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Tip #1070: What Determines Storage Performance?

Larry Jordan – LarryJordan.com

Aerial density, RPM and whether an SSD is in the mix all affect storage performance.

The Seagate logo.

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Seagate published an interesting article, titled: Choosing High Performance Storage Isn’t Just About RPM. This is a summary.

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 passed to a host computer. This is commonly referred to as data throughput and typically 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.

Higher revolutions per minute represent a faster hard drive, but the rate of media transfer is just as important for data storage solutions.

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, which leads 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, which results in higher data rates. Hard drives have been engineered with spin rates as low as 1200 RPM and as high as 15K RPM. But today’s most common RPM rates, in both laptop and desktop PCs, are between 5400 and 7200 RPM.

Given two identically designed hard drives with the same areal densities, a 7200 RPM drive will deliver data about 33% faster than the 5400 RPM drive. Consequently, this specification is important when evaluating the expected performance of a hard drive or when comparing different HDD models.

However, when moving to a solid state hybrid drive (SSHD), RPM is largely irrelevant. Why?

SSHD design is based on identifying frequently used data and placing it in the solid state drive (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 typically not a dependence on pulling this data directly from the spinning media in the hard drive portion.

Sometimes, however, data will be requested that is not in the NAND flash, and only during these instances does the hard drive 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.

In tests conducted by Seagate to illustrate this article, the fastest performance for an SSHD drive came from one where the platters only spun at 5400 RPM.

Here’s a link to the full article.


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