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
POST A COMMENT

91 Comments

View All Comments

  • abrowne1993 - Wednesday, July 06, 2016 - link

    Is that all for one second of CGI in the new Transformers movie? Reply
  • Michael Bay - Thursday, July 07, 2016 - link

    And some porn. Reply
  • Holliday75 - Friday, July 08, 2016 - link

    Is Megan Fox back? She could probably use the cash. Reply
  • JlHADJOE - Monday, July 11, 2016 - link

    8 Terra bits? So 1TB =) Reply
  • wumpus - Thursday, July 07, 2016 - link

    @JoeyJoJo123
    >It's ogre. HDDs are dead. SSDs won.
    >Just give up.

    Know how I know your data isn't backed up? There are two types of people. Those who have lost all their data, and those who are going to (possibly again).

    Sure, you might not know how Amazon is storing your data, but I'll bet that it just isn't backed up.

    Hard drives just moved up the data hierarchy. They are now stuck between tape (near the cost/TB without the huge entry costs) and SSD. They are also still ideal for NAS, especially consumer grade to fairly decent grade. Don't forget all the random access advantages HDDs have over tape.

    I also expect to watch the whole HDD/SSD dance play over again with SDD/3dxpoint. Although the more interesting story will be if 3dxpoint will be able to replace main/virtual memory, leaving multi-GB DRAM "caches" between the CPU and "main memory".

    Perhaps Mr. Ogre needs to consult with the tri-lambs. I'm sure they could find some uses for HDDs.
    Reply
  • Nagorak - Monday, July 11, 2016 - link

    I'm having difficulty understanding your comment but if you're implying that data stored on AWS isn't backed up then I find that almost inconceivable. Reply
  • hectorsm - Thursday, July 07, 2016 - link

    You're delusional. Today ~99% of PC shipments still have HDDs. Reply
  • romrunning - Wednesday, July 06, 2016 - link

    SMR was never a technology that ever interested me. I don't want more storage with measurably worse performance than existing ones. Seagate should have invested more into SSD tech than in SMR for HDDs. Reply
  • JoeyJoJo123 - Wednesday, July 06, 2016 - link

    You can do both. SMR for the platters with additional flash cache for frequently accessed data. Unfortunately HDDs are a one trick pony now, and they need to push in the one area they're better than SSDs in (relative amount of storage per $ spent). Reply
  • extide - Thursday, July 07, 2016 - link

    Yeah I always thought the combo of a SMR drive + some NAND cache was like super obvious, and I am wondering why we haven't seen more drives like that. Reply

Log in

Don't have an account? Sign up now