IBM gets the credit for inventing the concept of the hard disc drive (HDD) more than 50 years ago. But HDD technology has come a long way from washing machine-sized monstrosities with platters up to 14 inches in diameter spinning at a mere 1,200 revolutions per minute (RPM).
Since then, the industry has experienced dramatic innovation in shrinking the hard drive’s physical footprint while dramatically improving storage density and performance. But even as hard drive technology has matured, the way of measuring the performance of new HDD models has remained relatively consistent and closely related to two specifications:
- The density of bits storage on the circular platters—called areal density.
- The speed at which the platters rotate—called RPM.
The performance of a hard drive is most effectively measured by how fast data can be transferred from the spinning media (platters), through the read/write head and on to a host computer. This is commonly referred to as data throughput and usually measured in gigabytes (or gigabits) per second. In either case, data throughput is directly related to how densely data is packed on the hard drive platters and how fast these platters spin.
Comparing measurement methods
For the areal density specification, we can measure data density on a hard drive in two ways: bits per inch (BPI) and tracks per inch (TPI). As tracks are placed closer together, TPI increases. Similarly, as data bits are placed closer and closer to each other along a track, BPI increases. Together, these represent areal density.
As a rule, when areal density increases on a hard drive, so does data throughput performance. This is because the data bits pass by the read/write head of the hard drive faster, leading to faster data rates.
For the RPM specification, platters need to spin faster to increase performance in a hard drive. This results in moving the data bits past the read/write head faster, resulting in higher data rates. Hard drives have been engineered with spin rates as low as 1,200 RPM and as high as 15K RPM. But today’s most common RPM rates, in both laptop and desktop PCs, are between 5,400 and 7,200 RPM.
Given two identically designed hard drives with the same areal densities, a 7,200 RPM drive will deliver data about 33% faster than the 5,400 RPM drive. Consequently, this specification is important when evaluating the expected performance a hard drive or when comparing different HDD models.
Solid state hybrid drives make RPM largely irrelevant
It’s no surprise that when many people begin evaluating the expected performance of the new solid state hybrid drive (SSHD) technology, they look at the RPM specification, since an SSHD is basically an HDD with a bit of solid state technology integrated into the device. So RPM should still matter, right? The truth is, the RPM of an SSHD device is largely irrelevant. Here’s why:
SSHD design is based on identifying frequently used data and placing it in the SSD or NAND flash portion of the drive. NAND flash media is very fast, partly because there are no moving parts, since it’s made of solid state circuitry. Therefore, when data is requested by host computers, there is generally no dependence on pulling this data direct from the spinning media in the HDD portion.
Sometimes, however, data will be requested that is not in the NAND flash, and only during these instances does the HDD portion of the device become a bottleneck. Since the technology is so effective at identifying and storing frequently used data in the NAND area, SSHD technology is much more efficient in delivering data to a host computer quickly.
This result may be clearly observed by comparing the PC Mark Vantage storage scores of second- and third-generation Seagate SSHD technology and traditional 5,400 and 7,200 RPM HDDs.
Although third–generation SSHD technology is based on a 5,400 RPM HDD platform, the technology actually delivers faster performance than the previous generation product based on a 7,200 RPM HDD platform. Improvements in core SSHD technology and NAND flash systems explain such progress, and also exemplify why RPM is no longer as meaningful when evaluating SSHD technology.
When maximising the performance of your laptop computer, you don’t have to be bound by older storage technologies or performance criteria. Instead, let solid state hybrid drives take you to a higher level of digital lifestyle.