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.
| Products |
Western Digital Purple
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Seagate Surveillance HDD
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| Pricing |
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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.
In Western Digital's never-ending quest to snag every color in the rainbow, we now have the WD Purple, which is surveillance-specific. Right under WD's purple banner, there is a large QR code that sends you straight to the drive's webpage.
The Purple is a follow-up to the video- and surveillance-oriented WD-AV GP, which supported WD's SilkStream technology. The purpose of SilkStream was to facilitate up to 12 simultaneous HD video streams utilizing the ATA Streaming Command Set. With the Purple, WD rebrands SilkStream to AllFrame, which now supports 32 HD cameras/channels.
It's interesting to note the way WD is positioning this product. Unlike traditional hard drives, where access times and throughput are prominently displayed, the Purple employs the target market's vernacular, which is number of channels/cameras. To go along with its marketing change-up, WD provides some really nice tools on its website to help guide customers to the correct WD product.

- Compatibility: Select from over 20 surveillance system vendors and countless models to see which WD hard drives are supported.
- Product Selector: Select the number of cameras, drive bays, interface, and environment, and WD recommends the best product line.
- Capacity Selector: Enter the details of the video feeds, and WD lets you know how much capacity your system will require.
Although the Purple is meant strictly for surveillance, WD still recommends the enterprise Se and Re lines for large enterprise surveillance systems. As you will see in our test results, those two families aren't slouches in this segment, either. In fact, they are better than the Purple in almost every way, except for cost. That's really the point of the Purple, though; achieve enterprise levels of performance for a very specific task at a vastly reduced price point. Even though the Purple is well-suited to this particular workload, there are compromises you make.
| Purple | Se | Re | |
|---|---|---|---|
| Applications | Home/SOHO/SMB | SOHO/SMB/SME | SME/Large enterprise |
| Recommended # of Drives | 1-8 | 6+ | 6+ |
| Typical # of Cameras | 1-32 | 1-64 | Unlimited |
| # of Channels | Up to 32 | Up to 64 | 64 or more |
| Workload (TB/year) | 60 | 180 | 550 |
| Form Factor | 3.5" | 3.5" | 3.5" |
| Capacity | 1, 2, 3, 4 TB | 1, 2, 3, 4 TB | 250, 500 GB, 1, 2, 3, 4 TB |
| Mission-Critical | No | No | Yes |
| Power | 5.1 W | 9.5 W | 11 W |
| MTTF | 1 million-hr. | 1 million-hr. rack-mount 1.2 million-hr. table-top | 1.2 million-hr. |
| Warranty | Three-year | Five-year | Five-year |
| Interface | SATA | SATA | SATA or SAS |
| Performance | IntelliPower | 7200 RPM | 7200 RPM |
| RAFF Support | No | Yes | Yes |
For those of you who don't spend your days poring over datasheets, the workload rating may come as a surprise. SSD customers are accustomed to seeing a write endurance specification because NAND deteriorates over time, and its useful life can at least be estimated. With rotating media, there is an assumption that enterprise-oriented drives intended for 24/7 operation can be written to all day, every day, for their entire lifespan.
Recently, WD changed the way it rates its drives and now includes that workload specification, indicating how many terabytes of data can be written to the drive per year. Unlike the SSD world, this isn't related to the media's ability to hold data. Rather, it's more closely tied to the MTTF rating. Exceeding WD's number effectively reduces the longevity of the drive, which is guaranteed for three years. But of course, as with SSDs, the higher-quality components required to sustain increasingly taxing workloads drive up cost. The Re family is rated for a workload nine times higher than the Purple, and consequently is more expensive.
The next differentiator is the recommended number of drives. Specific to the Purple, WD suggests no more than eight in a system, and they're not intended for rack-mount applications. This is mainly due to a lack of Rotary Acceleration Feed Forward (RAFF), a technology that overcomes the effects of vibration introduced by other drives in an enclosure. In server applications, where dozens or hundreds of drives can share the same rack, RAFF is necessary. Clearly, WD doesn't believe that functionality is required for the Purple's target market.

