The HDD has been a work in progress since its inception in 1956. The evolution of the HDD has been fundamental in many aspects, with the same basic spinning platter design being further refined to produce more storage capacity and increase efficiency. Shrinking the size of the HDD has been the principal goal, and we have come from massive drives weighing hundreds of pounds, down to devices in 2.5" and 3.5" form factors.
The evolution of the HDD has focused on increases in areal density, and a whole slew of technologies have managed to pack more data onto the platter. SMR, along with helium-based drives, have made their way to the forefront in the density war. The near-term future might hold TDMR, while longer-term HAMR and MAMR technologies will likely provide tremendous increases in density.
There is no doubt that density has a clear path forward due to intense focus from manufacturers, but the real problem lies on the performance front. The spindle speed of the common HDD has settled at a top speed of 15,000 RPM, and there are no plans to increase this speed in the future. This stagnation leaves only incremental speed increases, borne of increased density, on the horizon. SSDs, on the other hand, have brought massive increases in speed and efficiency. Even the fastest of HDD's cannot hold a candle to the sheer speed of an enterprise SSD. This has led to a continuing assault in the performance segment of the enterprise HDD market, with SSD's gaining market share at a rapid pace.
Even with the evolution of the SSD leading to lower prices, they are still many orders of magnitude more expensive to deploy than HDD's. For years, the only method to increase storage performance was short-stroking the drive. This sacrifices a large portion of available capacity to force an HDD to operate on the outer tracks, where speed is highest. This also increases the price of the storage solution considerably.
Seagate's answer to the performance problem is the introduction of SSHD's (Solid State Hybrid Drive). SSHD's are only a fraction of the cost of an SSD, yet provide some of the same workload acceleration benefits by storing hot data in a NAND cache buffer. Seagate's first foray into caching HDD's was in the client market, and the lessons learned were applied to the new Enterprise Turbo SSHD.
The Seagate Enterprise Turbo SSHD is geared for use in a multitude of mission-critical environments, such as OLTP, VDI and SAP HANA. The drive comes in capacities of 300GB, 450GB and 600GB in a 2.5" form factor with a 15mm z-height. The Turbo SSHD features a 6Gb/s SAS connection and 128MB of multi-segmented DRAM cache, though this is only the first level of caching.
The Turbo SSHD is built upon the same 15,000 RPM HDD used in the Enterprise Performance 15K v4 HDD's, and sports the same design with three platters and six heads. The real ingenious aspect of the drive lies in the NAND caching, handled by 32GB of eMLC and an additional 8MB of NVC (Non-Volatile Cache). Hot data is held in the NAND and served to the host system at a much faster rate than the platters can provide, in many cases providing up to a three times improvement in application performance. The drive itself delivers up to 800 IOPS, a SDR (Sustained Data Rate) of 247 MB/s, and an average latency of 2.0ms.
The hardest part of testing an SSHD is quantifying the massive speed increases brought out by the cache. We have developed specialized tests that isolate and explore the performance of the NAND components. First, we examine the way an SSHD operates.