Introduction

The information industry is living through a revolution in data storage. Spinning disks whirling around the data center are being replaced by flash storage with no moving parts. At first glance, it seems easy to say that this is a Good Thing (TM) with no downside. However, reality constantly gets in the way of such statements and, although it has a lot of upside, solid state storage also has some clear downsides that storage vendors are working hard to mitigate. In this article, I will provide the first part in a series on solid state storage, beginning with the basics.

By the way, Wikibon’s Stu Miniman published published this great infographic on the evolution of flash memory a few months ago.

Just to be clear, I am discussing enterprise-class storage. Consumer-class solid state drives may not be as durable or as speedy as their enterprise brethren.

So, what is solid state storage?

When you think of data storage, you might think of hard drives, which have a whole lot of moving parts. Solid state storage, on the other hand, has none. Whereas data is stored magnetically on spinning hard drives, solid state storage uses electrical charges. Like RAM, solid state storage uses integrated circuits, but unlike RAM these circuits can hold their charge even when no power is being supplied to the device.

Why use solid state storage?

We’ll start with a simple list of the things that make solid state storage much better than traditional disk.

Solid state storage is fast and cheap in terms of IOPS

From a pure performance standpoint, solid state storage is a speed demon when compared to spinning disk. Boasting true random access and super-low latency, this kind of storage can read data at speeds that would require dozens or hundreds of spindle-based hard drives to match.

It eliminates the whole disk-head mechanism and with it the need for the storage device to wait for the disk head to pass over a section of disk that holds data. Data is simply pulled from the disk location on an SSD, providing orders-of-magnitude access speed improvements over spinning storage.

You might think I’m crazy for thinking that solid state storage is cheap in any way, but when you do the math, I think you’ll agree. In terms of IOPS-per-dollar, which is a valid metric used to gauge storage costs, solid state storage is generally less expensive than rotational storage. That makes it less expensive for smaller amounts of data that require massive IOPS, basically the same data that you want on Tier 0 storage. Of course it is more expensive in cost-per-Mbyte of storage, making disks still the better solution for the great mass of background data that is only accessed occasionally. Most companies, therefore, need the right mix of solid-state, disk, and probably tape storage.

Solid state storage is exceptionally robust

With no moving parts to worry about, solid state disks can withstand greater environmental stress than rotating disks. NASA and the military, that need highly durable technologies to survive extra-terrestrial and battlefield environments, were early adopters of solid state storage for exactly that reason.

Solid state storage is an energy sipper

With no moving parts, solid state storage requires far less power than disk. Imagine the power reduction that can be had when you start to replace entire shelves of disks with solid state storage devices.

So what’s the catch?

Before you run to the store to buy enough solid state storage to replace your entire data center, keep reading! You need to understand the downside before you jump into the solid state waters. (Would that be ice?)

Solid state storage is expensive in terms of dollars

You probably already know this, but when compared to rotating storage, solid state is pretty expensive when you consider buying enough to run an entire data center. Of course, cost-per-gigabyte is only one metric used to measure storage. Solid state storage is usually only cost justified when the cost of a millisecond of delay is enough to make dollars-per IOPS outweigh dollars-per-gigabyte. In these situations, for instance in currency trading, solid state storage looks far more attractive from a financial standpoint, and the capacity cost is coming down, as tends to happen over time with most technologies.

The life of a solid state disk used to be uncertain, but this is changing

Even though a rotating disk can die, spinning storage has proven to be relatively reliable when combined with appropriate data protection mechanisms. Solid state storage do eventually wear out due to writing and rewriting data, but the process takes a long time. Each time you write data to a portion of a solid state disk, the disk first needs to wipe and prepare that portion of the disk. This constant erase, write, and wipe process eventually takes its toll, causing sections of the chip to wear out.

Vendors use several techniques to battle this simple law of physics, and drives are getting better every day, extending the life of the storage medium, so this concern is becoming less critical as time goes on. One such technique is called wear leveling. This prevents the storage from using the same areas constantly, wearing them out prematurely. Data is spread out over a greater area, so wear and tear applies to more areas of the disk more equally, thus extending the usable life of the disk as a whole. In essence, wear leveling all but eliminates the problem of dead spots developing in the flash storage. Further, users can control this issue to a point by choosing the right kind of SSD for the job at hand. Don’t pick a consumer-grade disk to perform an enterprise job.

