How Shingled Magnetic Recording (SMR) Drives Up Data Center Capacity

As high-cap hard drive demand rises in tandem with the recent surge in data center construction, Seagate, Western Digital, and Toshiba are pressing ahead with ambitious HDD capacity roadmaps. With Heat-Assisted Magnetic Recording (HAMR) technology now delivering long-promised growth in areal density, the future of HDDs looks bright. But HAMR isn’t the whole story: Shingled Magnetic Recording (SMR) is a crucial component of many state-of-the-art drive designs. 

While the drawbacks of SMR drives are well-known, especially with regard to write speeds, shingled magnetic recording can help reduce total cost of ownership for the right use cases. Here’s a look at the impact of SMR drives in today’s data centers.

What is SMR (Shingled Magnetic Recording)?

Perpendicular Magnetic Recording (PMR) drives work by using a write head to flip bits on magnetic tracks. These tracks run side-by-side, with no overlap. 

In SMR drives, new tracks overlap with parts of previously written tracks like shingles on a roof. This doesn’t affect read speeds, because the read head can simply read the portion of the track which isn’t overlapped by any others. 

Reads are typically performed by an “MR head.” First developed in the 1990s, these heads use the magneto-resistant effect to reliably read small magnetic features. The upshot is that modern read heads are effective even when reading a narrow strip of the whole track.

This small difference in structure yields major differences in write behavior. When editing data, SMR drives don’t immediately overwrite tracks. Instead, like SSDs, they write the new data on empty tracks, and discard the overwritten data later during the drive’s idle time. 

Also like SSDs, SMR drives are much better at sequential writes than random writes. For a hard drive, sequential writing involves writing out whole bands, or “zones,” of tracks at once. The consequence is that SMR can’t take advantage of the increased flexibility random writes allow. Given these restrictions, SMR drives need system-level management in order to get the most out of them.

Shingled Magnetic Recording: The Drawbacks

Write Speed

It’s no secret that SMR drives pose certain challenges.

The biggest challenge is write speed. A PMR drive will write somewhere between 80 and 160 MB/s. An SMR drive is less predictable一it can write quite quickly onto a clean drive, but if it has too many write tasks queued, or has insufficient idle time to reorganize or discard overwritten data, then write speeds can be significantly lower than 80 MB/s. 

Reduced Flexibility

This speed challenge has other consequences. The idle time reduces availability for other tasks. When it comes to use cases, SMR drives are less flexible, working far better at writing data in bulk than performing non-sequential operations which involve editing multiple smaller files. Also, while solid data is hard to come by, anecdotally SMR drives have encountered performance issues when plugged into existing RAID setups. 

Care Needed for Sanitization

Another challenge is sanitization. According to standards such as IEEE 2883 and NIST 800-88, overwriting, block erasure, or cryptographic erasure are all secure methods of data sanitization when done correctly. However, BitRaser, a company that offers data erasure software, discovered a complication.

In a test done in partnership with Stellar, an Indian-based data recovery company, researchers took a 500GB WD SMR drive and used a professional data wiping tool to erase data according to the NIST 800-88 standard. While the hex viewer software revealed no traces of remaining data, detailed testing in Stellar’s laboratory allowed technicians to recover data, including whole images.

BitRaser’s conclusion is that overwriting won’t work on SMR drives. Thankfully, one can just resort to NIST or IEEE Purge Standards such as ATA secure erase and cryptographic erase.

Shingled Magnetic Recording: The Upsides

While SMR drives have some limitations, they come with real upsides.

Most benefits are downstream of the fact that SMR allows increased density. SMR drives can increase storage capacity by around 20% in some cases. This results in reduced cost-per-TB, perhaps the most obvious benefit of SMR drives. For hyperscalers looking for high-capacity drives by the pallet, SMR drives give you more for less. 

At the same time, denser storage helps reduce the cost of setting up and cooling extra racks of drives. This means you can raise capacity without increasing your data center’s energy footprint. In an era of increased public pressure for data centers to improve energy efficiency, that matters. 

Just ask storage service Dropbox, which transitioned to SMR drives back in 2018. Seven years later, 90% of its HDD fleet consists of SMR drives. It estimates that it has increased capacity by 10-20% at no extra cost, while reducing physical footprint. The firm’s power footprint for idle and random read workloads was slashed in half when it first adopted 14TB SMR drives, and its latest drives consume only 0.3 watts/TB idle and 0.5 watts/TB for random reads.

Speed also becomes less of an issue for certain use cases, since SMR drives work well for tasks involving sequential writes. Examples include cloud storage, archival tasks, and storing IoT data such as surveillance footage. SMR drives are particularly good for infrequently accessed storage, where write speed isn’t the most important factor.