For this review, we were not only able to test WD's Purple, but also its Red, Se, and Re, all with 4 TB of capacity.
While WD was busy color-coding its various product lines, Seagate took a red marker to the names it used to brand storage products. Barracuda, Constellation, and Savvio are no more, replaced by to-the-point labels like Enterprise Capacity and Enterprise Performance. So, it comes as no surprise that the surveillance-oriented product is called the Seagate Sureveillance HDD.

The Surveillance HDD is a seventh-generation device that builds on the previous SV35 series. Similar to WD, Seagate is able to stream from 32 cameras/channels simultaneously, and the Surveillance HDD is rated for 24/7 operation. While WD optimizes its firmware for simultaneous read/write operations, Seagate chooses to focus on write-heavy workloads, which are predominantly large block, sequential transfers. Seagate's claim is that writing represents up to 95% of a surveillance-oriented application. That's in stark contrast to WD's message.
While Seagate doesn't have any of the useful tools on its site to help you select a drive, the company does provide quite a few white papers that present similar ideas. In its Safe and Smart Surveillance Drive Selection Guide, you can see that the Surveillance HDD is positioned between the Video 2.5/3.5 HDD and the Enterprise Capacity 3.5 HDD. Seagate's Video 2.5/3.5 HDDs are lower-power, lower-cost alternatives, while the Enterprise Capacity 3.5 is meant for large enterprise deployments.
| Video 3.5/2.5 HDD | SV35 Series HDD | Surveillance HDD | Enterprise Capacity 3.5 HDD | |
|---|---|---|---|---|
| Applications | Low-cost, low-res, SMB | Central monitoring apps | High-res cameras and camera counts | Bulk storage, data center, corporate |
| Form Factor | 2.5", 3.5" | 3.5" | 3.5" | 3.5" |
| Capacity | 2.5": 250, 320, 500 GB 3.5" 250, 320, 500 GB, 1, 2, 3, 4 TB | 1, 2, 3 TB | 3, 4 TB | 500 GB, 1, 2, 3, 4 TB |
| Recommended # of drives | 1-6 | 1-6 | Up to 16 | 10+ |
| Data security | None | None | None | ISE feature in SED |
| Performance | 180 MB/s max data rate | 210 MB/s max data rate | 180 MB/s max data rate | 175 MB/s max data rate |
| Reliability | .55% AFR | 1 million-hr. MTBF | 1 million-hr MTBF | 1.4 million-hr. MTBF |
| Warranty | Three years | Three years | Three years | Five years |
This is where we can start to see differences between WD's and Seagate's drives. The Surveillance HDD has rotational vibration (RV) sensors. Their inclusion allows Seagate to support up to 16 Surveillance HDDs in a single system. Seagate also supports Idle3 spin control, decreasing time-to-ready so that the storage system can go into low-power modes more often without the threat of losing camera data during motion detection.
Like the WD Purple, the Surveillance HDD has a 1 million-hour MTBF and a three-year warranty. Seagate also offers a slightly higher ambient operating temperature (70 versus 65 degrees Celsius). But Seagate does not specify yearly workload. It only mentions a "normal I/O duty cycle", which isn't specific by any means.
Another item that jumps out compared to WD's Purple is power consumption. While the Purple has an average operating power draw of 5.1 W, the Surveillance HDD is pulling down 7.5 W. Even standby and sleep power draw are 0.25 W higher than the Purple. WD made power consumption a big bullet point when we talked, and now we can see why.

