Between the Racks takes on SSD v HDD Endurance
The key to a great product evaluation is always going to rely upon comparisons. Coming to conclusions based upon the performance of competing devices is the bread and butter of what I do for a living. Writing a conclusion is elementary, provided you keep it simple. One merely rehashes the stats and chats a bit about the performance of Device A compared to Devices B, C, and D.
One juxtaposition we rarely cover is the comparison of an SSD to an HDD. In most cases, it is unfair to compare the performance of an enterprise-class SSD to an HDD. The performance isn't in the same ballpark, and until lately the price hasn't been either. We tend to keep the comparisons confined to similar devices. Plainly put, SSD vs SSD is a fair fight. It just doesn't seem feasible to prop up an HDD in the ring for a faster and more powerful SSD opponent to pummel to death.
With the continued development of MLC, and even smaller lithographies in the near future, the price-per-GB of enterprise SSDs has fallen into striking range of performance 2.5" SAS HDDs (10K and 15K). The tremendous pressures placed upon the 2.5" performance spindle-based market drags the SSD vs HDD comparison back into light.
In a recent article I was making some basic comparisons of an entry-level SAS SSD and a traditional 2.5" 15K HDD. Aside from the comparisons on price, capacity, and other features, one item really stuck out. While crunching the numbers it occurred to me that endurance isn't quite as clear-cut as one would expect.
Which brings me to my heretical statement that enterprise SSDs have more useable endurance than 2.5" performance HDDs. When comparing the endurance of an HDD and an SSD it's important to note that both are covered by the same five-year warranty, and feature comparable MTBF and UBER ratings. In the majority of applications, a five-year lifespan may in fact be too long. The exponential gains in performance, density, and efficiency of both SSDs and HDDs merit a replacement cadence that grows ever shorter. As these three key parameters advance, it becomes a smarter financial decision to update hardware either at, or before, the five-year deadline.
Assuming the storage device will not be used longer than the five-year warranty period provides us a level playing field. We can also safely assume that most high-performance 2.5" HDDs will be used for random access, such as VDI, OLTP, and transactional-based workloads. This narrows down our comparisons to random write endurance.
Taking a look at an entry-level SSD we see that an endurance of 3 DWPD (Drive Writes Per Day) is pretty standard for a random workload. The sequential workload endurance can be much higher, from 7-10 DWPD, depending upon the SSD. While the glass platter substrate of an HDD has a very high tolerance for heavy workloads, the mechanics of the moving parts conspire to hinder the speed, and thus the useable endurance of the drive.
In our testing, the fastest 15K HDDs write at a speed of roughly 450 IOPS for both 4K and 8K random write workloads. For 4k this equates to roughly 148 GB's of data written per day. For 8K access, common to many server workloads, we arrive at roughly 296 GB of potential data written per day.
An entry-level SAS SSD, by comparison, can provide over 30,000 IOPS of 4k write speed and 18,000 8K write IOPS in steady state. This equates to nearly 9.6 TB of potential data writes per day for 4K access, and 11.5 TB's per day for 8K write access. This is a huge advance over the HDD, and could necessitate throttling of the SSD to keep it within the expected warrantied workload of 3 DWPD.
With the capacity of new SSDs touching 2TB, this can provide up to 6 TB of useable endurance per day for the SSD, in comparison to the lowly 148-296 GB attainable by today's fastest HDDs. The 2TB SSD can provide 10,950 TBs of 8K write activity over the warrantied period of five years, compared to the HDD with 534 TBs. The slower speed of the HDD negates its ability to take advantage of its nearly-unlimited endurance, while the SSD can write nearly 20X more data over five years.
Gartner estimates that by 2016 the average capacity of a deployed enterprise SSD will be more than 1.1TB. Even with smaller SSD capacities as low as 128 GB, and thus less available endurance per day, some entry-level SSDs still hold an advantage over the HDD in useable random write endurance during the warranty period. Mixed random read/write workloads will also muddy the water a bit, but it's safe to say that SSDs have a commanding lead in useable random workload endurance.
The picture for sequential workloads favors the HDD, with a 128K write speed of 250 MB/s providing roughly 20TB of data writes per day. This is well over the 14TB attainable from the largest 2TB SSD, providing HDDs a useable endurance advantage in sequential workloads.
Its hard to believe in this day and age there is still the archaic viewpoint that harkens back to the bygone era of, well, just a few years ago, that SSDs have endurance concerns. NAND wears out, that is an eventuality that isn't going to go away. However, advances in controller technology and a better understanding of NAND has created a solution that can provide more useable endurance per slot than its HDD brethren. Gone are the days of questionable SSD endurance. The new breed of SSD provides a compelling mix of density, performance, efficiency, and price.
Ruminating on the demise of the HDD is a fruitless activity, there simply isn't enough NAND fabrication capability for the HDD to be unseated by the SSD in the near future, regardless of performance advantages. There are a complex mix of factors that affect the purchasing decision, and the future of HDDs and SSDs is destined to be intertwined and complimentary to one another. In some areas, such as the performance segment, we can expect some heated competition for slots.
The HDD manufacturers surely aren't taking the challenge lying down, either. The advent of the SSHD is also going to alter the face of the datacenter, especially when it matures and expanded write caching is added to the list of compelling features. Until then, the SSD seems to have an advantage in an ever-increasing number of workloads, and is reaching price parity with performance HDD's. The purpose of today's article isn't to proclaim the winner, but merely point out that the competition is growing increasingly heated in the enterprise space.
The continued evolution of new technologies, such as HAMR, SMR, and helium, will keep the race between SSDs and HDDs hot for years to come. I for one look forward to watching the chess match.