HDD Manufacturers Push SMR

While HAMR technology plays a huge role in increasing areal density, SMR remains a central part of plans to increase capacity, and features prominently in the recent roadmaps of the Big Three HDD manufacturers.

Western Digital

SMR is central to Western Digital’s roadmap. While the firm has been using conventional SMR drives since 2013, it now focuses on so-called UltraSMR. This is a new variation on SMR which adds large block encoding and error detection algorithms in order to increase the number of tracks-per-inch on the disk. 

The first UltraSMR drive was the 26TB DC H670 drive, released back in 2022. Today, UltraSMR products are in over 50% of the firm’s portfolio. WD has also announced UltraSMR JBOD platforms for use in AI applications.

We see more and more customers adopting UltraSMR. We have our top three customers fully on board with UltraSMR drives already today, and we have another two to three more that are moving into a process of adopting UltraSMR. So we are likely to see the UltraSMR mix of our total nearline exabyte base continue to increase going forward. –WD CEO Irving Tan.

Seagate and Toshiba

The other two HDD manufacturers have also employed SMR to boost capacity. Toshiba has demonstrated 32TB and 31TB SMR drives. Toshiba’s 32TB drive employs HAMR, while its 31TB drive also employs microwave-assisted magnetic recording (MAMR) respectively. It remains to be seen whether combining SMR with other recording technologies becomes more common, or whether SMR falls by the wayside as HAMR becomes dominant.

Seagate uses SMR in the 32TB and 36TB models of its Exos M series, intended for hyperscalers. The technology is also used in some of its consumer Barracuda drives, and the lite version of its SkyHawk drives. However, Seagate has relied less on SMR than Western Digital. It instead emphasizes HAMR, which figures in many of its high-cap nearline drives.

Shingled HAMR?

With conventional drives, SMR gives a ~20% capacity increase compared to non-SMR versions. However, analyst Chris Mellor at Blocks & Files believes that when comparing SMR HAMR to non-SMR HAMR, the increase is closer to 6.66% for 30+TB drives. He bases this on the fact that Seagate’s 32TB Exos SMR HAMR is only a 2TB increase from the non-SMR version. Mellor conjectures that HAMR drives, being denser, just don’t have as much space to overlap, since the write tracks are already narrower.

When asked by Mellor, Seagate’s Jason Zimmerman agreed that SMR may have diminishing returns as areal density increases, but was quick to point out that track density itself, rather than HAMR specifically, was the culprit. In any case, Zimmerman suggested the capacity gains between SMR HAMR and non-SMR HAMR in Seagate’s Mosaic 3+ platform were more like 10%.

However, it’s worth noting that shingled HAMR is still fairly new. This means it’s probably too soon to say whether diminishing returns on capacity are a robust trend and whether there’s a way to get around it.

Zoned Storage

It’s one thing to say SMR needs careful host management, and another to spell out what this means. Enter zoned storage. 

Both SSDs and SMR drives use sequential writes, with chunks of data written on zones that must be written and erased as one. 

In an SMR drive, tracks generally overlap. However, they don’t always overlap: you can have gaps between different “zones” of shingled tracks.

The zoned nature of SMR drives means that it can benefit from standards which take this into account. Such standards spell out a set of commands, such as OPEN/CLOSE/FINISH ZONE, and REPORT ZONE. In a data center using zoned devices, it’s these standards which make it possible to manage writing and editing in a systematic way, rather than leaving management of zones entirely to the individual drives in which they figure. 

Just which standard is in play depends on the interface. For SCSI, the standard is Zoned Block Command (ZBC), and for ATA, it’s Zoned Device ATA Command Set (ZAC). SSDs can also benefit from zoned treatment: Zoned Namespaces (ZNS) is the relevant standard for NVMe SSDs. 

Zoned storage has a number of benefits. For SMR drives, zoned storage improves performance, optimizes space utilization, and manages complexity in large-scale storage systems. For SSDs, it also cuts down on the need for overprovisioning and minimizes garbage collection cycles. 

Zoned standards are crucial for the scalability of data centers making use of zoned devices. Implementing them can be tricky, however, as it requires updated software and operating systems. In order to speed up the adoption of zoned storage, WD’s Zoned Storage Initiative aims to foster an open-source community which makes such upgrades easier to perform. 

The Outlook for SMR

SMR enables denser storage, allowing manufacturers to ramp up capacity quicker than they otherwise could have. Innovations continues, with breakthroughs like UltraSMR helping to minimize common performance concerns. With appropriate host-management and judicious selection of use cases, even the write speeds become less of an issue.

Low cost has always been a major factor in keeping HDD the backbone of hyperscale data centers. This is especially true at a time when the data center construction boom is raising prices and causing shortages. It’s unsurprising businesses are increasingly finding SMR drives an attractive option.

Learn how Horizon Technology can help you get more out of your data center drives while lowering total cost of ownership (TCO).