Western Digital Reimagines HDD – Flash Integration with OptiNAND

Western Digital Reimagines HDD - Flash Integration with OptiNAND

The previous couple of years have seen loads of new improvements come up within the hard-disk drive market. For fairly a while, the HDD know-how roadmap was shared industry-wide – distributors launched new applied sciences at totally different deadlines, however they had been all comparable in nature. As a current instance, HGST (now, Western Digital) was the primary to market with helium-filled HDDs, however each Seagate and Toshiba adopted up with comparable drives inside a number of years.

Previous to 2017, there was consensus that heat-assisted magnetic recording (HAMR) would assist drive the rise in storage density for HDDs after conventional perpendicular magnetic recording (PMR) ran out of steam. Western Digital sprang a shock in This autumn 2017 by saying the choice to go along with microwave-assisted magnetic recording (MAMR) for future HDDs. Seagate, in the intervening time, has been all-in on HAMR and likewise launched 20TB HDDs based mostly on the know-how for enterprise prospects (these HAMR drives are but to hit retail, although). In the intervening time, Western Digital was promising MAMR drives for 16TB+ HDDs, however ultimately back-tracked in favor of energy-enhanced PMR (ePMR). Toshiba, then again, launched flux control-MAMR (FC-MAMR) in its MG09-series of enterprise 16TB and 18TB HDDs.

On the HDD Reimagine occasion right this moment, Western Digital is introducing OptiNAND – a novel structure involving the mixing of an embedded iNAND UFS embedded flash drive (EFD) on the drive’s mainboard.

In conjunction, the corporate can also be saying that it has been sampling its first 20TB non-SMR drives based mostly on OptiNAND-enabled ePMR to pick out prospects, and that it could be adopting the OptiNAND platform transferring ahead for all 20TB+ HDDs. The corporate additionally sees a path to 50TB OptiNAND-enabled ePMR drives within the second half of the last decade.

Whereas the corporate didn’t quantify the quantity of NAND in its OptiNAND drives, they’re stressing the truth that it isn’t a hybrid drive (SSHD). In contrast to SSHDs, the OptiNAND drives don’t retailer any person information in any respect throughout regular operation. As an alternative, the NAND is getting used to retailer metadata from HDD operation so as to enhance capability, efficiency, and reliability.


Western Digital’s OptiNAND announcement additionally conveys the truth that their 20TB 9-platter HDDs will proceed to make use of energy-enhanced PMR (ePMR). Along with using a triple-stage actuator to allow extra correct positioning of the heads over the tracks, the OptiNAND facet is being touted as the important thing to enabling 2.2TB capability for every platter.

The rise in areal density is being achieved by cramming the tracks on the platter nearer collectively (elevated TPI), whereas additionally transferring out a number of the metadata (each factory-generated and mid-user operation) out from the platter to the NAND. Particularly, Western Digital made a point out of the repeatable run out (RRO) recording of the top jitter / error place because the spindle revolves. This information (operating into a number of gigabytes) is generated within the manufacturing facility throughout manufacturing. It’s sometimes saved within the disk, taking on house that might have doubtlessly been used for person information. The OptiNAND structure strikes this to the NAND within the EFD.

One of many key challenges to packing tracks nearer collectively is the idea of ‘adjoining observe interference’ (ATI). This ends in the necessity to periodically refresh information within the platter’s tracks because it may get corrupted by writes to adjoining tracks. At the moment accessible HDDs triggered these refreshes on a track-by-track foundation based mostly on the recording of write operations on the track-level. One of many downsides to rising areal density by rising the TPI is the necessity to do extra frequent refreshes. From refreshing as soon as in 10000 write operations in early HDDs, the slim tracks now must be refreshed as often as as soon as each 6 writes. Past a sure level, it does not make sense to extend TPI any additional as a result of the rise within the frequency of ATI refreshes has an excessive affect on efficiency. In present-generation HDDs, these refreshes have been triggered on the observe stage by recording write operations at that hierarchy. The OptiNAND structure permits the write operations to be recorded on the sector stage. Because of this the refresh operations are extra unfold out each temporally and spatially, permitting the tracks to be packed nearer collectively with out sacrificing efficiency. In flip, this will increase the areal density.


