SMR (Shingled Magnetic Recording)

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.

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.

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 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.

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.

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.