basics we think we understand, adding in a little midrange, a dash of green, and a dollop of iSCSI to become a field of grapples, tangelos, limequats, and pluots.

Why EMC Shouldn’t Be Doing It

All that being said, I think it’s beyond perilous for a vendor to try to set up a standardized comparison of capacity, just like it is foolish to try to get some kind of meaningful performance statistic between such diverse platforms.

Even when (as appears to be the case here) a vendor tries to follow their competitors’ recommendations, they’ll likely not end up with the same configuration that an experienced Systems Engineer from that company would put together. Often, these smart guys know the real-world implications of the system and can put together a system that matches the requirements better.

Regarding EMC’s specific comparison, I do have some questions, however:

1. Does EMC really support using the five vault reserve disks on a CLARiiON to hold production data? EMC SE’s have suggested to me in the past that this is a bad idea…
2. Would EMC really suggest 8+1 RAID 5 for a production Exchange and SQL Server environment?
3. Is one hot spare per two DAEs (30 drives) really sufficient for a whole pile of 9-disk RAID 5 sets that are maxed-out with production data? I’d feel much more comfortable with a few more spares with such large RAID 5 sets.
4. There is no way 14+2 RAID DP is equivalent to 4+1 RAID 5, let alone 8+1. It’s in a different league of reliability.
5. Yeah, NetApp’s space reserve recommendation stinks. But you probably won’t need 100% in production – the real amount is something one would work out when testing and piloting and is probably substantially less than this.

I’m not trying to get into an argument about this, mind, just noting a few items that immediately jumped out at me. And if I could see these five issues in my quick read, I can just imaging what HP and NetApp will see! Watch out for the knives, Chuck! I know you mean well, but exercises like this just won’t ever work.

And where are HDS, Sun, and IBM? Plus, I would love to see 3PAR, Compellent, Dell/EqualLogic, LeftHand, and the rest jump in with their numbers! Maybe I should set up a sham RFP and ask the vendors to respond with their own systems for some real comparison!

(This post was updated for clarification and to add that last suggestion…)

© sfoskett for Stephen Foskett, Pack Rat, 2008. | Grapples and Tangelos: Why it’s Impossible to Compare Fairly
This post was categorized as Enterprise storage. Each of my categories has its own feed if you'd like to filter out or focus on posts like this.