Flash in the Data Center? Part 2 – Why not off the shelf SSDs?

by on September 30, 2011

In my last post I discussed a few of the technical aspects of flash that make it a unique storage media, particularly the complexity of garbage collection. Here we take a look at flash packaging and how that impacts architectural decisions. For this post I’ll focus on the question: “Should I use commodity SSDs?” and move on to PCIe cards and Enterprise Flash Drives in subsequent posts.

It’s easy to see why HDD form factor SSDs seem attractive: they have the same look and feel as a regular disk drive, they do usually weigh less, and they fit in the same HDD connectors you have in your existing server. That’s all you need to see great flash speed, right? Sure they look good in the benchmarks, and while some benchmark sites have become a lot more sophisticated in measuring SSD performance, the SSDs have also become more sophisticated at Specsmanship.

In the end the commodity SSD fails in the enterprise for a lot more reasons than just its inability to live up to exaggerated performance claims. The commodity SSD isn’t going to have the error protection or failed component recovery of an “Enterprise Flash Drive”.  The commodity drive is single bus attached so at best it can be used in simple server settings, but not in proper RAID enclosures with redundant controllers. The algorithms in commodity SSDs are optimized around the access patterns of your standard desktop machine and none of these optimizations are going to help your enterprise server.

Similarly, compression or dedupe built into the SSD sound like great ideas for the average desktop PC (and they sure do improve benchmark numbers) but not necessarily for the enterprise. Enterprise apps and services, or the storage arrays they are plugged into, are often already performing those functions. Additionally, encrypting the data right from the application is also becoming more and more common, which totally defeats these benchmark-oriented optimizations.

The commodity SSD isn’t meant to be sold to someone focused solely on performance, it’s meant to be as cheap as possible. So when a few months later you buy some replacements for a failed drive, or to expand your storage capacity, imagine your surprise when you find they have half the performance they used to have, because they got great pricing on Flash chips that were 2X as big, and so the vendor put half the number of chips in as the ones they sold you last month, half the chips, half the IOPs, but the price per bit is great isn’t it? The price per IOP? Not so great. (And yes, some vendor really did do that.)

So clearly packaging matters, particularly when you are talking about deploying in a demanding enterprise environment. The real objective is not so much about packaging flash, but optimally aggregating flash for the enterprise. I’ll get more into that in the next two posts.

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2 Responses to “Flash in the Data Center? Part 2 – Why not off the shelf SSDs?”

  1. We looked at SSDs mainly to increase simplicity and reliablilty for the most critical systems, where datasets are small but IOPS somewhat beyond a couple of local magnetic disks.

    We didn’t need “crazy” IOPS, 10k would fit just fine. And paying €40/GB for enterprise grade PCIe storage was just a little high, when even the daily overwrite rate was below 100%.

    Our loads were well within consumer SSD specs, but we wanted more reliability and nobody seemed to address that segment with the proper quality.

    With LSI purchasing Sandforce I expect that picture to change. Hybrid approaches like the OCZ Velo and Revo units may offer both default OS integration through “known” RAID and PCIe switch interfaces, while they may augment the Flash visibility and handling with customized/enhanced drivers: A bit like paravirtualized drivers for hypervisors.

    With FusionIO entering the appliance market you may have to swallow your pride and offer a PCIe solution, too. Or perhaps something smarter, that physically replaces all those unneeded drive cages while it offers plug-in upgradable Flash storage at PCIe speeds…

    • JCRB says:

      Thomas,

      I don’t know what you were doing with your storage, but if you only needed 10K IOPs and your dataset was small enough, and reliability wasn’t an issue then that’s not really a data center environment. Funny thing about performance, while you say you only needed 10K IOPs, I have yet to meet a customer who on discovering they could go 10X faster, sat there and said “you know 2X faster is fine”, there is always something more you can be doing with your data if only you had the IOPs to do it.

      Regardless of that, if you were looking at equipment priced at €40/GB then you were looking in the wrong place, so when you do find yourself needing to buy enterprise flash storage be sure and come talk to us instead.