Customers can function HDDs with the write cache within the gadget enabled or disabled. Regardless of the cache enablement, the HDD has to buffer up the incoming information. Within the disabled case, the quantity of knowledge that may very well be buffered up relies on the quantity of knowledge that may be safely flushed out to non-volatile storage within the case of an emergency power-off (EPO) state of affairs. The presence of serious NAND capability within the HDD implies that the drive can use the rotational power current within the platters to flush out extra information within the DRAM into the NAND (Current-day HDDs dump out the DRAM information into serial flash – round a few MBs value – in an EPO state of affairs). The flexibility to buffer out extra information on this case implies that the efficiency of write-cache enabled case and write-cache disabled case method one another in OptiNAND-enabled HDDs.

Western Digital additionally claims that the ‘write cache enabled’ case can profit on the efficiency entrance. That is an oblique results of the diminished refresh charges (referencing the observations within the earlier sub-section on how OptiNAND handles adjacent-track interference) that enables the HDD to spend extra time in servicing person information requests. Once more, there was no quantification of the development in IOPS for various entry patterns over non-OptiNAND HDDs in Western Digital’s occasion.


The features of OptiNAND used to boost the efficiency of the drives within the write caching disabled state additionally contribute to enhancing their reliability underneath EPO situations. By together with sooner non-volatile storage in comparison with serial flash, Western Digital claims that as much as 50x extra information could be flushed out in comparison with previous-generation HDDs.

Concluding Remarks

Western Digital claims that the vertical integration attainable with the HDD know-how from the WD / HGST aspect together with the flash know-how from the SanDisk aspect is crucial for the creation of a platform like OptiNAND.

There may be sure to be a cost-premium related to the drives as a result of NAND integration. New recording applied sciences (like HAMR and MAMR) require important funding into the design of the recording heads in addition to platters, and must be revamped each few generations. Alternatively, applied sciences like OptiNAND are impartial of the underlying know-how.

With out precise quantification of the rise in areal density enabled by OptiNAND, it isn’t attainable to offer comparative feedback on the Capability facet of Western Digital’s OptiNAND trifecta – besides that the corporate is now in a position to introduce 20TB laborious drives to the market with the identical ePMR know-how utilized in its 18TB drives (round 2.2TB/platter).

The Efficiency facet needs to be simpler to judge when OptiNAND drives hit retail. Whereas the advantages for the ‘write caching disabled’ case (the place the NAND can act as a secure cache in an EPO state of affairs) are simple to confirm (basically performing the identical because the ‘write caching enabled’ case), the pure ‘write caching enabled’ case needs to be far more attention-grabbing to research towards competing drives of the identical capability.

Western Digital indicated that each one of their 20TB+ HDDs transferring ahead will likely be OptiNAND-enabled. This will likely be throughout all market verticals – cloud deployment, enterprise drives (Gold), storage for surveillance recording (Purple line), and NAS (Pink line). It should be famous that the corporate has a 20TB SMR drive already available in the market that’s not OptiNAND-enabled. The brand new HDD structure with its versatile SoC and high-performance NAND integration will also be used to allow customer-specific enhancements sooner or later. The flexibility to make use of the NAND to dynamically remap sectors can enhance areal density and enhance efficiency far more in SMR drives. Based mostly on this, we will anticipate OptiNAND-enabled SMR drives to realize important capability benefit over CMR drives compared to what’s being seen available in the market presently.

The HDD {industry} is just not but in dire want of CPR, however Western Digital’s utilization of OptiNAND to handle the Capacity, Performance, and Reliability trifecta is yet one more distinctive facet within the innovation-rich hard-disk drive market. Western Digital has each HDD and full flash know-how (from NAND fabrication to controller) in-house, whereas the opposite HDD distributors shouldn’t have that benefit. As such, it’d take the opposite distributors a while to compensate for the benefits of utilizing NAND for HDD metadata.

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