While we weren't able to get our hands on a Seagate Video 3.5 HDD or Enterprise Capacity 3.5 HDD, we were able to put many of Seagate's 2.5" enterprise products in the comparison. These drives give us some interesting additional data, especially the high spindle speed, SAS-based models. Because of their form factor and performance, the pricing is an order of magnitude higher than the 3.5" drives.
| Test Hardware | |
|---|---|
| Processor | Intel Core i7-3960X (Sandy Bridge-E), 32 nm, 3.3 GHz, LGA 2011, 15 MB Shared L3, Turbo Boost Enabled |
| Motherboard | Intel DX79SI, X79 Express |
| Memory | G.Skill Ripjaws Z-Series (4 x 4 GB) DDR3-1600 @ DDR3-1600, 1.5 V |
| System Drive | Intel SSD 320 160 GB SATA 3Gb/s |
| Host Bus Adapter | LSI SAS 9300-8e |
| Tested Drives | WD Purple 4 TB: WD40PURX Seagate Surveillance HDD 4 TB: ST4000VX000 |
| Comparison Drives | WD Red 4 TB: WD40EFRX WD Se 4 TB: WD4000F9YZ WD Re 4 TB: WD4000FYYZ Seagate Savvio 15K.3 146 GB: ST9146853SS Seagate Enterprise Performance 10K HDD v7 1.2 TB: ST1200MM0017 Seagate Constellation.2 1 TB: ST91000640NS |
| Graphics | AMD FirePro V4800 1 GB |
| Power Supply | OCZ ModXStream Pro 700 W |
| System Software and Drivers | |
| Operating System | Windows 7 x64 Ultimate |
| DirectX | DirectX 11 |
| Driver | Graphics: AMD 8.883 |
| Benchmark Suite | |
| HD Tune 2.55 | Block = 8 MB |
| WD Surveillance Benchmark | 16 Cameras x 720p x 30 FPS 16 Cameras x 1080p x 20 FPS 32 Cameras x 720p x 30 FPS |
Testing a surveillance drive should be different than running a normal consumer or enterprise drive through its paces. Because the workload is so narrowly defined, we chose only tests that would mimic a surveillance environment. As a result, you won't see typical consumer or enterprise tests, such as SYSmark or enterprise server profiles in Iometer.
One test that we did include was provided to us by Western Digital. You know that we wouldn't accept tests or scripts directly from manufacturers without fully vetting them, and we've seen enough from this one that the results appear in line with other tests and our expectations. It should be stated, however, that WD's Surveillance Benchmark directly aligns with the type of transfers that WD is targeting with the Purple. The total workload is calculated by using the following equation:
Total Workload = (# of cameras) x (bit rate per stream) x 2 (for simultaneous record and playback) + (non-AV data)
The sizes of the read and write commands are matched with the host buffer sizes for each stream. Based on industry input, WD chose a medium-sized host buffer that correlates to 1024 sectors for write commands and 768 sectors for read commands. Non-sequential, non-AV data is injected in the write commands on a per stream basis.
The result of the test is a histogram showing what percentage of the read/write commands completed within a certain period of time. The test also displays the total drive idle time. According to WD, so long as idle time is greater than 20%, the drive passed for a given workload. Anything below that number cuts into any safety margin the system might have.

Since a vast majority of surveillance and video transfers are large block sequential writes, we took a look at those results to get a baseline for all of our drives.
Both surveillance disks perform almost exactly to their specifications. Seagate's Surveillance HDD is 20-30 MB/s faster than WD's Purple across the board. Also, as expected, the enterprise drives work their way to the top of the chart, with the high spindle speed Seagates pulling away from the pack.

The access time results are similar to the transfer rates, but with both the Surveillance HDD and Purple slipping a spot. Don't get too caught up in these numbers, though. Both drives have a very specific purpose, and excelling in synthetic benchmarks is not it. In fact, WD made it explicitly clear that the performance tweaks incorporated into the drive for surveillance data would adversely affect these types of tests. But this is just a line in the sand. The next set of benchmarks will ultimately determine the best surveillance drive.
As we stated earlier, WD considers each test case a pass if the idle time is greater than 20%.

