The amount of information that
data centers are storing—and that users are accessing or storing themselves—is
growing constantly, particularly with the demand for ever more multimedia
content. Companies, therefore, must struggle with the problem of how to best
store this mass of information. Hard-disk drives (HDDs) are a well-established
technology, but increasing use of solid-state drives (SSDs) raises the question of which is best for the data center. The
simple answer to this question is both, but the more critical question
is when each technology is best.
Basics of HDDs and SSDs
The fundamental difference
between HDDs and SSDs is their respective methods of storing data. HDDs store
data magnetically on spinning platters use a mechanical read/write head; SSDs,
on the other hand, store data electronically in semiconductor circuits. These
latter drives employ nonvolatile NAND flash memory as their storage medium.
(Unlike other silicon storage media, nonvolatile storage maintains stored data
even when powered down.) The difference between these technologies leads to a
number of fundamental advantages for SSDs—hence the growing interest in using
them for storage. But SSDs don’t hold the edge in every aspect, so before you
rush to replace all your HDDs, consider the entire story on these two
technologies.
Advantages of SSDs
Because SSDs store data
electronically and do not require the mechanical interface of an HDD, they yield
a number of important benefits. First, they are faster. According to
SorageReview.com (“SSD vs
HDD”), SSDs can read and write data at rates of
around 200–500MB/s (typically, writing is slower than reading), compared with
only 50–120MB/s for their HDD brethren. Furthermore, SSDs do not suffer from
fragmentation problems, which can plague HDDs. In high-performance applications
or where fast booting is needed, these advantages are tremendous. Mobile
devices, for example, can take advantage of SSDs to meet customer demand for
faster on/off capability in their gadgets. Another benefit is that SSDs use less
power owing to their exclusive reliance on silicon rather than mechanical
operation. StorageReview.com estimates at least a 50% cut in power consumption—a
boon to mobile devices, which seek to maximize battery life.
In addition, SSDs do not
produce noise or vibration like HDDs, and they may offer better resilience if,
say, you were to drop your laptop. For an HDD, such an event can be disastrous,
whereas an SSD may suffer less damage. (Of course, this line of argument has its
limits: what if you drop your laptop into a pool—which drive is better?) Because
SSDs are offered in the same form factors as HDDs, they can be substituted
easily, enabling simple upgrades.
So, why not rush out and
replace all your HDDs with SSDs? Here’s why.
HDDs Rule in Price and Capacity
Naturally, SSDs have their
downsides: most importantly, price and capacity. SSDs are much more expensive
than their mechanical/magnetic counterparts. Although it’s difficult to pin down
a single number to express the relative price—the difference depends on a number
of factors, such as capacity and performance—Forbes.com (“No, Solid-State Drives Are Not Going To Kill Off Hard
Drives”) notes that “the HDD industry in 2011
shipped about 350 exabytes (350 billion gigabytes) of capacity for a total
annual revenue of about $33 billion. The cost of the equivalent amount of flash
memory would be over $250 billion and would require a manufacturing capacity
over 10x larger than exists today.” That would mean SSDs are roughly 7.5x more
expensive per bit than HDDs. Again, however, this doesn’t take into account the
differences that can arise for extremely high-performance drives or SSDs with
relatively high capacity. PCMag.com (“SSD vs HDD: What’s the Difference?”),
for example, notes that “For the same capacity and form factor 1TB internal
2.5-inch drive, you’ll be paying about $100 for a HDD, but as of this writing,
you’ll be paying a whopping $900 for an SSD”—a 9x price difference.
The other major factor that
hurts SSDs is capacity. Whereas HDDs below about 500GB are becoming rare (the
terabyte range is now commonplace), SSDs seldom exceed about 500GB, although
some reach 1TB or so—and that for a hefty premium. Thus, although SSDs offer a
number of performance benefits over their mechanical competitors, they cannot
beat HDDs for inexpensive mass storage capability. HDDs are therefore more than
a few steps from the technology graveyard. “Just as HDDs have not replaced tape
for certain key archiving enterprise applications, as was speculated for the
last 20 years, so too will HDDs retain their function for the foreseeable future
as the primary form of storage from a cost and performance standpoint for large
capacity storage,” says Forbes.com.
Furthermore, the cost per bit
for SSDs versus HDDs may not fall sufficiently fast to guarantee a wholesale
replacement of HDDs in the broader market. The price of HDD storage per bit is
also falling, and the speed of the price drop for SSDs may not be sufficient to
catch up, barring some unforeseen development in the market or
technology.
Who Wins in the Data Center?
So, how should data centers
resolve this contest? Should you pay an arm and a leg to get the benefits of
SSDs, or settle for the lower-performance (but much cheaper) HDD? Obviously,
such a question is a false dichotomy. Hybrid storage systems that employ SSDs
where the fastest read/write rates or boot times are required, but HDDs where
standard storage space is sufficient, are the best solutions. In this way,
companies can balance the cost and performance factors to achieve a storage
implementation that provides high performance when needed but standard
(inexpensive) capacity otherwise. According to Forbes.com, the adoption of “SSDs
[in] the enterprise data center has not been nearly as large and as fast as many
had predicted. In enterprise systems, SSDs and HDDs will likely coexist with a
relative balance depending on the application requirements for performance
versus cost. Generally, though, the relative number of bytes stored on SSDs will
be less than 10% of the total.”
Mobile devices are one
segment, however, where the benefits of SSDs outweigh the cost factor: their
lower power consumption, faster read/write performance and durability are an
excellent match to help meet user demands in this segment.
Conclusions
Although SSDs offer several
fundamental advantages over HDDs, the older technology still has a strong edge
where it counts for many applications: cost and capacity. Assuming SSDs are
unable to overtake HDDs in cost per bit of storage, the hybrid approach is the
most appealing implementation in data centers. Rather than relying on one
technology or the other exclusively, many companies are using (or will use)
both, employing SSDs when faster data read/write speeds are necessary, but
relying on HDDs when less performance will do, since these drives offer much
cheaper and more capacious storage. In the data center, look for a
balance—rather than dominance by one or the other—of these
technologies.
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