A study of high-end hard drives based on drive track technology by Blocks & Files shows that while SMR (Shingled Magnetic Recording) does offer some benefits, these improvements can fall off massively at the high-end of storage technology, especially as HAMR's layout (Heat-Assisted Magnetic Recording) makes it harder to save space. This is illustrated in the hypothetical diagram below.
The example drives pointed out by Blocks & Files exist on the current high-end of HDD storage. 24 TB HDDs from Seagate and Western Digital using conventional PMR (Perpendicular Magnetic reading) become 28 TB when upgraded to SMR, which offers a 16.66% increase in capacity. This is roughly in line with the upgrade we typically expect from SMR.
Where things get more interesting is when the 30 TB Seagate Exos, a HAMR drive, gets a SMR upgrade. HAMR drive tracks are closer together and narrower than PMR, and have a different ratio of read track width to write track width. Ultimately, this means that SMR applied to PMR nets an up to 17% capacity increase— while SMR applied to the newer HAMR drive tech tops out at a mere 7% improvement, at least with the current high-end of HDDs.
This difference between HAMR and PMR drives with and without SMR enabled is what's illustrated in the above diagram. Each PMR and HAMR example takes up the same amount of space, but PMR can manage 6 shingled tracks in the same physical space of 4 non-shingled PMR tracks, while HAMR can only manage 5.
According to Jason Zimmerman, Seagate's Sr. Director of Product Line Management whom Blocks & Files contacted for its article, "The Mozaic 3+ product offerings are not necessarily completely indicative of the overall SMR vs CMR (PMR) gains at a fundamental technology level." He says the company is seeing roughly a 10% capacity gain when opting for SMR over PMR, so the 6% gain isn't fixed or always what we should expect in terms of capacity gains with SMR on HAMR drives.
"The observation is generally correct that with higher TPI (narrower tracks) on any CMR will result in less SMR gains, Zimmerman continues. "However, this relationship is independent of recording technology (HAMR or PMR) [...] The label 'HAMR lowers shingling capacity addition', does not seem fundamentally true."
Since Seagate seems to insist that HAMR itself isn't to blame, the most likely culprit at this point is the sheer density of these ultra-high capacity HDDs. With narrower tracks required to increase storage capacity and less vibration tolerance in turn, the gains of HAMR with SMR are running into some hard limits compared to traditional, albeit lower-capacity, CMR drives with SMR. In other words, for platter-based storage to continue significantly increasing in capacity, it seems likely that some new density-increasing tech tricks will need to be invented and enabled, or mechanical storage is going to need a more fundamental technological rethink.