For this round of benchmarks, we considered three different cases. The first was 16 cameras running at 720p/30 FPS, adding up to slightly less than 6 Mb/s per stream in both directions for 16 cameras using H.264 compression. Only the WD Red was unable to keep up with the workload. And now we can start to see the difference WD's AllFrame technology brings to the table. While the Purple idled nearly half of the time, the Surveillance HDD was idle less than 30%. Surprisingly, the Savvio 15K.3 ended up in the middle of the pack. We reran the test multiple times, but recorded the same result.
The next test case was 16 cameras at 1080p/20 FPS. The lower frame rate was a limitation of WD's benchmark, and not a conscious decision on our part. This equals a per-stream bit rate of nearly 8 Mb/s. And we can already see the limitations of both surveillance-oriented drives. Only the enterprise disks were able to get above the 20% mark; WD's Re falls just short.
Finally, we expanded the first test case from 16 to 32 cameras, at which point all of the drives land under the pass/fail criteria. Seagate's Savvio put up a good fight, but was only idle 9% of the time.
This is, by far, the most strenuous test for these drives. Even though the bit rates aren't very high, the read/write cycles put a damper on their performance.
The WD Surveillance Benchmark also displays histograms of the read and write completion times. When you start to stress the drives, writes take priority over the read operations, and their completion times start to drift upwards. This is what you want to see; if you have to drop data, you'd rather lose information played back than the stream you're trying to record. For all drives, write completions were always less than 20 ms.

In the graph above, we are looking solely at read completions times.
In the first test case, all products perform well. Everything except the WD Red has 50% of its read commands complete in less than 20 ms. Moving to the more demanding test cases proves to be problematic for the non-enterprise drives.
The Seagate Surveillance HDD has slightly better read distributions than WD's Purple, and in the next section, we'll see how the actual distribution looks for both disks.
Looking at the enterprise drives, you can see that they are better equipped to handle more demanding workloads. In fact, the Seagate Savvio 15K.3 just plows through the tests. Even though the idle time for the 32/720p/30 FPS lands at 9%, almost 100% of the writes and reads are still under 20 ms.

For the first test case, WD is the clear winner with 59% of its reads completing in under 5 ms. The Surveillance HDD has its read completion times distributed in the 10-40 ms span.

In the next two test cases, Seagate takes a slight lead, with a higher percentage of completions in the 10 to 20 ms range. The WD Purple also reports a much higher percentage of completions greater than 80 ms. We were hoping that the great results from the first test case would carry over to others, but both drives look similar in these subsequent runs.

Pricing was determined by checking multiple reputable online retailers. And if you're still on the fence between the Purple and Surveillance HDD, the information on this page won't be a lot of help. The WD Purple sells for around $180, while the Surveillance HDD is slightly higher at $187. Those prices are pretty much in line with other specialty 4 TB 3.5" drives. In fact, the Purple is actually only a few dollars more expensive than the WD Red.

Once you start moving into the enterprise space, prices quickly skyrocket. While the WD Se and Re are roughly 50-80% more expensive than the Purple, the enterprise Seagate drives are on a different planet altogether. As I noted earlier, those Seagate models are in there as a performance reference; they wouldn't normally compete in the same space as the Purple and Surveillance HDD. They're 2.5" disks with much higher spindle speeds and a SAS interface, which account for the massive price premium. We just wanted to add them because the 10K and 15K Seagate offerings perform so well, considering they're rotating drives.
Even though the surveillance storage market would appear to have a very narrow focus, both WD and Seagate made interesting design choices that match their vision of the market.

With WD, you have the Purple line of surveillance-oriented hard drives that are optimized for simultaneous read/write performance. Looking at our performance results, workloads that stress the Purple up to a certain point are handled with great responsiveness. This is largely thanks to the company's AllFrame technology, which streamlines performance based off of a known workload. Once you exceed the drive's performance boundaries, less ideal results start cropping up. One big advantage favoring the Purple, however, is power consumption. It draws one-third less power than the Surveillance HDD.
Seagate, on the other hand, is going after pure write-intensive applications with its Surveillance HDD. In our transfer rate test, the Seagate disk is 15-20% faster than WD's Purple. And when you stress the Surveillance HDD, it responds more gracefully, kicking back fewer outliers. Seagate also wins points for including RV sensors that enable RAID arrays of up to 16 drives in a single system. As a result, the Surveillance HDD is more scalable than the Purple.
Overall, deciding between the Purple and the Surveillance HDD comes down to your application. With fewer than eight drives, under a known read/write workload, the WD Purple stands out. Going beyond eight drives with a write-only workload definitely favors the Seagate Surveillance HDD. In both cases, the two drives offer great surveillance performance, exceeding what you'd see from mainstream desktop drives, while selling for far less than their enterprise-class siblings.