Seagate to Expand Usage of SMR

Today, the vast majority of HDDs are based on perpendicular magnetic recording (PMR) technology, which is sufficient for today’s applications in terms of areal density and performance. Several years ago makers of hard drives believed that PMR technology would not support areal densities of over 1 Tbit per square inch (Tb/inch2) because of physical limitations and yields. However, in the last couple of years, a lot of progress has been made and it looks like PMR technology will continue to evolve towards that goal (albeit slowly).

To increase areal densities significantly, Seagate started to use shingled magnetic recording (SMR) technology several years ago. The SMR technology enables areal densities higher than 1 Tb/inch2, but brings a number of challenges. HDDs that use shingled recording write new tracks that overlap part of the previously written magnetic tracks. The overlapping tracks may slow down writing because the architecture requires HDDs to write the new data and then rewrite nearby tracks as well. For this reason, Seagate’s implementation of device-managed SMR groups adjacent tracks into bands, where shingling ends. This optimizes the number of tracks that need to be rewritten after writing operations and thus promises to help provide deterministic and predictable performance of SMR HDDs in typical scenarios. Ultimately, environments that involve a decent amount of writing might not be impressed with SMR performance, but the key figure here is density.

Grouping into bands is not the only way to conceal peculiarities of SMR. In fact, every SMR drive has zones that use PMR recording technology with relatively fast writes. Those zones are used to quickly record data and perform other necessary operations when needed. Eventually, information from PMR zones is automatically moved to SMR zones without any actions from the user or the operating system. One can think about it as some sort of garbage collection that needs to be triggered by the firmware. Seagate does not disclose actual configurations of its SMR bands or capacity of PMR zones, but notes that such configurations depend on types of applications that the HDDs are designed for (i.e., consumer drives and drives for cold storage have different configurations).

To further ensure optimal writing performance, SMR-based HDDs can also integrate DRAM and/or NAND flash buffers. For example, Seagate’s Mobile 2.5”/7mm hard drive with 2 TB capacity has a 128 MB DRAM cache and an unspecified amount of SLC NAND flash memory. The SLC NAND buffer has a rather high writing performance, which means that when small amounts of data are recorded on an SMR-based drive, the latter can boast with a very high write speed. Since the amount of NAND flash is not very high (less than one gigabyte in the case of the mobile 2.5” 2 TB HDD), it does not help a lot with large files, but for a typical home user storage environment it should be helpful.

One of the areas Seagate is proud of is the iterative product design for optimizing writing performance of SMR-based drives since the company first introduced them several years ago. One might argue that the claimed performance numbers for the Seagate Archive 8 TB and Seagate Mobile 2 TB are not that impressive. This hides the implementation of SMR management in the Seagate Mobile 2 TB, which involves three levels of caches/buffers (DRAM, NAND, PMR zones), and demonstrates the complexity of such HDDs. The architecture of SMR-based consumer drives requires controllers with advanced computing features to manage buffers, transfer data from PMR zones to SMR zones and perform other operations to guarantee expected performance in different workloads. We have seen similar problems with TLC NAND-based SSDs, which use pseudo-SLC buffers to ensure fast writes. Depending on Seagate’s plans for the future, the device-managed SMR HDD architecture seems to be expandable for future performance benefits.

Seagate plans to adopt SMR rather widely going forward. In the near future, Seagate will introduce SMR-based HDDs specifically for video surveillance applications (Western Digital's Purple line of HDDs spring to mind as the competition there). Later on, more hard drives featuring “shingled” platters for client PCs can also be expected. We are not sure whether SMR-based HDDs are set to be offered to performance-demanding applications given the evolution of PMR and inevitable emergence of other technologies, but we might see hybrid variants that a partial SMR and partial PMR to keep performance high. Still, Seagate made it clear that SMR is not reserved for cold storage.

The Evolution Continues, New Challenges Arise Helium Will Remain Exclusive for High-Capacity Applications, For Now
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  • Zak - Wednesday, July 06, 2016 - link

    Forget enterprise. I use 4GB drives as local backups and planning to go up to 6 or 8. Show me affordable 8TB SSD I can use for backup. Reply
  • inighthawki - Wednesday, July 06, 2016 - link

    8TB no, but I'm sure I can find you a few good 4GB drives :) Reply
  • cm2187 - Wednesday, July 06, 2016 - link

    WD Reds? Reply
  • cm2187 - Wednesday, July 06, 2016 - link

    Actually even cheaper if it is for backups: seagate 8tb archive drives. Reply
  • Samus - Wednesday, July 06, 2016 - link

    First off, GREAT article Anton. This is what AT is all about.

    I don't have a single HDD in my house anymore. Between 11TB on AWS and 800GB in OneDrive, it all comes down to the data centers which will all be using this technology.

    Meanwhile the 480GB SSD's that cost $100 running my PC's and laptop have made magnetic storage irrelevant for my consumer use, so who can blame Seagate for not targeting me?
    Reply
  • trivor - Thursday, July 07, 2016 - link

    What everyone seems to be missing it NASes for the home with LARGE MEDIA collections. When you're looking at 2 GB for DVD rips and 4-5 GB for Blu Ray rips you need Terabytes of storage for $30/terabyte (or less hopefully) that SSDs can't touch. Even for full Blu Ray rips (some people want this) you're still looking at only needing 50 Mbps without any compression and even a lossless rip with Makemkv will take it down to 20 GB and will easily stream from a NAS with any decent spinning drive. When SSDs which are currently around $200/GB (for a consumer commercially available drive) to compete with spinning drives (say 3 TB @ $94 for a Toshiba or 3 TB for WD Red @ $109) then we won't see much in the consumer space. Not to even talk about 8 TB drives for around $200-$250. We are a long way from the demise of consumer spinning drives. Reply
  • CaedenV - Thursday, July 07, 2016 - link

    No kidding! I love my SSDs, but they are not going in my Nas any time soon. I have 5 3tb drives in a raid 6...that would cost a mint in SSDs still. Maybe I'll get there eventually, but it is going to be a long time.

    Still, it is a sin to sell a pc with a hdd as a system drive these days. Really wish manufacturers would stop that
    Reply
  • JlHADJOE - Wednesday, July 06, 2016 - link

    IBM showed us that magnetic storage can store a bit using as few as 12 atoms. That's far denser than any type of memory developed so far.

    http://www.wired.com/2012/01/ibm-scientists/

    SSDs will replace HDDs for most of the consumer market, but HDDs will stay around for bulk data.
    Reply
  • Cygni - Wednesday, July 06, 2016 - link

    It's 'ogre'? Is shrek around or something?

    Also if you read the article, you will see that this isn't exactly focused at the same market as enthusiast SSDs.
    Reply
  • Michael Bay - Wednesday, July 06, 2016 - link

    Tell that to 8Tb of media I have copying to the new HDD now. Reply

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