Hard Disk Drive (HDD), Solid State Drive (SSD) and Solid State Hybrid Drive (SSHD)
Dongyoung Jung
Abstraction
A couple of decades ago, most of computer games were installable with floppy disks. However,
nowadays, computers need to read DVD disks to install games. We borrowed video tapes to watch
movies, but DVD enables us to enjoy them with a high-resolution quality. As seen from the example of
floppy disks, video tapes, and DVD, the paradigm of data management have been changed significantly.
At the same time, the quantity of data has been increasing significantly. Also, how safely they are
manageable has been emphasized between electric device users. Users need more space to save data.
They need a faster process to retrieve data. Users need a secure storage to block others from stealing data.
As a starting point, a Hard Disk Drive played a role to give people convenience of a data management.
However, as users desire another kind of data storages with a bigger, faster, and safer capability, Hard
Disk Drive has been losing its position in an electric storage market. It is unable to satisfy people as much
as it did a couple of decades ago. Two new types of information storages started replacing Hard Disk
Drive. They are a Solid State Drive and a Solid State Hybrid Drive.
Introduction
A Hard Disk Drive (HDD) is the most common type of data storage device. It is used in
devices such as computers, video camcorders, cell phones and tablets, so on. While it is broadly
used and has a large storage space, the first HDD, manufactured by IBM in 1953, could save
only 5 MB of data. You could purchase it at $10,000 [1]. At present, an internal HDD with 2 TB
space costs about $100 [2]. However, IT device users are now facing a new paradigm of data
storage. It is a Solid State Drive (SSD). In 1976, a memory chip manufacturing company,
Dataram, made a 2 MB data storage, BULK CORE, which resembles a SSD. However, it was
$9,700 in 1977, which was as expensive as the first HDD [3]. Now, you can purchase a 180 GB
SSD for around $200 [4]. In terms of price, it is more reasonable to purchase a HDD rather than
a SSD because it is able to store about ten times as much data to a HDD as to a SSD for a
cheaper price. Nonetheless, in reality, the SSD market has poached users from the HDD market
[5]. Why is this happening? To begin with, a comparison in durability, accuracy, security and
price between a HDD and a SSD is presented in this paper as the first topic. However, many
users still demand large data storage for a cheap price. For this purpose, a memory chip
manufacturing company, Seagate, showed a hybrid model of a HDD and a SSD. It is a Solid
State Hybrid Drive (SSHD) 2011.
As the second topic, the SSHD is introduced with a
clarification on which part of it is better than a HDD and a SSD.
Durability
A HDD is composed of disk platters, an actuator,
arms, a spindle and a disk case as shown in Figure 1.
When the HDD writes or reads data, the disk platters
starts to spin. The actuator locates the arm above a
designated sector on a designated platter. Since there is a
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gap called an air bearing, which is about 5×10-4 mm
Figure 1 : Cross Section of Hard Disk Drive (HDD)
between the arm and the designated platter, the read/write arm can avoid colliding with other
adjacent sectors on the platter [6]. Since 5×10-4 mm is such a short distance, a small turbulence
could leave cracks on a platter when it is in contact with the head and destroy the sector on the
HDD. The head cannot read or write data on the destroyed sector. If a HDD has many damaged
sectors, it will eventually become unusable.
The spindle holds platters vertically while the actuator moves the arm. However, the arm
cannot reach to the entire platter because the actuator is stationary. This is why a motor inside the
spindle spins platters so that the arm can move to any sector of a platter. The speed of a HDD is
estimated by how fast the motor spins platters in RPM (revolution per minute). This capacity has
grown rapidly with a technical development. However, the speed enhancement slows gradually
because of the difficulty of locating the arm on a faster platter precisely. In addition, the spinning
speed could affect the distance of the air bearing, which could lead to an inaccurate transit of bits.
In addition, some hard drives are equipped with more platters for more data space, which could
negatively result in more weights on the spindle.
