Western Digital reinvents (hybrid) HDD

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Western Digital manufactures both “traditional” hard disk drives (HDD) and flash drives. Nobody else in the market does this. So it makes pretty good sense for it to bring a hybrid drive to market. However, it is a fundamentally different hybrid drive than we have seen before.

The concept of the hybrid drive, which combines HDD and flash, is certainly not new. The first models came to market just after flash had become reasonably available. However, those were drives where both HDD and flash contained user data. The idea was that frequently used data would be written to the flash part, while data you needed less often would be put on the HDD part.

The new type of hybrid drive announced by Western Digital today does not resemble the “old-fashioned” hybrid drive in any way. In fact, Western Digital itself is not talking about hybrid drives, but flash-enhanced drives. We explain below what the company means by this.

Embedded flash for more capacity

The new flash-enhanced (hybrid) drives consist of a traditional component, namely the platters in the HDD, and a flash portion. However, the flash part is not used to write user data to it. Western Digital uses it to increase the TPI of the platters in the drive. TPI stands for Tracks Per Inch, the number of tracks per inch on which the drive can write data. These tracks can be put closer together through clever use of metadata stored in the embedded iNAND flash. As a result, the areal density of the platters, and thus of the drive as a whole, goes up. The SoC in the disk also plays an important role in this, of course. Western Digital optimized this to interact with the iNAND.

In addition to more capacity, the new drives also offer higher performance, in part by reducing adjacent track interference (ATI). This is of course necessary when you put tracks closer together. The reduction of this interference also seems to be partly due to the addition of flash memory to the new type of drive. That is, Western Digital speaks of “vertically integrated iNAND and SoC innovations’. This seems to indicate that the iNAND flash (called OptiNAND in these new drives, by the way) also works together with the SoC here. The same goes for the reduction in the number of write cache flushes in the new drives. That too seems to be the result of the cooperation between iNAND and SoC.

The fact that smarter controls increase performance isn’t new for HDDs, of course. Earlier this year, Western Digital presented an interesting case in point.

Finally, Western Digital claims that the new flash-enhanced drives have become a lot more reliable than before. Especially during power outages, it is important that HDDs lose as little data as possible. Western Digital claims that the new drives retain up to 50 times more customer data than previous generations. Again, the flash-enhanced nature of the drives plays a role in this.

Flash-enhanced and ePMR

We briefly discussed TPI as a way to get more data onto a HDD’s platter above. However, there is another way to accomplish this, and that is through the BPI route. BPI stands for Bits Per Inch and is about how many bits of data you can fit per (square) inch on a platter. In this area, Western Digital has also recently developed something new, namely ePMR. This is an extension of PMR, or Perpendicular Magnetic Recording. The “e” added by Western Digital stands for “energy-assisted.”

In PMR, an electric current creates a magnetic field. This magnetic field causes the read/write head to become magnetic. It can then move the magnetic granules in the platter in a certain direction, thereby creating a stored bit. By passing an additional current through it, ePMR makes this process even more precise. The result is that the drive can write bits closer together. This also means that an HDD can fit more bits per inch on a track, resulting in an increased the disk capacity. Western Digital already achieved 2.2 TB per platter, and the first 20TB disks have already been delivered to customers. In the second half of this decade, Western Digital expects to grow to 50TB drives.

The new drives announced by Western Digital today combine enhanced TPI performance with the addition of OptiNAND with improved BPI performance resulting from ePMR. The target audience for this new type of drive is the enterprise market, from cloud companies and service providers through video surveillance companies to NAS vendors. The new drives will be available in different variants. More clarity on this will come later this year.