Write performance does not always equal read performance

Although solid state storage is well known as a speed demon, that reputation comes primarily from the technology’s ability to read data at lightning-fast speeds. The write side of the equation is a different story, particularly after a solid state disk has been in operation for a short period of time. In the previous bullet, I mentioned that the use of SSDs comes with in a constant erase and write cycle. These erase operations add latency to the write process, which can be significant. Performance with small random writes can be particularly poor with SSDs.

But this is all relative. Even a slow-writing SSD is faster than the fastest rotational hard drive.

Underlying NAND-based SSD technology

Whereas rotational storage uses magnetism to store data bits, flash storage is a non-volatile system based on a cell structure that is electrically erased and programmed.

NAND-based flash storage comes in two primary versions – single level cell (SLC) and multi level cell (MLC). Both have a place in the SSD-based world and serve very different needs.

Although the CIO probably doesn’t need to know the deep tech that makes up SSD, as you’re looking at price differences or capability differences and wondering why things are different, the technical underpinnings can provide a place from which to begin understanding.

Single Level Cell (SLC)

SLC SSD’s store just one value per single layered cell. The cell value can be in just one of two states, making SLC very fast and very robust. Why fast? It’s easy for the system to determine the basic “Yes” or “No” status of a cell in SLC.

When you need raw speed and are willing to pay for it, SLC is where its at. SLC is fast, uses less power than MLC and has high endurance, making it a great fit in harsh environments and where speed is king.

Multi Level Cell (MLC)

MLC SSD’s can store multiple states (generally four) per cell, increasing the potential density of the storage medium to two bits per cell. However, this capacity advantage comes with some drawbacks. MLC is not as fast as SLC, since the system has to do more work with each cell and because it takes longer to determine the value of a cell. Further, MLC has to spend more time preparing a cell for writing, so MLC is not as fast as SLC on the write side of the equation, either.

That said, MLC is still orders of magnitude faster than rotational storage, and it has a place in the storage market, particularly when capacity is favored over raw performance.

Today’s solid state market

Now that you know the ins and outs of the underlying technology, let’s take a look at some of the ways that flash-based storage is used today. With the sheer number of variations that are available and with the number of different ways that vendors have chosen to implement different flash-based solutions, organizations have many options at their disposal when it comes to choosing a product.

Last September, Wikibon’s David Floyer wrote an article entitled 2011 Flash Memory Summit Enterprise Roundup. In that article, David described five different approaches that vendors are taking with regard to flash storage.

When considering storage, CIOs need to consider two primary questions:

• How much storage capacity do we need?
• How much storage performance do we need?

Every solution will have a different set of answers to these questions. Further, while capacity and performance used to be difficult to achieve in singular solutions, the deep melding of rotational and flash storage systems is providing new options that can provide better fits for many applications.

The information below is current as of July, 2012. Note that this list does not contain entries for each and every vendor in the SSD market. I had to start somewhere and I plan to add new vendors to the list at least quarterly, if not more often. Here are some additional caveats:

• All information was gathered from information publicly available from the listed vendors.
• Performance figures may not be directly comparable at present. Different vendors use different metrics. In a future update, I plan to normalize these figures as much as possible.
• In blank spots, I was not able to identify appropriate information for that area, or there is nothing for that area. Again, over time, I will fill in the blanks as I talk with vendors.
• Please feel free to leave comments with corrections (or, make the corrections yourself!) if you find an error.

Fusion-io

ioMemory

Kaminario

K2

Nimble

CS

Nimbus Data Systems

S-Class

E-Class

Pure Storage

FlashArray

SolidFire

SolidFire

Tegile

Zebi

Expansion Chassis

Tintri

File:Tinitri-logo.png

VMstore

Virident

FlashMax MLC

FlashMax SLC

WhipTail

Accela

Invicta

XIO

HyperISE

Action Item: For those watching the flash market and considering purchase options, keep watching this article. It will be updated with new information on a regular basis.

Footnotes: