I’m working on my CET 751 paper/presentation on solid-state drives. Here are some sources:

  1. Samsung marketing video that made me go “Holy crap! I want SSD!” (stay tuned for the trampoline!).
    1. Samsung SSD three-story drop test (2008)
    2. Intel SSD laptop roof drop (2010)
    3. This guy showed his Macbook SSD opening 50 apps. He got over three million views.
  2. Wikipedia on solid-state drives (no, really: Wikipedia is pretty good, especially on tech stuff) and, for comparison, on hard drives.
  3. Lucas Mearian (2010), “Why Aren’t SSDs Getting Cheaper?” NetworkWorld.com.
  4. Logan G. Harbaugh (2010), “Storage Smackdown: Hard Drives vs. SSDs,” NetworkWorld.com.
  5. Mark Kyrnin (2007… I think!), “SSD — Solid-State Drives: A Hard Drive Alternative Based on Flash Memory,” About.com.
  6. Elexis Marie (no date given), “How Do Solid-State Hard Drives Work?” eHow.com
  7. Jeff Atwood (2009), “The State of Solid-State Drives,” Coding Horror.
  8. Linus Torvald (yes, the Linux inventor — 2008), “…so I got one of those new Intel SSDs,” Linus’ Blog.
  9. Leo Noteboom (2009), “Can a USB Thumb Drive ‘Wear Out’?Ask-Leo.com.
  10. no author cited, “OLPC XO Laptop: First Look,” ConsumerReports.org

The term solid-state has evolved just a little over the past few years. In older usage, every computer in this room is “solid-state”: our machines all run on semiconductors, not vacuum tubes. Currently, though, solid-state is used primarily to distinguish memory devices with no moving parts from conventional spinning disk drives.

The standard hard drive has two motors. One motor spins one or more magnetic platters, the disks on which data is stored. That data storage layer is a magnetic coating 10 to 20 nanometers thick — several thousand of these layers stacked atop one another would equal the thickness of one sheet of paper. Another motor moves an actuator arm that sweeps the read/write heads across the platters. These motors do some serious work: the platters spin at 4200 to 15000 rpm, while the tip of the actuator arm experiences accelerations as high as 550 g’s. These high-speed movements in tight tolerances leave a hard drive susceptible to crashes caused by sudden jolts, contaminants, temperature, and even changes in air pressure (operating a typical hard drive more than 10,000 feet above sea level increases the risk of a crash).

A solid-state drive looks much like a hard drive, since it is designed to swap in for traditional hard drives (Kyrnin, 2007). They come in the same chassis as 2.5-inch or 3.5-inch hard drives, ready to plug in to the same ATA or SATA interface.

The big difference is that only movement in a solid-state drive is the movement of electrons. There are no spinning platters or swinging arms. Instead of storing data in alternating magnetic fields, the solid-state drive moves electrons in and out of tiny semiconductor transistors. A transistor that is full of electrons and cannot accept electrical flow reads as a binary 0; a transistor that accepts flow is a binary 1.

The absence of moving parts presents several advantages:

  1. Reliability: Fewer moving parts mean fewer things that can go wrong. Hard drives require gentle treatment, lest a sudden jolt send heads grinding into platters. Mobile devices may require accelerometers to sense sudden movement and stop hard drive operations before a shock causes a crash. While users should not get too reckless with solid-state drives, sudden movement does not affect the reading of transistors. Solid-state drives can withstand various effects that would cripple hard drives, which makes them superior memory solutions for mobile devices.
  2. Performance: Hard drives must spin the platters to the proper position for the heads to read or write the data. That means hard disk reads and writes include some spin-up and seek time during which data isn’t moving. Solid-state drives experience no such delays. Recognizing these time savings, programmer Jeff Atwood characterizes solid-state drives as “the most cost effective performance increase you can buy.” Linus Torvald offered the following positive assessment of Intel’s SSD in October 2008:

    The whole thing just rocks. Everything performs well. You can put that disk in a machine, and suddenly you almost don’t even need to care whether things were in your page cache or not. Firefox starts up pretty much as snappily in the cold-cache case as it does hot-cache. You can do package installation and big untars, and you don’t even notice it, because your desktop doesn’t get laggy or anything.

    By the numbers, a solid-state drive can perform 16,000 Input/Output operations per second (IOPS). A top of the line 15,000-rpm enterprise-grade Fibre Channel hard drive will perform 200 IOPS (Mearian, 2010). The solid-state drive is 80 times faster. Think of it this way: suppose it takes me 15 minutes to deliver this presentation. Replace me with a solid-state drive, and you could get the message in 11 seconds. Currently, consumer-grade SSDs offer a much smaller advantage, “only” twice as fast.

  3. Power usage: Motors eat electricity. Practically, energy usage may not hinder a desktop computer’s performance, but mobile device us”ers demand every power savings they can get to extend battery life. Unlike toning down screen brightness, a solid-state drive saves power while improving performance, a nice bonus for mobile device users.
  4. Noise: what noise? With no moving parts, no mechanical energy is lost to movement turning into sound waves. Of course, if you like that little grindy-grindy sound when you put your computer through the paces, then solid-state drive silence will bug you the way the Prius bugs some people as it starts noiselessly.

No device is perfect. One weakness of the solid-state drive is the vulnerability of transistors to wearing out. They can take only so many fillings and emptying of electrons — in this case, only so many read and writes. You can use your thumb drive as a sort of solid-state hard drive, but you would wear it out the same way. Estimates for flash memory lifetime range from 10,ooo to 100,000 read/writes. For copying and backing up files, that lifetime isn’t bad. But if you start using your flash drive for various applications, especially database applications, reads and writes can add up quickly (Noteboom, 2009). Solid-state drives can have compensating mechanisms, but one bad bit can be enough to render the entire drive unreadable (Noteboom, 2009).

To extend the life of a solid-state drive, engineers use wear leveling, a technique that distributes read/write activity roughly equally around the disk. If a frequently accessed file were kept in a static location, the transistors in that “sector” of the solid-state drive would wear out sooner than other transistors that were lucky enough to be assigned some archied file. Wear-leveling moves files around and even breaks them up from read to read and rwite to write, spreading out the abuse to all portions of the drive roughly equally.

Wear leveling creates some security issues. If a user loads, encrypts, and saves a file, the solid-state drive does not overwrite the original file. The drive instead creates the desired encrypted copy in a new location, leaving the original unencrypted and possibly accessible. Users also can not overwite files directly, as the solid-state drive will distribute new files to locations that require wear-leveling rather overwriting already existing files in more often-used areas. Solid-state drives can get around these problems, but the solutions require some processing overhead.

Solid-state drives also cost more than hard drives. One analyst predicts that for manufacturers, solid-state drive costs will drop from $1.90 per gigabyte today to $1.70/GB next year, while online consumers will likely see prices stay steady at $3.00 to $3.30 per gigabyte (Mearian, 2010). Prices did drop 60% in both 2007 and 2008, but manufacturers were making no money. When the recession hit, manufacturers could not afford to take that hit any more. They slowed production, tightening supply even as demand increased, which flattened out the price curve and drove some price increases (Mearian, 2010).

Mearian (2010) finds 1-terabyte hard drives available for $90, compared to Intel SSD’s for $400. Mearian (2010) cites one expert from Gartner who says solid-state drives will never match hard drives on price per gigabyte. Mearian (2010) suggests that if that cost difference persists, a viable option may be hybrid systems with a lower capacity SSD boot drive combined with a big external drive for multimedia storage.

Cost likely means solid-state drives will not fully replace hard drives. However, solid-state drives can  meet important needs in a number of realms. For some mobile users, the increased reliability and lower power consumption may be worth the cost overhead. Military and aerospace users are big purchasers of solid-state systems to provide reliable computing power in high-performance situations. Even the One Laptop Per Child project incorporated a solid-state drive in its XO laptop at the end of 2007. The XO had a 1GB SSD with 800 MB available. OLPC was trying to provide a cheap yet rugged laptop for use in isolated and low-income locations around the world. In OLPC’s case, the advantage of a machine that would withstand rough conditions with little to no opportunity for tech support or replacement outweighed their very strong determination to hold down up-front costs.

The same cost-performance tension OLPC faced also applies to our own educational environment. Distributing mobile computers to hundreds of K-12 students is no task for the timid. Computers in the hands of kids will get shaken and stirred. Solid-state drives offer one more avenue to provide students with technology that will withstand their abuse, but we still have to decide whether we can afford to spend the extra dollars for that advantage.


Bonus link: Kate Bush, “Rubber Band Girl“: not related, but fun to study by.