How IT Works: Flash Storage


May 13, 2016

Flash storage is just now becoming a competitive alternative to hard disk storage in business deployments, but variations of the technology have been in use for thirty years. Memory cards, USB drives, smartphones, digital cameras, video game consoles, and more all utilize flash storage to some degree.

It’s all around us, but do you know how it works?


Storage Basics

To understand data storage, you must understand binary code, the fundamental language of computing. At this core level, everything in a computer can be boiled down to a series of 1s and 0s. Data storage is just the process by which those 1s and 0s can be recorded and read by a computer.

Over the years, it has been accomplished in a variety of ways. Technically, you can qualify devices reaching all the way back to Thomas Edison’s Phonograph as rudimentary storage devices. More modern incarnations include magnetic tape, optical disks (CD-ROM, DVD, Blu-Ray), and hard disks.

It sounds simple, but putting this idea into practice comes with a number of challenges, especially as the scale increases.


How a Flash Drive Stores Data

The short answer is that flash drives are able to maintain an electrical charge in their transistors after a power supply is removed. It traps a tiny portion of electricity in some of its transistors and neglects to do so in others, and so a disparity is created.

Any computer reading the device can then correspond the transistors with a negative charge as a “1” and any transistor without a negative charge as a “0.” Through this process, a computer can rapidly scan the drive and read the series of transistors with and without charges and react to that information.


The Process

The process of actually trapping that electrical charge requires a slight modification to a MOSFET transistor. A transistor normally has two roles in electronics. It can act as an amplifier or a switch. As an amplifier it can take a small electrical current and amplify it into a larger one. As a switch, a small electrical current through one part of the transistor can activate a larger current in another part.

The problem with transistors is that when an electrical current isn’t running through them, they revert to their original state which has no charge. To prevent that, a second gate, the floating gate, is added.

Now, when an electrical current is flowing between the source and drain points, a few electrons can be drawn upwards and stuck to the inside of the floating gate. These two gates are separated by oxide layers which prevent any current from passing.

In this state, the transistor will hold the charge almost indefinitely. Over time, the charge can weaken, but for our purposes as a storage device, it retains the information it needs to.

To clear the floating gate of electrons, a negative charge needs to be applied to the Wordline. Because identical charges repel each other, this will push the electrons out of the floating gate.


Data Density and Degradation in Flash Storage

As you might image, these flash drives must be able to carefully organize their transistors, or in this context, their cells. A flash drive arranges each individual cell into pages of about 4KB or 8 KB, and then the pages are grouped into blocks of 128KB to 256KB. By this process, a 1GB flash drive is composed of approximately 3906 blocks of 32 pages for a total of 124,992 pages in the whole drive.

The properties of solid state storage dictate that individual cells can be changed from a 1 to a 0, but not the other way around. For that, the drive must rewrite the entire block. While this doesn’t seriously impact the speed of a flash drive, it does impact its overall lifespan.

Every time a charge is added to a cell, it degrades it by the slightest bit, and each time a block is rewritten it adds unnecessary wear on cells that do not need to be altered. Developers have found ways over the years to slow or counteract this consequence.

One such technique that can extend the life of a flash drive is known as wear leveling:

  • Wear Leveling: When employing wear leveling, the drive will not rewrite the same block if a cell needs to be changed. Instead, it will write on a completely new block that is currently unused. This causes the drive to wear down evenly across every block at approximately the same speed and thereby extending the overall lifespan of the drive.


Flash Storage for the Enterprise

All flash storage for a data center has long been an appealing prospect, but the cost has always outweighed the benefit. Flash storage has always performed at far faster spends than hard disk drives (HDD), but it was too expensive to use in large scale deployments.

That is changing this year. For the first time, all flash storage arrays are at a competitive price point with HDD storage. Organizations large and small can now accelerate their business with a 1000% increase in IOPS.

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For Further Reading:

NVMe Vs SATA SSDs: A Saga of Solid State Storage

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