Hybrid drives: not so fast!

Hybrid drives: not so fast!

Summary: Vendors are pimping hybrid drives of flash memory and rotating disks. Disk drives and flash memory have very specific performance profiles.

TOPICS: Hardware

Vendors are pimping hybrid drives of flash memory and rotating disks. Disk drives and flash memory have very specific performance profiles. And they don't match up that well. Here's the scoop.

The disk problem Disks are great for capacity and for reading and writing large chunks of data at high speeds. What they aren't so good at is small reads and writes, the kind your computer does all the time.

Disk drives used to be capable of 1 I/O for every 10 MB of capacity - disks were a lot smaller then - but today most SATA drives can only do 1 I/O for 3-5 GB of capacity. Luckily we keep a lot of rarely accessed stuff on our drives so the changing I/O ratio hasn't had an impact on consumers. Corporate databases are another story.

The common operating systems - Windows, Mac OS and Linux - have their roots in the old days when disk I/O limits weren't so bad. Which is to say they don't play well with today's large capacity disks.

The flash problem I've investigated flash performance issues, trying to get behind the wall of uninformative industry happy talk (see Flash drives: your mileage will vary, 5 things you never knew about flash drives and my StorageMojo flash and SSD articles.

Flash is very quick for small reads: where a disk will take 6-15 milliseconds, flash will take less than 1 ms. And Samsung has optimized its flash disks to handle large reads and writes at disk speeds.

The problem is small write speeds: they can be much worse than disks. Further, the small write performance depends on how well the software translates disk commands into flash storage. This translation layer is proprietary to each flash drive vendor and can have a massive impact on flash performance. Expect highly variable performance until vendors figure out the best way to handle the translation.

How bad? Over at Microsoft Research 4 very sharp guys, Andrew Birrell, Michael Isard, Chuck Thacker, and Ted Wobber, looked at the issue. In their paper A Design for High-Performance Flash Disks they spend some quality time analyzing a translation layer's write performance.

It isn't a pretty picture. In fact, here's a picture: picture-161.png

Even best case, single writes take as long or longer than most disks. But double writes can take much longer than any disk.

With journaled file systems - NTFS, HFS+ and most Linux file systems - such paired writes are very common. Open up a document and the document meta data and the journal get updated. That is a pair of writes.

The Storage Bits take The flash memory in hybrid drives is currently about 10x per GB over disk storage. With operating systems weighing in at several GB is it worth an extra $40-$50 to put the OS on flash for faster startup? That would almost double the price many disks.

Most notebook users would simply prefer a sleep mode that works reliably - which I haven't seen since my Windows 3.1 HP Omnibook 300 - but maybe with Vista Microsoft and the PC vendors have broken the code. And the performance benefits depend on getting often-read data on to the flash. Who will see to that?

Flash disks have some good performance stats, but they aren't a panacea. When they finally do come out I'll want to see some very detailed performance tests before I'll be tempted to buy something that is only slightly faster and much more expensive.

Comments welcome, as always. And if you are a storage geek I highly recommend the Microsoft Research paper. Update: One of the comments suggested a couple of clarifications, which I've added above. Hope it is clearer.

Topic: Hardware

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  • Thoughts on these benchmarks

    What are your thoughts on these benchmarks testing a solid state drive against some hybrid drives:

    Aside from the price, which is bound to fall over time, it seems like the ssd performs well.
    • Large sequential R/W work well on flash


      Thanks for the link. After a quick look at the testing, I noted that all the test favor
      large sequential I/O where you can expect flash to do a great job.

      Flash has a problem with small writes which are quite common in the age of
      journaling file systems. I didn't see anything in these benchmarks that addresses
      that issue.

      The big surprise is how well the Momentus does against the flash drive in
      performance and against the hybrid drive in power consumption. It looks like a
      price-performance champ. The few watts the flash drive saves translates into
      minutes of battery life, not hours.

      R Harris
  • Hybrid drives solve a real problem for the mobile world

    Hybrid drives solve a real problem for the mobile world. It extends battery life by caching those tiny read/write jobs that would ordinarily wake a hard drive. You only need about 256 or 512 MB to do the job in a hybrid hard drive. They're very expensive right now because it's a premium product, but prices will come down over time.
    • intel robson technology Fash Memory

      george is this the same thing, - not a hybrid drive.

      Overcoming Disk Drive Access Bottlenecks with Intel? Robson Technology
      By Michael Trainor

      Overview: Intel? Robson Technology Offers Extended Cache Capability
      Robson is the code name for a new Intel? platform technology that uses non-volatile memory (Flash memory) to increase system responsiveness, make multi-tasking faster, and extend battery life. Robson technology will be available on Intel?s forthcoming Santa Rosa platform?the next-generation mobile platform building on the power savings and performance benefits of Intel? Centrino? processor technology. Robson technology is designed to support two new features of the upcoming Microsoft Vista* operating system: ReadyBoost* and ReadyDrive* technologies.

