SSDs dominate the headlines in the storage industry, but every so often, a new mechanical drive catches our eye. Today, it's HGST's Ultrastar C10K1800, which crams 1.8TB into a 2.5" form factor. That storage density isn't terribly remarkable by itself, but the Ultrastar spins its platters at 10,000 RPM, which is a lot faster than most HDDs. It also has a novel "media cache architecture" that uses a portion of the platters as a drive cache. Oooh.
The media cache is separate from the Ultrastar's 128MB DRAM cache. HGST hasn't revealed how large it is or whether data is cached speculatively for reads. However, the company claims a "a significant improvement in write performance even at high workloads when compared to solutions with limited NAND or flash-based NVC." (NVC refers to a non-volatile cache.)
Note the "limited" language. Although the new Ultrastar is supposed to have 2.5X the random write performance of its predecessor and 23% higher sequential speeds, it's not in the same class as modern SSDs. The drive's access times are measured in milliseconds—not fractions thereof—and its sequential throughput tops out at just 247MB/s. Given those figures, the 12Gbps SAS interface seems excessive. But I guess that's what you use when building an exotic mechanical drive for datacenters.
Make no mistake, this thing is exotic. The C10K1800 stacks four platters in a case just 15 mm thick, and its rotational speed is roughly double that of comparable consumer drives. The latest mini Ultrastar also has 50% more capacity than the previous generation. More detailed specifications are available in the official datasheet (PDF).
Update: HGST has confirmed that the media cache has no impact on read performance.