Pushing the boundaries with SMR

November 24, 2014 | Engineering

Recently, we posted a blog about 4Kn and how to properly take advantage of the technology for performance gains and efficiency. On a related storage topic, I want to take a moment to introduce Shingled Magnetic Recording, or SMR. SMR is yet another technology developed to tackle the push for higher and higher storage densities. Major hard drive manufacturers are investing in these drives for the long haul, and while this new breed of drive is laser-focused on capacity, the trade-off is performance. If you’re wondering if SMR would be a good fit for your application, here are a few things you should know.

Is SMR the right choice for your application?

An SMR drive differs from a normal drive in that the tracks of sectors (the sequential path of data area) overlap one another, allowing for vastly more capacity on the same physical media. This of course brings some new challenges to the table. Much like a misaligned partition, an SMR drive write typically will cause the drive to read larger areas of sectors, update the data, then rewrite those sectors to maintain the existing data while merging the new data onto the drive. Because the tracks are overlapped, a single write could have severe performance implications as it could affect large volumes of physically proximate sectors. Think of them like shingles on your roof: unless you are talking about the ones on the eave, you can’t simply replace one without affecting the surrounding shingles.

This sort of inherent inefficiency speaks to the ideal use case of such a drive. Applications requiring high bandwidth random write scenarios, for example, are not well suited for an SMR drive. Where an SMR drive really shines is with sequential read/write situations where data is not overwritten frequently or where latency is not a key factor. Think of them as your massive cold storage device: you trade speed and flexibility for unparalleled storage capacity.

There are three basic types of SMR usage scenarios.

New drives, new tools. While the ‘drive-managed’ (more information to follow) SMR drives will essentially be a drop in replacement for standard drives, there are a few new scenarios that can hold promise for unlocking the potential of an SMR drive. These tools and frameworks will require a bit more on the implementation side but can provide a new level of control that was previously abstracted away in traditional hard disk drives.

The first type of SMR is drive-managed. The ideal use case for drive-managed SMR disks is archival storage or storage where performance is not a critical issue. A drive-managed SMR drive will work like any other drive, but much like an SSD, it may have inconsistent performance depending on the workload. For example, random writes are generally a terrible match for an SMR drive since you will typically be invoking a massive number of read-modify-write operations on the disk to write your small random data blocks. The good news is that a drive-managed scenario ensures that you have a backward compatible data store.

The second type is host-aware. This type of SMR has the potential to provide better performance and efficiency at the expense of implementation effort. A host-aware setup involves an SMR drive that can manage itself with some structure and preemption from the host system. The driver and/or storage framework on the system can preemptively query the drive and locate optimal places to write data that will cause the least amount of rework and yield the best performance. In order to do this, the OS will have to use the requisite SMR commands to communicate to the drive. In the event the OS issues a non-optimal request, the drive will execute the request, but performance may be impacted. Host-aware SMR volumes are also backward compatible with non-host-aware systems.

Lastly, we have host-managed.  Host-managed SMR has the potential to give the user total control over the drive operation. In the right hands, this could be a boon to extracting the most from the SMR platform.  Using the SMR commands, the host will order all data and requests to the drive. The drive can refuse any non-optimal commands which could be an issue if implementation of the SMR command is poor. Also note that host-managed SMR volumes are not backward compatible with non-host-managed systems.

The Future of SMR

We have a feeling there will be more discussion about SMR in the future, as the technology and tools surrounding it are still in their early stages. Don’t let this scare you off, however. The major players in the industry, Seagate and HGST, are 100% committed to SMR as a mainstay storage technology, and it isn’t just a passing fad. The tools they have available right now are robust and perform as described, though obviously, much consideration for performance, cost, and density must be made on a case-by-case basis.

Have questions? We’ve got answers.

As always, MBX engineering staff is always on hand should you want to discuss the migration to 4K, SMR, or anything else technology related. We live for this kind of thing, and every step into the future is lined with excitement for mastery and what might come next!

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