      Early testing in Intel?s labs shows that Robson technology on a Santa Rosa platform1 running Microsoft Vista (pre-production hardware and software) improves multitasking performance with 2x improvement in application load and run time as well as a 2x improvement in resume time from hibernation over systems without Robson. Robson technology also offers increased power savings?extending battery life by an estimated 20 minutes in lab testing. And perhaps best of all, these benefits may improve as the software and hardware are tuned for production release.

      Systems with Santa Rosa and Robson technology are expected to launch in the first half of 2007.
      not of this world
      • No, Robson is similar to Readyboost using USB flash

        Intel Robson technology is a replacement for readyboost USB flash, it's not a replacement for flash on disk. The flash on USB and Robson is typically 4-phase flash that doesn't quite last as long as 2-phase more expensive flash used in hybrid hard drives. The flash in hybrid hard drives is smaller and more expensive per MB than normal flash. It's used to buffer everything on the hard drive whereas Robson and ReadyBoost are used to buffer temporary reads/writes.

        ReadyBoost operations call on the disk and the flash at the same time and takes the first response it gets. Hybrid drive flash allows the hard drive to rest while it delivers the data independently which allows the drive on a laptop to stay spinned down a lot longer which conserves power and extends battery life. That's the primary purpose of hybrid hard drives.
        • small differential 7200 rpm = 5400 rpm

          "Hybrid drive flash allows the hard drive to rest while it delivers the data independently which allows the drive on a laptop to stay spinned down a lot longer which conserves power and extends battery life."

          so a 5400 rpm drive would be just as effective and also
          more so - conserve energy and heat.
          not of this world
          • 5400 RPM equals more power consumption than 0 RPM

            A 5400 RPM drive spinning at 5400 RPMs uses a LOT more power than a 7200 RPM hybrid drive at 0 RPMs because the data is being fed from the flash. Furthermore, the 7200 RPM drive will perform much better than a 5400 RPM drive.
          • Yes, but...

            As soon as you need a write or have a cache (flash buffer) miss, the drive would have to spin up to write back the data from cache to the disk or to refresh the cache with the needed data.

            The flash buffer on the hybrid drives is relatively small (about 256 MB), so the chances of a cache miss are pretty high.
          • you don't understand what I said

            both 5400 and 7200 drives are both hybrid.

            i suggest that 5400 rpm would consume less power to start up
            given the fact that sata 1.5GBs is faster than needed anyway.
            when loading applications or basic cashing when switching apps in windows. also consider heat and vibration.
            not of this world
  • what?

    The many errors in this piece make it hard to read. Some of these sentences I read three times before I finally realized that they simply didn't make sense. For example "Further, the small write performance depends on how the software that translates disk commands into flash storage." huh? or this "The flash memory in hybrid drives is currently about 10x per GB over disk storage." what? or how about this one: "That would almost double the price many disks."

    I am no storage geek but this stat seems highly questionable: "today most SATA drives can only do 1 I/O for 3-5 GB of capacity" what does that mean? Are you saying that the software can read 3-5 GB from the disk into memory at a time? I just don't buy it especially when most machines still have less than 2G of RAM. What do you really mean here?

    Sorry for the ranting. I must just be in a bad mood.
    • i agree

      my computer bursts 6MB data in less than 1 sec.
      when making mutiple copys across partitions.
      and im using an ata100 with 2mb cash 7200 rpm.

      its moving arround in windows that i notice performance lags !

      but your right, this article makes no real sence.
      not of this world
    • Made sense to me...

      The flash memory providers must use a translation layer of software that make their various versions of flash memory appear as a removable "drive". They do it differently and some do it very inefficiently.

      Flash memory is 10x the cost per GB of comparable disk storage.
      The increased cost of adding the flash will be about twice that of a comparable sized drive without the flash.
      Uber Dweeb
  • Forget Flash Hybrid! More RAM!!

    Forget about using Flash memory as a hybrid with drives. Given the (relatively) low cost of RAM, what would it take to outfit a drive with a large enough cache that could store the entire OS? You'd still have to deal with slow boot times as the cache gets filled, but as soon as that happens, all the OS operations happen at RAM speed.

    I'm suggesting this rather than a solution that would require a major re-write of both the BIOS and Windows, i.e., make it so that Windows can be burned into a ROM. Of course, that will happen when M$ makes it so Windows will allow/require that data and programs be on separate partitions so you could actually backup all of your data.