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Solid State Hard Drives vs. Traditional Hard Drives

Traditional Hard Drives vs. Solid State Drives

Figure 1: Traditional Hard Drives vs. Solid State Drives

Traditional Hard Drive

Traditional Hard Drive

Magnetic Recording

Figure 4: Quantum Tunneling in Flash Memory

There is alot of buzz over solid state hard drives. I recently purchased the Intel X25-M 120 gig solid state drive on black Friday at a killer price. This blog entry describes  how a traditional hard disk drives (HDDs) and solid state disks (SSDs) work and how they are different.

A traditional hard drive is a non-volatile (retains data when powered off) storage device featured in most computers today. It is a random access medium, which means that data can be read or written to non-sequentially. The opposite is a sequential access medium like cassette tapes. A read/write head, attached to an actuator arm, stores and retrieves data on magnetized platters (Figure 2). The head magnetizes sectors on the platter’s recording layer to store data. The orientation of the magnetic regions represents a 0 or 1 (Figure 3). The actuator arm moves the head radially to position the head anywhere on the platter.

A file is stored across many sectors. The total time it takes to read/write to a sector is called access time. Here the access time has 3 components: seek time, rotational latency, and transfer time.

Seek Time: The time it takes for the head to move to the location of the target sector.

Rotational Latency: The time between when the head is in location and when the first bit is read. Once the head is in position, the platter starts to spin. If the first bit is right under the head, rotational latency is 0. If the head just missed the first bit, the platter has to make one full rotation until the first bit is read.

Transfer Time: The time between when the first bit is read until the end of the sector.

The total time is defined: Taccess = Tseek + Trotation + Ttransfer

The time it takes the head to move to the target sector and the platter to position the first bit under the head (Tseek + Trotation) is what makes HDDs really slow. Think about having many small sectors scattered throughout your hard drive and how much time would take to get to each of them. The relatively slow performance of HDDs is a consequence of their mechanical nature.

Solid state hard drives, also non-volatile and random access, will be the staple storage device on most computers really soon. This technology is entirely different. A traditional hard drive is an electromechanical device that stores data on magnetic platters. SSDs, on the other hand, store data on microchips called NAND flash memory. No moving parts no fat chicks.

NAND flash memory is composed of floating gate transistors (Figure 4). A floating gate stores electrons that represents binary data (0 and 1). The voltage reading from these gates determines the data stored in it.

Technology behind writing data to NAND flash memory rely on a quantum physical phenomena called quantum tunneling. If you run straight towards a wall, there is a very very small chance that you will pass right trough. It’s kind of like magic… basically all quantum physics is magic. The less massive an object the higher the probability of it to tunnel. Thus an electron has a much higher probability of tunneling than say a human.

To write data onto flash memory, electrons are tunneled through an electric insulator and trapped in a floating gate (Figure 4). These electrons remain trapped there even after the hard drive is powered off (non-volatile). A gate can retain its electrons for a few years! Unfortunately, the number of times a cell can be written to is limited to a couple hundred thousand times. After that, the cell cannot be used anymore. However, new algorithms minimize wear on cells by spreading write operations across other cells instead of using the same ones.

As you can see, reading and writing data to SSDs is entirely electrical. There’s a lot more detail to how these drives work, but it is beyond the scope of this blog entry. Because there are no moving parts, today’s SSDs take about 0.1 ms (milliseconds) to start transferring a file as opposed to 5 – 10 ms for HDDs. These time differences are unnoticeable to humans but they add up when transferring many files scattered throughout the hard drive. SSDs also have faster transfer rates.

SSDs make no noise, are lighter, and use less energy. No mechanical parts means no breakdown of mechanical parts. Thus SSDs are much less likely to fail and can better withstand unfavorable conditions such as shock, altitude, vibration, and extreme temperatures. Because they have no magnetic platters, they are unaffected by magnetic radiation. These properties make them ideal for military applications.

Even though SSDs will replace traditional hard drives very soon, they do have their drawbacks. The limited amount of times a cell can be written to degrades the speed and capacity of the drive over time. A fresh SSD will run better than one that is a few years old. HDDs do not degrade and can last for many years. SSDs are way more expensive. As of December 2010, SSDs cost about 2 – 4 dollars per gig while HDDs cost about 5 – 10 cents per gig. Because of this, computers will likely come with an SSD and HDD. The SSD will store speed sensitive files such as the operating system and most commonly used programs. HDDs will store everything else such as your documents, videos, music, backups, etc and less commonly used programs.



Disclosure: I have no positions in any stocks mentioned, and no plans to initiate any positions within the next 72 hours.