Introducing Surveillance-Specific Hard Drives
When you look at the video surveillance market, you find a wide array of products and solutions. At the high end, there are companies serving large business applications that can include dozens, or even hundreds of cameras. Naturally, they need hard drives by the truckload for implementations selling for six figures. At the other end of the spectrum are all-in-one home surveillance kits that you can buy online for a few hundred bucks. What both systems share in common is a need for dependable storage.
Surveillance, like many other storage applications, is all about the scale and usage. What I mean by that is most home security systems follow a write continuously/read rarely model. One compressed, 1080p stream has a ridiculously low data rate (<2 MB/s) compared to the performance of a typical hard drive (50-150 MB/s). Even when you increase the number of cameras, you still have ample headroom to record data. In more enterprise-oriented systems, the requirement is for simultaneous record and playback. And when you think about how a mechanical, rotating drive works, performing both tasks at the same time can be incredibly difficult, depending on the physical location of data on the disk. These sort of challenges are why we are seeing the major hard drive manufacturers introduce products aimed specifically at the surveillance market.
So what can a disk vendor do to streamline the types of transfers typical of surveillance? The main lever available to pull is a full implementation of the ATA Streaming Command Set. When a drive is formatted, multiple zones are created that include a varying number of sectors per track, based on location. The Streaming Command Set includes tables that describe these zones and the average seek time from track to track. By knowing exactly how long it will take to access data, the surveillance system can optimize its transfers. There is also a Configure Stream command that allows the host to set the number of simultaneous read or write streams. This knowledge lets the drive configure its buffers accordingly.
At the end of the day, surveillance customers don't care about how ATA commands are implemented. They don't care about access times or data rates. They care about recording and, in turn, playing back video. Not to sound too dramatic, but the difference between a bad guy getting caught and him getting away can be a few frames of a capture. If your surveillance system cannot keep up with every single frame of video, you are taking on risk.
As part of our surveillance hard drive showdown, we're matching the Western Digital Purple against Seagate's Surveillance HDD. Both drives sport almost identical specifications and target markets. With roughly 90% of the rotating disk space split between the two, there aren't many moves that one company makes without the other one matching.
The surveillance market is no different. In a case of one-upmanship, Seagate announced its Surveillance HDD one day prior to the WD Purple announcement. But before we crown a surveillance champion, lets take a closer look at each drive.
