Short Stroking: How It Works
Fast, SLC flash-based SSDs are superior to every traditional hard drive when it comes to answering demanding I/O workloads for databases, Web servers, or workstations. Hard drives typically deliver up to a few hundred I/O operations per second, while professional flash SSDs can deliver thousands of I/Os per second. If you now think of transaction-intensive applications, the difference is really significant, as it not only increases performance, but may also make entire storage arrays obsolete. These will increasingly be realized using a small number of flash SSDs versus dozens of hard drives in complex RAID arrays. There are even significant power savings available along the way as well.
However, not all so-called online storage applications require maximum performance, and most customers would trade performance for maximum reliability. This has been a clear benefit for conventional hard drives.
Containing The Operating Range
Short stroking aims to minimize performance-eating head repositioning delays by reducing the number of tracks used per hard drive. In a simple example, a terabyte hard drive (1,000 GB) may be based on three platters with 333 GB storage capacity each. If we were to use only 10% of the storage medium, starting with the outer sectors of the drive (which provide the best performance), the hard drive would have to deal with significantly fewer head movements.
The result of short stroking is always significantly reduced capacity. In this example, the terabyte drive would be limited to 33 GB per platter and hence only offer a total capacity of 100 GB. But the result should be noticeably shorter access times and much improved I/O performance, as the drive can operate with a minimum amount of physical activity.
First Thoughts
I think we all agree that the capacity loss is not an issue—in the end, we’re looking at maximized performance, and storage capacity is available in abundance anyway. Considering the capacity of enterprise flash SSDs, which is still typically limited to 32 GB, limiting a hard drive to roughly the same capacity appears legitimate. In addition, short stroking setups usually consist of multiple hard drives in performance-optimized RAID arrays, so overall capacity should really not be an issue. But short stroking may also have a positive impact on the life expectancy of a hard drive, as reducing physical activity reduces overall wear and tear. It’s hard to predict the performance impact, though, so let’s look at the test hardware and the benchmark results.