Then how can a SSD improve from a HDD? To
begin with, a SSD consists of several flash memory
chips, a controller, an interface, and a Dynamic Random
Access Memory (DRAM) chip as shown in Figure 2.
The flash memory chips are where data is saved. The
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Altered by Dongyoung Jung
controller ensures that data goes to the right place inside
Figure 2 : Solid State Drive (SDD)
the flash memory chips. The interface helps to convert the data into an acceptable form to make
no misinterpretations and wrong exchanges of data between the host and the controller. A
DRAM chip is another small unit of storage where data with a more frequent usage is retained.
Before searching in the flash memory chips, the controller checks the DRAM chip to find data
upon a host’s request. If the data is found in the DRAM chip, it is returned to the host. Otherwise,
the controller searches flash memories for the data. This feature shortens overall processing time
of a SSD since the distance between the controller and the DRAM chip is shorter than between
the controller and the flash memory chips.
Inside the flash memory chip, there are a lot of cells, each of which has one bit of storage.
A voltage inside the storage determines which bit, 0 or 1, goes into a cell. Generally, the voltage
is not prone to elastically change as a result of exterior damages. Also, the controller, the
interface and the DRAM chip run electrically. If the controller does not work correctly, the
interface, the DRAM chip and the flash memory chips cannot operate normally. In summary, as
mentioned above, an arm of a HDD is the most susceptible part. In contrast, a controller of a
SSD is better at protecting against damages than the HDD, which gives the SSD more durability.
Security
When data is deleted in a HDD, people usually assume that nobody could ever see the
data again. However, as soon as the data disappears from the trash bin, it is not really deleted.
Each deleted icon only has a starting address point. It does not represent the whole data. A
computer normally remembers the starting point so that when a user needs data, it reads from the
starting point. A data deletion normally means that the computer forgets the starting address
point. In other words, if the point is found again, anyone can access the data. This is a big issue
of security. Then what is a more correct way to delete data? It is overwriting repeatedly.
Overwriting means to record data into storage that already contains a piece of data.
Nonetheless, this is not a perfect solution. According to Dr. Peter Gutmann at the University of
Auckland, only one method of overwriting does not prevent data recovery. To illustrate, you
write a letter ‘P’ on a paper. Next, you write a letter ‘D’ right on the ‘P’ letter. It could be hard to
recognize both letters. The point is that there are still possibilities to recover the data. Because of
this issue, Dr. Gutmann suggested that we try overwriting 35 times [7]. Does a SSD follow a
same step repeatedly? Not necessarily.
Each cell in a flash memory chip plays a role in capturing the voltage inside it. This
enables the flash memory chip not to forget the bit even if the power is off. To change the bit
inside the cell, a different voltage is injected. This new voltage is the new data. At this point, the
previous data cannot be recovered. Since a HDD records physically, data must be deleted several
times. However, an electrical deletion does not need such a process. Therefore, a SSD is more
secure than a HDD.
Size
The biggest issue in the current IT market is who can make products in a smaller size.
Since a laptop is commonly used amongst a broad spectrum of people, most laptop firms
consider a laptop size as one of their greatest priorities. To expand a HDD’s size, more platters
should be added with more arms for each platter, which makes the HDD’s size bigger. An
expansion of a SSD does not. A SSD does not stack up flash memory chips like the HDD.
Memory chips are placed horizontally on the SSD. To overcome the weakness of the HDD,
instead of adding more platters, HDD companies try to make each platter have a higher density
of sectors. However, physically, if a HDD gets denser and denser, a boundary between sectors
could become overlapped by other boundary, which could damage data.
