In a recent posting, Curtis Preston wrote some pretty insightful words. I was waiting for the appropriate moment to reference them, and this seems as good as any. Since Val Bercovici referenced them recently, it seems doubly appropriate. Curtis wrote:
I treat vendor blogs as a printed version of a sales rep talking to me. I expect the sales rep to tell me all the best things about their product and none of the bad. I expect them to try to bash the competition at some point (although I don't like it when they do), but I also believe that the best source about their competition isn't them; it's their competition. So when they tell me that the other vendor's blood is green, I don't believe them. I grab the nearest sales rep and prick them with a needle and look very closely.
My thoughts on this? This is all true, with one important exception: where a product has a specific and significant shortcoming. To (unfairly?) perpetuate a stereotype of sales reps, we wouldn't expect that rep to immediately admit, or even admit under anything less than extreme duress, a critical failing of a product, would we? So, after thinking a lot about this, I think there is a valuable role for the competitive blog post: to expose a critical short coming. Hopefully this can be accomplished without any unnecessary histrionics, vaudevillian showmanship, or needless hyperbole. I regard this as similar to the role of the press in a democracy, or the role of the opposition in government: to provide checks and balances, and a contrary opinion. To keep everybody honest.
In the blogosphere, we have the added luxury that this can be a dialogue. If a competitor disagrees, there is every opportunity to engage in conversation, and point out the failings of the post. Does the reasoning hold up or does it not? Is it sound and relevant, or is it much ado about nothing? And ultimately I hope the readers bring their critical reading skills to the table so they can judge each position on its own merit.
So, when I posted that the NearStore VTL was practically guaranteed to lose data, I hoped that the analysis would: a) stand up to scrutiny by NetApp, b) that they would engage in dialogue if, in their opinion, I was wrong, and that, c) it was relevent. Well, like Meatloaf said, 2 out of 3 ain't bad.
Alex McDonald, to his credit, did attempt some dialogue. Which amounted to claiming that the assumptions they used to bash RAID 5 are not right. Hrm. More or on this in a bit. David Chapa, unfortunately, didn't even make that attempt: his response was mainly to indirectly impugn the math in my analysis, attempting, if you will, to imply that if it is on the internets, it must be false. Or perhaps that the math was similar in complexity to that which Maximillian Cohen wrestled with in Pi--so byzantine that it could only drive you mad, and certainly beyond the capabilities of any mere blogger from EMC.
So here is a fuller analysis. A complete understanding of how dangerous the NetApp VTL is to your data, if you use its deduplication capability. The percentages in the table represent the chance of complete data loss (loss of the entire array) over 5 years. The numbers are based on NetApp's own marketing materials, blogs, and technical papers. The assumptions are RAID 5 (identical to VTL RAID in terms of probability of catastrophic failure), 7 disk RAID groups, deduplication capability on.
The left hand column is the number of RAID groups present. I ended at 91 simply because if you divide the maximum useable capacity of the NetApp VTL by 6 TB (the useable capacity of a 7 disk RAID 5 group), you would have 91 RAID groups. So that is full system. The percentages in the top row are the probability of catastrophic failure of a RAID 5 group with seven disks over five years. 6% is the number used by NetApp in their marketing and blogging. Recently, they suggested that might be a bit of an exaggeration, and implied that 3% might be a better figure. For the sake of completeness, I am providing numbers for various probabilities of failure.
Here is the conclusion: with NetApp's own numbers, a full system is 99.64% likely to experience complete data loss over 5 years. If you half the likelihood of failure for a single RAID group, that only declines to 93.74%. In fact, even if you drop to a 1% failure rate on a RAID group, a full system is still 59.93% likely to experience total data loss. So, if you are thinking of buying a NetApp VTL: "you've got to ask yourself one question: Do I feel lucky?"
Here is the table:
And, until such time as NetApp has some genuine data to contradict this reasoning, or can point out any real issues with the probability calculations, this is my last word on the subject.
Well, do ya? ;)