WD explains hybrid tech behind Black SSHDs

Ever since WD announced its first SSHDs last month, we’ve wondered about the specifics behind the firm’s hybrid technology. Yesterday, the drive maker shed a little more light on the subject, revealing that its Black SSHDs will use iSSD flash components supplied by SanDisk. The SanDisk iSSD is a standalone SATA device meant for mobile and embedded applications; however, WD provided no details about how the iSSD is integrated into its hybrid drive.

Curious, we spoke to Matt Rutledge, Vice President of WD’s client computing group, who was kind enough to explain the hybrid system in greater detail.

Existing hybrid solutions from Seagate and Toshiba manage caching at the drive level, with the firmware determining what gets put in the flash. WD has taken a different path: it uses driver software to control the caching scheme. According to Rutledge, WD wanted its solution to be closer to the user and to the file system. This driver-based approach doesn’t have low-level visibility on how data is organized on the SSHD’s mechanical platters. However, it receives "hints" from the operating system about whether files contain so-called hot data that should be cached. Data for both read and write requests can be cached.

This scheme sounds reminiscent of Intel’s Smart Response Technology, which enables standalone SSDs to work as a cache for mechanical hard drives. While WD has consolidated solid-state and mechanical storage on a single SSHD, those components are still somewhat independent. The SanDisk iSSD has an integrated controller, and so does the mechanical portion of the SSHD. The two controllers are hooked up to a Serial ATA bridge chip that directs the flow of data between them and the host system. What we have here is a highly integrated take on dual-drive caching.

WD’s Black SSHDs will be available with 8-24GB of iSSD flash. Interestingly, the NAND has a dual-mode arrangement similar to Seagate’s Laptop Thin SSHD. While the bulk of the flash is addressed as MLC NAND, some of it is configured as SLC memory. These "SLC zones" should offer better write performance and endurance than the rest of the NAND. We’re waiting to hear back from WD on whether the portion of NAND dedicated to SLC zones is fixed or can be adjusted dynamically.

Although WD’s hybrid development started with an off-the-shelf iSSD, we’re told the firmware has been heavily customized since. WD’s customizations are proprietary, of course. The caching concept is not, and we may see other drive makers introduce products that resemble the Black SSHD. For example, Seagate told us in March that it’s working on a new hybrid drive designed to work in conjunction with Intel’s caching software.

PC makers using the Black SSHD will be able to choose between WD’s proprietary driver and one provided by Intel. The drivers behave similarly, according to Rutledge, but they’re not identical. The WD driver was developed in-house and works with both Windows 7 and 8. In an interesting twist, that driver also employs system memory as part of the caching scheme. We’ll probably have to wait until the Haswell launch next month to find out exactly what Intel is bringing to the table.

For now, WD is restricting its hybrid tech to mobile offerings. Rutledge argued that, unlike slim notebooks, desktop PCs have plenty of room for dual-drive solutions, which blunts the appeal of integrated SSHDs. He has a point. A dual-drive approach gives you the flexibility to run separate system and storage drives—or to combine the two with software-managed caching. That said, Rutledge conceded that a single hybrid drive might have some appeal for the average desktop user. I suspect we’ll see WD offer hybrid desktop drives before too long.

In any event, I’m curious to see how WD’s hybrid tech stacks up against the other implementations on the market. Having up to 24GB of flash on a 5-mm, 500GB notebook drive is certainly tantalizing. SanDisk’s iSSDs can purportedly hit read and write speeds of 450 and 350MB/s, respectively, and Intel’s SRT has already proven that software-based caching can deliver substantial performance gains.

The only potential fly in the ointment is the Black SSHD’s spindle speed. Rutledge wouldn’t put a number on it, contending that rotational speed doesn’t matter for hybrid solutions. That probably means we’re looking at something around 5,400 RPM. As we saw with Seagate’s Laptop Thin SSHD, such a low spindle speed can really hurt performance in some situations. Let’s hope the Black SSHD’s larger flash cache can mitigate the damage.

Comments closed
    • Aliasundercover
    • 7 years ago

    I have read many a complaint of how hard it is to get data from a RAID in a fouled up computer. Before the file system works you need all the RAID configuration set correctly. You don’t just pop your drive in a working computer and pull the data off. This two device disk drive will be similarly fragile, no driver, no data.

