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April 08, 2014




While I wouldn't say that VSAN REQUIRES 3 copies of data I would say that in a distributed mirror architecture using standard servers like VSAN, or most object stores for that matter, that I wouldn't sleep well at night with less than 3 copies.

When that VMware admin takes a node offline to add capacity, as you suggest they can, if they took the defaults there would be just 1 copy of each VM on 1 disk drive. Should that drive fail at the wrong moment the poor admin would be faced with data loss.

To get resiliency comparable to a dual controller array with RAID 3 total copies would be required.

I would also note that per-VM data services are not unique to VSAN or even ServerSANs. Tintri for one has per-VM data services at lower cost in performance or capacity than VSAN. I hear some others do as well including Isilon.

I, as you well know, also work for storage vendors as an analyst. Tintri has loaned me a storage system but has not, yet hopefully, engaged my services.

- Howard

Chuck Hollis

Hi Howard -- yes and no.

Just to restate, the default is to have two copies on separate separate server nodes, but this can be increased or decreased on a per-VMDK basis. But let's assume 2 copies for now.

You are right in that if I take a server down *and* my second copy failed *and* I didn't reprotect the original storage objects before powering down the server, *and* if I couldn't bring back the first server I took down, *then* I might experience data loss.

But that's a choice, not an inherent limitation. I would also have the option of relocating the storage objects prior to powerdown, preserving two copies despite the missing server. Or, as you point out, establishing three copies on a per-VMDK basis as needed.

I'd have to balance the resources required for the automatic move against the need: the tantalizingly small probability that -- for some reason -- my server didn't come back up *and* my second copy failed.

And, then again, there are hot-add servers.

The good news here is that VSAN isn't forcing you to do things one way or another. Certainly, there will be people who feel as you do that 3-way protection is the way to go for everything. Others will be more selective in their approach, balancing cost and risk.


-- Chuck

Chris Evans

Howard, 3 nodes (2 data + 1 witness) is the minimum requirement, however I suspect you're following the line of VMware's bloggers with the logic posted here - http://www.yellow-bricks.com/2013/10/24/4-minimum-number-hosts-vsan-ask/ - which makes sense. After all, server maintenance now means reduced storage resilience, which needs to be catered for. Whether done externally or in the server, there's still a price to pay for data protection.



I think we're in agreement, and for applications where a 1-2% chance of data loss is OK and there are many, 2 copies, and the current VMware admin processes, would be fine. Therefore I appreciate the choice.

For most of my apps I'd say 3 copies, for some 2 copies and modified process to migrate data off before power down.

As a Steely-eyed Storage Guy paranoia is my job.

George Crump

Thanks for the professional response. Frankly, I have a few issues with your criticism of the article so I have to ask for your patience as I construct my equally professional response to your blog. I must say, however, that I find it a bit perplexing that you took umbrage about the lack of disclaimers in our article; despite the fact that it was clearly listed at the bottom of the piece.

Furthermore,to say that I have been stubborn in my defense in the light of "substantial criticism in a number of forums" is at best, artistic license. Interestingly, the ONE forum you link to has had as much support for what we had to say as it did for defending the virtues of the VSAN offering. In fact, the public and private comments to our site have been roughly equally divided between proponents and detractors of VSAN. Even the recent comments on your own forum show that the verdict is still out on the VSAN solution. Again, a more detailed response will be forthcoming on our our part shortly.

That said, I do appreciate the work that must have gone into your response.

Chris Ramseyer

The writing is on the wall for pay to play. On the component side, not only would pay for play not fly, end users would revolt against the content. It doesn't matter how catchy the name is or what it represents. You simply cannot be independent while pointing the finger with the check to do so in the other hand.

Independent reviews and editorials are coming to this market. Unlike the pay to play guys who write mainly for industry insiders, the 'review sites' write for the decision makers. The content is the key. Too many times I've seen single or two page previews and laugh while wondering who would write a check based on the same information you get on the manufacture's product page.

Here's our initial take on vSAN in news form: http://www.tweaktown.com/articles/6140/a-major-shift-in-the-data-storage-market-is-on-the-horizon/index.html

Kelly Murphy

Chuck -

It is unfortunate that this is how the industry works today. However, I am surprised to hear this statement coming from EMC/VMware considering there is not an analyst group you have not paid and this is the industry largely created by EMC. Its both amusing and hypocritical.

On the article itself - the content is 99% about the architecture of server-side storage - and there are various flavors of this. VSAN is the most recent of many and obviously topical at the moment. It’s also fair to say that not all arguments made apply to VSAN, and it is clearly not worded that way.

For a more technical response, please see my blog at Gridstore.

Chuck Hollis

Hi Kelly

I understand you're very passionate about your product, something I can certainly empathize with. I've certainly been there before :)

Just to make sure we're being accurate here, I work for VMware, but I write my blog independently. My content is neither reviewed nor approved by VMware.

To the best of my knowledge, neither EMC nor VMware have ever paid an analyst to intentionally fabricate facts. While you might want to hide behind the fig leaf of "well, we weren't really talking about VSAN", I would direct you to the title of the published article.

Look Kelly, I'm sure you're a good guy with a lot to say -- but you went down a dark path on this last escapade.

I'm more than willing to discuss the pros and cons of different architectures -- that's fun for me, and useful for everyone -- but paying hired guns to do your dirty work isn't exactly kosher.

Server-side storage is a big deal in the industry, and deserves a lengthy exposition. Coming from the array side of the business (EMC), I see both sides, and am quite comfortable discussing the pros and cons of each approach.

Let's look forward to a productive discussion, shall we?

-- Chuck

Kelly Murphy


I appreciate that you posted my response on your blog and your willingness to discuss the technical merits of each approach - you are right - that would be a good thing for everyone.

There are similarities between Gridstore an VSAN - both are built specifically for virtualization and we both operate in the kernel of the host - many of the advantages we both offer come from working in the kernel - close to the work load and being able to provide storage that is optimized per VM. There are also significant differences.

I would love nothing more than to do a head to head benchmark of the two architectures, same workloads, same platforms - and let the results speak for themselves. Unfortunately we do not operate on ESXi because VMware is not open to allow us to work in the kernel like we have done successfully on Micorosfts Hyper-V for the past year.

I would however like to take you up on your offer to openly discuss the two architectures. That would be useful and productive for everyone.

Let's discuss where and how we can do this.


The comments to this entry are closed.

Chuck Hollis

  • Chuck Hollis
    SVP, Oracle Converged Infrastructure Systems

    Chuck now works for Oracle, and is now deeply embroiled in IT infrastructure.

    Previously, he was with VMware for 2 years, and EMC for 18 years before that, most of them great.

    He enjoys speaking to customer and industry audiences about a variety of technology topics, and -- of course -- enjoys blogging.

    Chuck lives in Vero Beach, FL with his wife and four dogs when he's not traveling. In his spare time, Chuck is working on his second career as an aging rock musician.

    Warning: do not ever buy him a drink when there is a piano nearby.

    Note: these are my personal views, and aren't reviewed or approved by my employer.
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