SMR (Shingled Magnetic Recording) increases storage density

Our world is becoming more and more data-intensive – just think of all the mobile devices, streaming applications, cloud services, and servers we use. Estimates suggest that the average household will soon be consuming one terabyte of data per month! But what is going to fulfill this growing demand for more storage space? Well, Shingled Magnetic Recording (SMR) could be the answer. But what exactly is it? How does it work? And how much data can SMR drives actually store? Let’s find out.

The track to Singled Magnetic Recording (SMR)

For a long time, it seemed as though four terabytes was the absolute maximum that hard drives could handle, and no significant advances were made. Then along came Shingled Magnetic Recording, or SMR for short, making eight terabyte disks a reality – and an affordable one at that. The best thing about SMR hard drives is their improved areal density. Physically, they aren’t any bigger than other drives, yet they offer over 25% more storage capacity.

Note

To understand SMR, you need to be familiar with the geometry of a hard drive. Hard drives contain platters that are divided into cylinders, tracks, and sectors. When data is written to the drive, it is broken down into little blocks and stored in the sectors. The platters are double-sided disks, with a read/write head for each side. The surface of each platter is split into multiple concentric tracks which, stacked together, form cylinders.

So, what is SMR? Put simply, Shingled Magnetic Recording is a hard drive technology that reduces the space between adjacent tracks on the platters. This is achieved by making the tracks overlap one another like roof shingles – hence the name. Thanks to this layering, more data can be stored in the same physical space. In SMR drives, the read heads are significantly narrower than the write heads, meaning that all of the data can still be read despite the overlapping tracks.

Although the technology is still new, it’s very promising and could soon replace the current standard, Perpendicular Magnetic Recording (PMR). In PMR drives, the tracks are arranged parallel to one another, and the poles of the magnetic elements (which represent bits of data) are perpendicular to the surface of the platter. The magnetic tracks do not overlap one another, so the only way of increasing the storage capacity is to increase the physical size of the drive. This once again highlights the tremendous potential of Shingled Magnetic Recording. SMR is an extension of PMR and offers greater areal density. From a physical point of view, SMR drives look just like their PMR counterparts, but they have much more storage space inside. However, given the difference in architecture between PMR and SMR drives, the user experience is different too.

SMR hard drives – features

SMR drives take a new approach to overcoming the physical limitations of conventional drives. The first difference is that the read/write heads are fairly wide. In fact, they’re much wider than the tracks themselves. Data is therefore written to the tracks sequentially, and the existing tracks are partially overwritten. However, because of the overlapping, if adjacent tracks contain data, they also have to be overwritten each time. In theory, this would mean overwriting the entire SMR drive whenever a single byte of data was modified, but the process would then take so long that Shingled Magnetic Recording would simply be unusable.

To get around this dilemma, SMR tracks are divided into “zones” or “bands”. This limits the number of tracks that can overlap, so fewer tracks have to be overwritten each time. In addition, each platter has an internal buffer zone for temporary storage, and a cache. Data is initially stored in these two temporary areas and only written to the correct positions on the track later on. This explains why SMR drives often keep running for a long time after a write operation.

Shingled Magnetic Recording uses a highly powerful write element to create very narrow tracks and achieve greater data density. This makes it ideal for continually writing data to a hard drive in long sequential writes.

How does SMR compare to other storage technologies?

SMR is still a relatively new technology and one disadvantage is that hard drive manufacturers do not always indicate which of their drives use it. As a result, it often turns out that hard drives that are labeled as being suitable for NAS actually use Shingled Magnetic Recording. Now in theory you can use an SMR drive in a NAS environment, but with changing workload requirements, the inherent technical limitations of these drives quickly become apparent.

If you’re using several hard drives, for example to upgrade a server with SSD, it’s best to avoid combining PMR and SMR drives, because some users have reported performance issues with certain models.

In principle you can use an SMR drive just like any other hard drive. You can format it in FAT32 and run it with Windows for example. Compared with a conventional PMR hard drive, for the reasons mentioned above, the data transfer rate is very erratic, especially when writes are irregular. However, there is no negative impact on speed if the processes remain linear, and for reads there is no noticeable difference at all. Although Shingled Magnetic Recording is better than its reputation might lead you to believe, that won’t be enough to end the dominance of SSD hard drives when it comes to gaming and programming. But that isn’t really what SMR is intended for.

Where is SMR used?

SMR really comes into its own when a system continuously writes data to the drive. These kinds of long sequential writes are necessary for server applications, backups, and archiving, for instance in cloud archives and object storage architectures.

For private use, for example as a large hard drive for your home computer, SMR drives are not recommended because they generate more noise and heat, and consume a lot of energy. These factors are due to the reorganization process described above. At first, new data is stored in an empty part of the platter and the old data is left on the existing tracks. Then, when the SMR drive goes into idle mode, the old data bits are replaced by the new ones. This means that a drive that uses Shingled Magnetic Recording needs idle time between subsequent operations. If it has to continuously write data to random locations on the disk (known as “Random Write”) over a longer period, the drive can become noticeably slower.

SMR drive manufacturers are working to develop firmware to address this issue, but for now, SSD drives are still the best option for most private users, especially since they are becoming more affordable.

Nonetheless, if you want to create a data archive on your server, SMR is a good solution. It saves you (physical) space and is cost-effective, and reliable.


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