    I like knowing my drive is just a drive and it works from Windows or Linux, my rescue CD or my bootable USB stick. I will not buy a driver based hybrid disk. Perhaps one of those Seagate models which presents itself as a single device.

    Using an SSD on one computer but a lot of memory, 16+GB, in any which will take it I find the SSD’s main advantage is on first use. Being memory rich the computers without SSDs are just as fast after they warm up a bit. I do like the fast start but having lots of memory gives both speed and the option to run piggy programs or even whole virtual machines. Why is it the vendors want to sell me 4GB soldered on non-upgradable RAM yet the disk be it SSD or hybrid gets so much attention and money?

    • Flatland_Spider
    • 7 years ago

    I wonder if this will work like Apple’s Fusion drives.

    Pushing the cache management up to the OS level seems to be the best solution, provided the manufacturer put enough flash on the drive.

    • BIF
    • 7 years ago

    Me no want hybrid devices. Just make ’em SSDs big like buffalo.

    Who needs a buffalo that looks like a chicken but is really an armadillo under the … feathers?

      • chuckula
      • 7 years ago

      I am intrigued by your ideas and would like to subscribe to your newsletter.

    • HisDivineOrder
    • 7 years ago

    It really is a shame that WD is calling any drive with 5400-ish RPM’s Black.

      • indeego
      • 7 years ago

      Yeah because color coding my devices is how I keep track of how fast they are.

        • DancinJack
        • 7 years ago

        Historically you have been able to do so with WD consumer offerings.

      • albundy
      • 7 years ago

      i would have stopped at shame. thats all WD deserves.

    • shank15217
    • 7 years ago

    yet another reason for manufacturers to stick with 4GB ram limits on ultra books.

    • crabjokeman
    • 7 years ago

    Wonder if this works with Linux..

      • just brew it!
      • 7 years ago

      I doubt WD will provide Linux driver support for this tech any time soon. Maybe they will at least release technical info on how their driver works, allowing the Linux community (or perhaps a corporate entity like IBM or Redhat) to implement the functionality. Until then… meh.

        • Flatland_Spider
        • 7 years ago

        If this is exposed as two different drives, a driver might not be needed for Linux. There are already caching daemons that could take advantage of the SSD part, or something could be added to a filesystem that would allow drives to be “layered”. A 128GB SSD could be placed “on top of” a 1TB HD, but the drive would report as a 1TB drive.

        We don’t know yet what kind of magic the driver is doing. It could be a simple service running in the background caching reads and writes, the SSD could register itself with superfetch and readyboost, or it could be something more complex like some sort of weird software RAID that hooks into superfetch and readyboost.

    • ludi
    • 7 years ago

    I look at that graphic…and I see the head of an angry ant robot.

      • NeelyCam
      • 7 years ago

      Wtf?

      • BIF
      • 7 years ago

      Me too, and he’s holding an orange light saber in one hand!

    • FuturePastNow
    • 7 years ago

    Well now that’s interesting.

    I enjoy the speed of my SSD, but I am also weary of its limited capacity, and large SSDs are still out of my price range. I’ve considered switching to a hybrid drive since I saw those Seagate drives a couple of years ago. I look forward to seeing how much these cost. Now might be the time.

      • Stickmansam
      • 7 years ago

      You can only use this drive though if you have support for that connector, don’t think most laptops have them

        • FuturePastNow
        • 7 years ago

        No, but my desktop has plenty of room for an adapter. And they’ll probably make a version with a more common form-factor- be it a 2.5″ notebook drive or a full-size desktop drive- soon.

    • jjj
    • 7 years ago

    They got a SFF-8784 edge connector so there isn’t much of a point of selling this in retail anytime soon and makes the drive so much less interesting.

    • Chrispy_
    • 7 years ago

    Well, the z77 and z68 SRT caching is the best hybrid solution I’ve come across so far, it’s leagues better than MomentusXT drives and not reliant on dubious software a la OCZ Synapse.

    I dont even care if it runs on steam and voodoo as long as it reliably makes mechanical storage faster, specifically in the 4K IOPS department.

    • Ryu Connor
    • 7 years ago

    A driver, eh? No way this could ever end with kernel panics/BSOD and corrupt or lost data.

