On The Stroke Of Performance: Hard Drive Short Stroking
Flash SSDs dominate the news at present. If you believe the coverage from multiple Internet tech magazines, flash-based devices are about to take over the storage market. They’re perceived as being faster and more efficient than hard drives—and maybe a bit more expensive. The truth is different, though; flash memory still has only a tiny market share for many reasons. The good old hard drive isn’t dead, and we just found another reason for them to live on for a while: short stroking technology.
HDD Vs. SSD
I want to make very clear that we do not favor hard drives versus flash SSDs or vice versa. Each technology has its pros and cons, and each makes a lot of sense when deployed in a smart way. The advantages of SSDs are almost nonexistent access times and high I/O performance. In addition, flash memory has become extremely cheap for capacities of up to 32 GB. Finally, flash SSDs can be amazingly power efficient. Unfortunately, many flash SSD products on the market just are not there yet.
To understand these products, you need to know about the difference between single-level cell (SLC) and multi-level cell (MLC) flash. MLC allows for quick read operations, but the technology is not suitable for high-speed write operations, especially when they occur randomly. SLC flash is superior, but also more expensive. Some MLC flash SSDs are affordable, but the really juicy SLC flash SSDs are still very pricey.
Efficiency and reliability are additional issues. Performance and power efficiency are highly dependent on the quality of the controller used by a flash SSD product, as does power efficiency. There are some amazing products on the market, but they come at a price.
HDDs Holding Up
Conventional hard drives have become extremely affordable, with terabyte drives approaching—or even dropping below—the $100 mark. High capacities, low cost and ready availability clearly are the main reasons to keep using hard drives. And despite being based on magnetic, mechanic, and electronic components, HDDs are typically reliable and comparatively robust. In fact, enterprise class hard drives have to endure torture testing and demanding validation processes before they can be used for mission-critical applications. In contrast, the long term reliability of flash SSDs is still uncharted territory, and hence, they're avoided by many enterprise customers.
Fighting The Physical Disadvantage
The main disadvantage of conventional hard drives is the time delay that occurs each time the read/write heads are required to move. While this is not an issue for sequential read or write operations—where conventional hard drives are often still faster than most flash SSDs—it becomes a serious performance issue for random read and write operations.
Throughput of more than 150 MB/s for fast 15,000 RPM enterprise hard drives can decrease to only kilobytes per second once the hard drive has to access random information that may be distributed across the entire magnetic medium. In other words: the drive or the application loses a lot of time spent on head repositioning operations. What if this mechanical activity could be minimized? It can! We looked at an approach that is referred to as short stroking and tested a set of Hitachi hard drives that aim to reducing mechanical activity by utilizing only a fraction of the hard drives’ capacities.
