Western Digital
At Western Digital’s HDD Reimagine Occasion yesterday, the corporate launched its latest arduous drive structure—a hybrid spinning rust/NAND flash design it calls OptiNAND. However as WD President of Know-how and Technique Dr. Siva Sivaram advised Ars in an interview, OptiNAND bears nearly no resemblance to the much-maligned hybrid SSHD drives first launched in 2011 and 2012.
As a substitute of promising SSD-like speeds by way of caching of buyer knowledge, OptiNAND presents elevated areal density by eradicating firmware-accessible metadata from the disk itself and storing it on NAND as a substitute.
20TB per disk with out SMR
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Like final yr’s 20TB SMR drives, the OptiNAND drives make use of EAMR within the type of a further present to the principle pole of the write head.
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The triple-stage actuator—very like the three knuckles of a human finger—permits fast, high quality head positioning in each final yr’s and this yr’s 20TB drives.
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Final yr, Western Digital’s 20TB drives wanted Shingled Magnetic Recording to succeed in their excessive areal densities. This yr, the added edge comes from offboarding metadata as a substitute of shingling sectors.
Western Digital
Essentially the most tangible milestone achieved by Western Digital’s newly introduced structure is a nine-platter, 20TB drive that doesn’t require Shingled Magnetic Recording (SMR) methods. The brand new disk makes use of a subset of Western Digital’s EAMR expertise, which has been rebranded ePMR—presumably to emphasise that it is not SMR, which has extreme efficiency and value implications for a lot of widespread workloads.
The brand new drive makes use of the identical EAMR and triple-stage actuator expertise as final July’s 18TB and 20TB choices however will get its enhance in areal density from offloading onboard metadata to the flash aspect of the OptiNAND structure. The metadata we’re speaking about is not filesystem metadata—it is {hardware} metadata, accessible by the drive’s firmware however not uncovered outdoors the drive itself.
Repeatable Runout (RRO) is an outline of a rotational system’s inaccuracies that may be predicted forward of time—for instance, a gentle wobble brought on by the microscopically imperfect alignment of a rotor. RRO knowledge is particular to every particular person drive and is generated on the manufacturing facility throughout manufacturing and saved on the disk itself.
In typical typical drives, RRO metadata is interleaved with customer-accessible knowledge on the platters themselves, decreasing the general areal density of the disk as a consequence of decreasing the variety of tracks per inch (TPI) accessible for buyer knowledge. OptiNAND structure permits Western Digital to maneuver this metadata off the platters and onto the onboard NAND.
With the intention to hit 20TB on a nine-platter drive with present recording applied sciences, you want a next-generation “edge” of some kind. In final yr’s 20TB drives, that edge was SMR—on this yr’s latest fashions, it is OptiNAND.
Sync write acceleration
Though Sivaram opened up our interview declaring that OptiNAND just isn’t for storing buyer knowledge in any respect, there may be one exception—the contents of the drive’s DRAM cache may be flushed to OptiNAND within the occasion of an sudden energy loss.
This has an enormous affect on the drive’s efficiency when the drive is requested to carry out sync writes—a particular mode of operation sometimes requested by databases, digital machines, and most NFS exports. When an software requests a “sync write” to a drive, the entire regular write aggregation and caching operations change into unavailable—the applying tells the working system, “I am not doing the rest till you confirm that this knowledge is saved completely and safely on disk.”
OptiNAND structure permits drives to finish these sync() calls instantaneously with out mendacity concerning the security of sync write knowledge. As a substitute of pausing the whole lot the drive is doing to hunt the heads to the right locations and write the info to the magnetic medium, the drive can merely say, “Your knowledge is protected” as soon as the writes are accepted into the drive’s personal DRAM buffer.
Usually, this is able to be thought of an unsafe violation of write barrier mandates. However on the OptiNAND drives, as much as 100MiB of “cache-disabled” writes may be accepted into DRAM as a result of, in an emergency power-off occasion, the drive’s onboard capacitor can preserve the DRAM viable for lengthy sufficient to flush the write cache to OptiNAND. On restoration of energy, the soiled knowledge quickly written to OptiNAND is learn again and instantly dedicated to rotational storage.
Permitting the drive to “fudge” on write obstacles by counting on capacitor energy to flush soiled writes from DRAM to OptiNAND can enhance efficiency for these writes drastically—in lots of instances, most likely by an element of ten or extra.
Use instances and availability
In response to Sivaram, the brand new OptiNAND drives can be helpful wherever typical arduous drives are used. With out the efficiency bottleneck of both drive or host-managed SMR, the drives may be anticipated to operate as plug-and-play replacements for older, smaller drives in the whole lot from the NAS to the info middle.
The 20TB OptiNAND disks are at the moment within the early levels of manufacturing, with samples transport to pick out Western Digital enterprise clients solely. Nevertheless, the expertise is anticipated to function “the muse for future designs and improvements” throughout Western Digital’s complete rotational storage product line, with market-specific merchandise turning into accessible later this yr.
We have now requested product samples for direct testing and overview.
