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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  • nandnandnand - Wednesday, July 06, 2016 - link

    They explained how to mitigate SMR performance issues in the article. Reply
  • romrunning - Wednesday, July 06, 2016 - link

    Sure, you can try to mitigate the performance, but I feel the initial performance design should have been aiming higher than existing PMR performance. That's why SMR is disappointing to me. I feel like they were dumping R&D into HAMR, and they weren't getting results as fast as they wanted. So they go the SMR route to get the desired results (more storage density) in the interim.

    It's not like SSDs. While SSDs were lower in capacity initially, the speed increase was dramatically better. SMR-based drives are only slightly better in capacity, but they are more noticeably worse in performance. Thus it didn't have the same impact - it doesn't have enough of the "wow" speed factor to help overlook its performance shortcomings.

    It's like when Honda introduced the new Insight hybrid. It wasn't better or even at the same level of "MPG" as the Prius; that's why it didn't fare well.
    Reply
  • JimmiG - Thursday, July 07, 2016 - link

    Well, SMR is a stopgap solution, but it's here now and it works. HAMR sounds great on paper, but show me where I can buy an 8 TB HAMR drive today, for under $250?

    My 8TB Seagate Archive has the lowest cost/GB of any drive at the time, and it works fine as a secondary storage/backup drive. Just manually dumping files to it or running scheduled backups work great, with performance that doesn't really "feel" any different than my 5900 RPM 4 TB PMR drive.
    Reply
  • romrunning - Wednesday, July 06, 2016 - link

    I also say this coming from someone who liked the Seagate 600 Pro SSD. They could have done a lot more years ago to compete for the market lead in consumer SSDs; basically Samsung dominates right now. Reply
  • StormyParis - Wednesday, July 06, 2016 - link

    Even as an individuaI, I mostly don't care about storage performance. OS and apps and "live" data files get an SSD. That <5% of my storage. Media, archives and backups get an HDD. That's >95% of my storage. I don't care about performance for SSD nor HDD, both are OK for the uses I dedicate them too even in their cheapest crappiest incarnations.
    I'm more interested in cost, space, reliability, and durability.
    Reply
  • serendip - Friday, July 08, 2016 - link

    Amen to that, beyond the tiny but vocal enthusiast community there are loads of users who want cheap, huge and reliable storage. I hope the SSD and HDD manufacturers don't forget this market and just concentrate on enthusiasts or enterprise customers. Reply
  • paulemannsen - Saturday, July 09, 2016 - link

    +1. I could even live with 10/10 mb read/write speeds, just give me more reliability and space. Reply
  • Nozuka - Wednesday, July 06, 2016 - link

    There are already 13TB SSDs... currently extremely expensive, but it's only a matter of time... Reply
  • XZerg - Wednesday, July 06, 2016 - link

    you mean 16TB - from Samsung - http://gizmodo.com/samsungs-16tb-ssd-is-now-an-act... Reply
  • Lolimaster - Friday, July 08, 2016 - link

    The 13TB was from fixstars (japan based). They were the 1st in multiterabyte SSDs 3-6-10-13TB. Reply

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