Western Digital again shows HDD – Flash integration with OptiNAND

Many new innovations have emerged in the hard drive market in recent years. For some time now, the roadmap for HDD technology has been shared across the industry – vendors have introduced new technologies at different times, but they have all been similar in nature. As a recent example, HGST (now Western Digital) was the first on the market to deliver helium-filled HDDs, but both Seagate and Toshiba followed similar drives within a few years.

Prior to 2017, there was a consensus that heat-assisted magnetic resonance imaging (HAMR) would help increase hard disk storage density after traditional vertical magnetic resonance imaging (PMR) ran out of steam. Western Digital prepared a surprise in the fourth quarter of 2017 by announcing the decision to use magnetic recording with the help of a microwave oven (MAMR) for future HDDs. Seagate, meanwhile, has been all-in on HAMR and has also launched 20 TB technology-based HDT drives for business users (these HAMR drives are yet to appear in retail). Western Digital, meanwhile, promised MAMR drives for 16TB + HDDs, but eventually switched back to energy-enhanced PMR (ePMR). Toshiba, on the other hand, introduced flow control-MAMR (FC-MAMR) in its MG09 series of business 16TB and 18TB HDDs.

U HDD Reimagine event Western Western Digital presents OptiNAND – a new architecture that includes the integration of the built-in iNAND UFS built-in flash drive (EFD) on the drive motherboard.

Together, the company also announces that it has sampled its first 20TB non-SMR drives based on optiNAND-enabled ePMR for customer selection, and will adopt the OptiNAND platform for all 20TB + HDDs. The company also sees a path to 50TB of OptiNAND-enabled ePMR drives in the second half of the decade.

Although the company has not quantified the amount of NAND in its OptiNAND drives, they emphasize the fact that it is not a hybrid drive (SSHD). Unlike SSHDs, OptiNAND drives do not store any user data at all during normal operation. Instead, NAND is used to save metadata from HDD operation to improve capacity, performance, and reliability.


OptiNAND’s announcement by Western Digital also conveys the fact that their 20TB 9-disc HDT drives will continue to use energy-enhanced PMR (ePMR). In addition to using a three-stage actuator to allow more precise positioning of the heads above the tracks, the OptiNAND aspect is promoted as a key to enable a capacity of 2.2 TB for each board.

An increase in areal density is achieved by accumulating songs on the board (increased TPI), while some metadata (factory-generated and working with the middle user) is moved from the board to NAND. Western Digital specifically mentioned the Repeated Ejection Recording (RRO) recording of the position of the vibration / error of the head as the spindle rotates. This data (multi-gigabyte) is generated at the factory during production. It is usually stored on disk, taking up space that could potentially be used for user data. The OptiNAND architecture moves this to NAND in EFD.

One of the key challenges for track packing is the concept of “adjacent track interference” (ATI). This results in the need for the data in the boards to be refreshed periodically as they can be corrupted by writing to adjacent records. Currently available HDDs have launched this track-based refresh based on recording write-level write operations. One of the disadvantages of increasing surface density by increasing TPI is the need for more frequent refreshments. Since refreshing once in 10,000 write operations on early HDDs, narrow records now need to be refreshed as often as once in 6 records. After a certain point, it makes no sense to further increase the TPI because increasing the ATI refresh rate has a tremendous impact on performance. In current-generation HDDs, this refresh is triggered at the record level by recording write operations in that hierarchy. The OptiNAND architecture allows sector-level writing operations to be recorded. This means that refresh operations are more widespread both temporally and spatially, allowing songs to blend harmoniously without sacrificing performance. In turn, this increases areal density.


Consumers can manage HDDs with record cache enabled or disabled. Regardless of enabling the cache, the HDD must buffer incoming data. In the disabled case, the amount of data that could be stored in the clipboard depends on the amount of data that can be safely emptied into permanent storage in the event of an emergency shutdown (EPO). The presence of significant NAND capacity on the hard drive means that the drive can use the rotation energy present on the boards to flush more data into DRAM NAND (today’s HDDs output DRAM data to serial flash – about a few MB worth – in an EPO situation). The multi-buffer capability in this case means that the performance of the cache-enabled case and the cache-disabled case are closer to each other on OptiNAND-enabled HDDs.

Western Digital also claims that the “write cache enabled” case can benefit from performance. This is an indirect result of the reduced refresh rate (referring to the observations in the previous subsection on how OptiNAND resolves interference on the adjacent track) that allows the HDD to spend more time servicing user data requests. Again, there was no quantification of IOPS improvements for different access patterns compared to non-optical HDDs in the Western Digital event.


Aspects of OptiNAND that are used to improve the performance of drives in the state of disabled caching of records also contribute to increasing their reliability in EPO conditions. Including faster non-volatile storage compared to serial flash, Western Digital claims that up to 50 times more data can be sent compared to previous-generation HDDs.

Concluding remarks

Western Digital claims that vertical integration is possible with HDD technology on the WD / HGST side along with flash technology on the SanDisk side necessary to create a platform like OptiNAND.

Due to the integration of NAND, there must be a cost premium associated with the plants. New recording technologies (such as HAMR and MAMR) require significant investment in the design of recording heads as well as plates, and need to be renewed every few generations. On the other hand, technologies like OptiNAND are independent of the underlying technology.

Without an accurate quantification of the increase in surface density provided by OptiNAND, it is not possible to make comparative comments on Capacity aspect of OptiNAND trifecta Western Digital – except that the company is now able to introduce 20 TB hard drives on the market with the same ePMR technology used in its 18 TB drives (about 2.2 TB / plate).

The Performance the aspect should be easier to assess when OptiNAND drives arrive at retail. While the benefits of the ‘write caching disabled’ case (where NAND can act as a secure cache in an EPO situation) are easy to verify (essentially acting the same as the ‘cache write enabled’ case), a pure ‘caching’ write enabled ‘case should be much more interesting for analysis compared to competing plants of the same capacity.

Western Digital has indicated that all of their advanced 20TB + HDT drives will be equipped with OptiNAND. This will apply to all market verticals – cloud deployment, business facilities (gold), surveillance storage (purple line) and NAS (red line). It must be noted that the company already has a 20TB SMT drive on the market that is not enabled by OptiNAND. The new HDD architecture with flexible SoC and high-performance NAND integration can also be used to enable user-specific improvements in the future. The ability to use NAND for dynamic sector mapping can increase areal density and improve performance much more in SMR drives. Based on this, we can expect that OptiNAND-enabled SMR drives will gain a significant advantage over CMR drives over what is currently seen in the market.

The HDD industry does not yet need CPR, but Western Digital uses OptiNAND to solve the problem Ccapacity, Pperformance and RThe reliability trifecta is another unique aspect of the innovation-rich hard drive market. Western Digital also has HDD and full flash technology (from NAND creation to controller), while other HDD manufacturers don’t have that advantage. Therefore, it may take some time for other vendors to see the benefits of using NAND for HDD metadata.

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Naveen Kumar

Friendly communicator. Music maven. Explorer. Pop culture trailblazer. Social media practitioner.

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