Speed & Accuracy
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Figure 3 HDD vs SSD
Since its first appearance in 1976, a SSD has not been widely used by users. The price
was unaffordable and the available data space was not as great as a HDD. Even now, it has not
overcome the shortcomings. Nevertheless, the number of people who want a SSD instead of a
HDD seems to have risen. Current business trends accelerate this current. Competitions have
hastened the need for a quick acquisition of information. As shown in Figure 3, a SDD is
dominant with a HDD in the time of boot up, restart, and Photoshop launch. The random read
latency to take data from flash memory chips to the host shows that HDD has more delays. In
other words, the faster system speed enables a fast decision in a business, which could give the
business more benefits.
Ultimately, a failure rate is more
critical in deciding which one is more
beneficial. As shown in Figure 4, a HDD
shows a higher failure rate than a SSD. If a
storage device fails more frequently, it will not
satisfy users even if the speed is getting faster.
What does this mean? The SSD has not only a
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Figure 4 : Failure Rate Comparison
faster speed but also a lower failure rate. In terms that business seeks to work in a fast and
accurate way, it seems to be a good idea to choose a SSD.
Solid State Hybrid Drive (SSHD)
What is a SSHD? It is a combination of a HDD and a
SSD. Developers of the SSHD decided to move an operating
system in flash memories since a computer normally assigns
many resources to its operating system. They put other less
significant applications in platters, which transit data at a slower
pace. Therefore, this results in an overall faster speed. However,
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Figure 5 : Solid State Hybrid Drive (SSHD)
it still has some weaknesses which are passed down from a
HDD. The SSHD still has arms and platters. The air bearing is still an issue, which could make it
sensitive to damages. The security could be an issue if important data is left on the platter. In
short, the SSHD takes both good and bad characteristics from a HDD and a SSD. It cannot be a
perfect solution.
Conclusion
Which device is the best for you? A HDD is cheap and gives you a large data space. In
contrast, a SSD shows a faster speed, and you can use it securely but it is expensive. As an
alternative, a SSHD could be a good choice. However, you should consider that it still has a
durability and security issue. As mentioned above, the operating speed is the most significant
feature for users, and also for businesses. In the manner, a SSD seems to be the best option. A
HDD has a long history. The long period of development enabled users to obtain it for a cheap
price. A SSD has just been getting the popularity in public. The next generation of a SSD would
come to the market with a cheaper price.
Then how does the data storage industry change? As discussed above, a SSHD is not the
perfect choice. How about a SSD? Can it be the best substitute for a HDD and a SSHD? A SSD
is advantageous in terms of security, accuracy, durability and size. The only issue is the storage
space. However, the SSD industry just started developing in a fast speed. This development will
enable one flash memory chip to have more space someday soon. With the assumption that the
SSD keeps its superiority over a HDD and a SSHD, the HDD and SSHD market will get in a
down phase. Once the price of a SSD becomes affordable, it seems hard for the HDD and SSHD
market to struggle to win this combat.
Reference
[1] PCWorld, Timeline: 50 years of Hard Drives [Online]. Available:
http://www.pcworld.com/article/127105/article.html
[2] Fry’s Electronics, Hard Disk Drives [Online]. Available:
http://www.frys.com/category/Outpost/PC+Components/Hard+Drives+-+Internal/
[3] PCWorld, Evolution of the solid state drive [Online]. Available:
http://www.pcworld.com/article/246617/evolution_of_the_solid_state_drive.html
[4] Fry’s Electronics, Solid State Drives [Online]. Available:
http://www.frys.com/category/Outpost/PC+Components/Solid+State+Drives+-+SSD/
[5] DailyTech, SSDs Become More Attractive as Cost per Gigabyte Continues to Decrease
[Online]. Available:
http://www.dailytech.com/SSDs+Become+More+Attractive+as+Cost+per+Gigabyte+Continues
+to+Decrease/article25000.htm
[6] Abdullah Al Manun, GuoXiao Guo and Chao Bi, “Components of a Hard Disk Drive” in
Hard Disk Drive: Mechatronics and Control. Boca Raton. Florida: CRC Press, 2006.
[7] University of Auckland, Secure Deletion of Data from Magnetic and Solid-State Memory
[Online]. Available: http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html