Chances are high that if you or your organization use Domain Name System services, you’ve dealt with OpenDNS. With over 30 million users globally and 32 billion daily DNS requests, many of OpenDNS’s clients opt to have the company store their DNS traffic data for better security and subsequent analysis. This obviously isn’t transactional processing; access to the data needs to be quick but not necessarily instantaneous. The most important thing is that OpenDNS’s storage needs to be ultra-reliable. Mixed in with performance and affordability, the need for bulletproof consistency is paramount.
OpenDNS chose Silicon Mechanics to supply its latest storage platform, and Silicon Mechanics, in turn, selected Seagate’s 3TB Constellation ES.2 drives as being the optimal fit for the environment. Why? In answering that question, we’re going to take a look at the core concepts of what distinguishes an enterprise hard drive from its traditional desktop counterpart. It may be that desktop drives are the best choice for many businesses. At the same time, there are an ever growing number of applications that can realize greater return on investment and better satisfy IT service level expectations by adopting true enterprise-class storage.
Let’s look at the five main reasons for selecting an enterprise-class hard drive.
Reason #1: Rotational Vibration
Quite simply, if it rotates, it rattles. Feel any laundry machine’s spin cycle for proof. Unlike a laundry drum, though, the platters on a conventional, high-capacity hard drive spin at 7,200 RPM. Some slight imbalance is unavoidable, and when examining a single drive in isolation, this vibration is well within the device’s tolerance range, so no performance impairment occurs.
The question is how much rattling can a hard drive withstand before it starts to sustain errors in the data read/write process. The answer may be less than you think. Sun Microsystems’s Brendan Greg famously demonstrated this in a YouTube video (www.youtube.com/watch?v=tDacjrSCeq4) in which he caused substantial disk latency within a data center JBOD array by literally yelling at the drive. The vibrations from his loud voice were sufficient to make the read/write heads move out of alignment with the underlying media tracks, resulting in the drive controller needing to realign the heads and attempt another read/write process, much like trying to reposition the needle into the correct groove on a skipped LP record.
Admittedly, most IT staff don’t make a daily habit out of shouting at their drives. (There are exceptions.) However, they do run HDDs in metal enclosures, stuffing drives in close proximity to one another. The vibration of one drive alone may be insignificant, but two or more drives operating in close proximity and rigidly connected by metal bars may start to develop vibrational harmonics, especially when those drives are under load. Barbara Craig, senior product marketing manager for Seagate enterprise storage products, notes, “Any time you’ve got four drives or more in an enclosure, we feel you should be concerned with rotational vibration.” In addition, vibrational forces outside the cabinet (power supplies, air conditioners, etc.) may only exacerbate the problem.
Quality enterprise drives integrate multiple sensors in order to detect this vibration and respond accordingly. Typically, the drive will sense a vibration causing motion in one direction, then a controller will instruct the drive’s actuators to move the heads in a way that allow them to keep centered in their track and move with the vibration. Seagate was a pioneer in developing rotational vibration (RV) tolerance technologies and has been employing them in its enterprise drives for over a decade.