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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  • Notmyusualid - Wednesday, July 06, 2016 - link

    Just lost my Seagate 4TB 2.5" internal disk last week.

    Luckily my most precious things were backed-up elsewhere, but dam, 2.4TB lost.
    Reply
  • jwcalla - Thursday, July 07, 2016 - link

    I'm honestly fed up with the poor reliability of HDDs. Of the three ones I still had in service, two are dead, and I'll never buy those two brands again. Reply
  • Michael Bay - Thursday, July 07, 2016 - link

    Out of not so idle interest, what were they? Reply
  • kamm2 - Thursday, July 07, 2016 - link

    This was in the back of my mind the whole time I read this article. What good is any of this if the damn drives keep dying? Maybe things are different on the enterprise side but I've given up on Seagate drives. Reply
  • jbrizz - Wednesday, July 06, 2016 - link

    Can we just have 5.25 inch hard drives again? I don't care so much about density at home, but I need to hoard more files! Reply
  • rstuart - Wednesday, July 06, 2016 - link

    I like most have move to use SSD exclusively for the, hmm, "boot drive", But I do use HDD's for media storage where only the biggest highest density drive works. SSD's are currently about an order of magnitude more expensive per byte in this area. At a wild guess SSD's might reach price parity in a decade, but for now the HDD's are the only sane choice.

    I'm currently struggling to fit in 6TB. My guess is 10Tb would be enough for the foreseeable future, and 20Tb would cover everything I am every likely to need. So I rather pleased to see Mr Re say they will hit 20Tb in a few years.
    Reply
  • patrickjp93 - Wednesday, July 06, 2016 - link

    Samsung is hitting 15TB this year, so Seagate is done in enterprise. The performance/watt/$ is just vastly superior for SSDs. Once storage density is also in the wheelhouse of SSDs, the scales will tip and never go back. Reply
  • gospadin - Thursday, July 07, 2016 - link

    absolute density (TB/m3) is already in SSD's advantage

    The biggest 2.5" HDD you can buy today is 2TB. In that form factor, Samsung is currently selling 4TB SSD, with 8TB/16TB drives announced.
    Reply
  • Lolimaster - Friday, July 08, 2016 - link

    Do like me:

    4x6TB WD blues, so should be covers for some time.
    Reply
  • anactoraaron - Thursday, July 07, 2016 - link

    This paragraph was enlightening -
    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.

    Which explains why my 5tb seagate with this tech can't seem to get past 40MB/s when writing to the drive.

    Then read this - Ultimately, environments that involve a decent amount of writing might not be impressed with SMR performance, but the key figure here is density.

    Is anyone getting a high capacity drive going to be impressed with 2-40 MB/s?

    No. No they will not.
    Reply

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