    :rolleyes:

      • Chrispy_
      • 7 years ago

      Newsflash:
      Even plain-Jane mechanical disks need drivers.

        • Ryu Connor
        • 7 years ago

        Controller drivers are already one layer in and are not always perfect.

        This caching mechanism is a second layer on top of that.

        You do understand the concept of what happens when you stack complexity, yes?

        Not as if SRT has a perfect records of avoiding BSODs to begin with.

        • just brew it!
        • 7 years ago

        …which conform to the AHCI interface standard, and are included “out of the box” in pretty much every OS released since 2005 or thereabouts. This is not even remotely the same thing.

        I can see the rationale behind this: Caching algorithms can be made more efficient if they have some knowledge that only the OS’s filesystem logic has access to. Absent a mechanism to communicate this info down to the drive itself, it needs to be implemented up at the driver level. But this also means that the enhanced capabilities are only available on platforms for which WD has deigned to develop and release a driver.

        As a user of a non-Windows OS, I am probably not a member of the target audience for these drives…

          • Ryu Connor
          • 7 years ago
          • Flatland_Spider
          • 7 years ago

          There is Flashcache and Bcache for Linux, which can use SSDs as a filesystem cache.

          Flashcache:
          [url<]https://www.facebook.com/note.php?note_id=388112370932[/url<] [url<]https://github.com/facebook/flashcache[/url<] Bcache: [url<]http://bcache.evilpiepirate.org/[/url<] Benchmarks: [url<]http://www.accelcloud.com/2012/04/18/linux-flashcache-and-bcache-performance-testing/[/url<] AskUbuntu Entry: [url<]http://askubuntu.com/questions/252140/how-do-i-install-and-use-flashcache-bcache-to-cache-hdd-to-ssd[/url<]

        • Ryu Connor
        • 7 years ago

        [quote=”Intel”<] Intel RST Version:12.5.0.1066, Released: 3/22/2013 Intel RST Version: 12.5.0.1064 [b<]Resolved Issues[/b<] 4628805 SSHD: BSOD when loading RST driver on Win7 system 4627886 BSOD during hybrid sleep cycle with SRT enabled + SD Media card 4628338 Attaching SSHD while system is in hibernation induces DPC Watchdog violation BSOD upon resume 4161045 System Black Screen happened during S4 Longrun on Samsung PM830 128GB SSD 4628018 BSOD 0x7E occurs when Liteon LCS-256M6S SSD is used in SRT configuration 4628123 BSOD occurred while the recovery disk with password protected is plugged-in in RRT mode 4628214 BSOD at S3/resume with Intel Cherryville SSD 4627964 WHQL: 0xD1 BSOD during Disk Stress / Disk Verification (BLOCKING) 4628015 System BSOD 7B on boot on WTM when RTD3 is enabled[/quote<] I can find more BSOD, a lot more, if I keep combing through the Intel RST notes. Also notice there are SRT and SSHD specific BSOD in that list!

        • designerfx
        • 7 years ago

        newsflash:
        this wasn’t his point or implied argument.

    • willmore
    • 7 years ago

    If you’re going to provide SSD performance by putting a complete standalone SSD on your spinny device, I think I’d rather just split them into two distince devices and let the OS/driver do the hard work.

      • Star Brood
      • 7 years ago

      That’s not always possible in a SFF/portable solution. I look at it as a convenient Smart Response, but I wouldn’t call this a Hybrid Drive as much as I’d call it a two-in-one drive.

        • Airmantharp
        • 7 years ago

        mSATA makes it possible almost everywhere.

          • Star Brood
          • 7 years ago

          Provided that the manufacturer has thus made provisions. I have seen plenty of laptops that do not.

        • cynan
        • 7 years ago

        In line with Airman’s post, with mSATA it definitely should be possible. Right now there are micro SSDs being introduced that perform every bit as well as their full size 2.5″ counterparts, that cost roughly the same amount (ie, the Plextor M5M drives [url=https://techreport.com/news/24745/ihs-isuppli-most-ultra-slim-notebooks-to-feature-hybrid-storage-configs?post=728355<]I posted about[/url<] in a related topic). If there is room for a mechanical HDD in a computing device, there should also be room for a micro SSD (Edit: as well). The things take up about as much space as a SODIMM.

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