Hard Drive FAQ

Q: Why are there so many drive models to choose from now?

A: As the usage of the hard drive has been expanded over the years, so has the diversity of how they are used. Hard drive makers have invested heavily in analyzing and understanding the stresses involved with unique usage cases. Where the usage was unique, and the market was significant, the drives have been tailored, and the result is the wide range of product offerings you see today.


Q: Do I have to use a higher spin speed drive, to get higher performance?

A: Not necessarily. It depends upon some other factors, including the type of data you are storing, and some specifics of your application. Drives today are using an “advanced format”, which is making drives much more efficient. This may mean that you can satisfy your data throughput needs with a lower spin rate than was previously possible.


Q: Why have the Solid State drives not taken over the market from magnetic drives?

A: There are multiple reasons, but cost and “write cycle wear-out” issues top the list. SSD’s are very fast, and offer a great choice for system/OS drives in performance intensive applications, but don’t expect them to compete with hard drives in cost/TB anytime soon.


Q: What “wears out” in a hard drive, and how long does it take?

A: Many wear-out mechanisms that existed in hard drives early on have been successfully addressed, including the flexible printed circuits and ball-bearings motors. Today, the only known wear-out is the write element on the recording head, which can be designed with varying levels of robustness. To avoid this one from becoming an issue, pay attention to the manufacturer’s specifications on workload, as “overworking” the lower cost drives is a common issue.


Q: Hard drives are often specified to have One Million or more hours of reliability, but then last only about 5 to 7 years, which is only 61,000 hours. Are the drive companies honest about the specification?

A: The drive makers are typically very accurate with these specifications. It is a common misconception that the “Reliability” of the product is directly related to the “Useful Life” of the product. In fact, these two are completely unrelated. To learn about this, check out Making Sense of Reliability Specifications.


Q: Why are hard drives sensitive to altitude and temperature changes?

A: The read/write heads fly over the media on a very thin cushion of air. The density of this air will change with altitude and temperature changes. Since this affects the head fly height, the drive’s magnetic recording system will need to compensate for this change, which it is enabled to do, but only within a predetermined range. Operating outside this range will cause the magnetics to become ineffective, and operating at the extremes of the specified range will significantly increase the likelihood of errors.


Q: What difference is there between “video” drives, and “surveillance” drives?

A: The original Consumer Electronic AV (now also called just “video”) drives were designed for home use in a set-top box. Early products showed significant failure rates as the drives were not designed for 24/7 operations, or tuned properly for this use case. Years of study and refinement have led to today’s broad mix of hard drive offerings, including an industrial surveillance video drive. While these two use cases are similar, they have been proven to be different enough for drives to benefit from features unique to each model. By using the drive type recommended by the drive makers, you are staying in step with the “state of the art”.


Q: What should I know about hard drives, when writing my custom software application?

A: Know that when it comes to tasking drives, the details matter. It is highly recommended to utilize a bus trace analyzer to record a significant string of actual commands, and then have the resulting data reviewed for stressful access patterns, and any unnecessary repetitive tasks. You should also know that the ATA 7 Streaming Command Set was developed specifically to reduce drive stresses, and has been proven to enhance drive reliability in streaming AV applications. Your drive manufacturer of choice may be able to assist with this analysis.


Q: What do “RV sensors” do, and why would I want them?

A: In high density chassis’, drives are exposed to multiple sources of chassis-borne vibrations, including each other’s activities, system cooling fans, etc. As the workload goes up, so does the frequency of the vibrations. The RV (Rotational Vibration) sensors are a supplemental feature that allows the drive to individually measure, analyze, and partially neutralize these external forces. You can witness their effectiveness in performance testing, but what they really do for you is to lower the number of retries, which in turn lowers the probability of an error.


Q: Why do I need a more expensive drive just because I’m using RAID?

A: This is a complex issue, but fundamentally, when you utilize RAID, you increase the workload for each of the drives, which adds up. In any RAID system that includes parity, the seek and write workload are both increased for every drive, and none of the drives get idle time, which essentially means you are voluntarily wearing the drives faster in trade for the insurance against data loss in case of individual drive failure. It is highly advisable to utilize drives that are rated for RAID usage, and since these drives are more robust, they will be more expensive.


Q: Is there an optimal operating temperature for hard drives?

A: Yes. Since drives are designed to be used in the indoors environments, where people generally are, they are designed around typical “indoor ambient” temperatures. This translates to the following assumptions. Room temperature = 25oC. System temperature rise characteristics of 10-15oC. So, the “ideal” operating temperature for most hard drives is 35-40oC, measured at the drive base casting.


Q: I have been told it is best to keep drives cool. How hot is too hot?

A: The reality is that heat is bad for the reliability of ALL components and technologies. It is commonly accepted that adding heat to any test population is the fastest and lowest cost method of failure acceleration. The goal should be to keep all components well within their temperature specifications. With hard drives, this translates to 35 to 40oC when measured at the base casting, and if the population were allowed to run 20oC higher than this, you could expect failure rates to increase by up to 100%.


Q: Why not just use PC drives in my system?

A: The “Standard” PC hard drives are not up to the task of 24/7 operations, and they are not designed, or tuned for other applications. It is common for failure rates to be 3 to 5 times higher on PC drives, when used in high workload applications, such as NAS devices, workstations, DVR/NVR and servers. Performance related functionality issues, such as dropped frames in an A/V application, are also common.