Home > Disk, Price > Where are Disks Headed?

Where are Disks Headed?

I started my career at a different Texas company – Texas Instruments – and I remember the 1” drive division aimed at the mobile device market.   It didn’t take very long for this to get end-of-lifed.  It was a neat product and a serious feat of engineering, but it just couldn’t compete with Flash.  At first it was because Flash was smaller, more rugged, and used less power. However, it was ultimately just because Flash was cheaper!  (Compare the disk-based iPod Mini and the Flash-based iPod Nano.)

Disks have a high fixed cost per unit and a small marginal cost per GB .  Physically bigger disks have a lower cost per GB than smaller ones.   This is very different from the other storage media like Flash and tape.  So it was bothering me recently – since each generation of disks takes on a smaller form factor than before, why are mainstream disks still shrinking, from 3.5” to 2.5”?  If the disk market was just concerned with cost per GB and “tape is dead”, this is crazy – disk should be getting bigger!  Why do disks continue their march towards smaller form factors when that just makes SSDs more competitive?

I originally thought that this was just a holdover from the attempts to make disks faster.  Bigger disks are harder to spin at a high speed, so as the RPM rate marched forward disks had to get smaller.  The advent of cost effective SSDs, however, has stopped the increase in RPMs. (Remember the news in 2008 of the 20k RPM disk?)  The market for performance storage at a premium has been ceded to SSDs.

After spending some time thinking on it I think there are a few basic reasons disks continue their march:

  • The attempt to have a converged enterprise, desktop, and laptop standard.
  • The need for smaller units to compose RAID sets, so that during a rebuild the chance of a second failure is not too high. I understand this, but RAID-6 is an alternate solution.
  • Disks are not just for storage, they are for both performance and long term storage.

Simply because disks store data on a circular platter, every time the bit density increases, the capacity grows by a power of 2 function, but the ability to access randomly doesn’t change, and the bandwidth only grows by a power of 1 function.    At some point the need for capacity is more than adequately met so the performance need takes over and disks shrink to get the performance and capacity more in sync.

Neither tape cartridges nor Flash suffer the fixed cost problem or geometry induced accessibility issues of disks.  With the new high density cartridges coming online tape continually avoids being supplanted by disk for pure capacity requirements.  TMS even recently had a customer that was able to leverage a Tape + SSD deployment and skip disks altogether.

Is the future of storage SSD + tape?

No.  While this works for a streamlined processing application, tape just isn’t ever going to be fast enough for data that needs to feel like it is instantly available.  There is just too much data that probably won’t be needed much, but when it is, it must be instantly available.  However, disks are much faster than the ~1 second response time needed for a user facing application.

With SSD handling more and more of the performance storage requirements  it will be interesting to see if disks stop their march toward smaller form factors and head in the other direction by becoming bigger and slower and fully cede the “tier 1 storage” title to SSDs.

Categories: Disk, Price
  1. August 3, 2011 at 4:53 pm

    While I was working on this post a great blogger, Howard Marks, put out the article “Is Flash The New Disk?”
    http://www.networkcomputing.com/servers-storage/231001451 that has some interesting counterpoints that are worth a read and I commented on.

    • September 27, 2011 at 4:11 pm

      Wow, he called me a great blogger. I’m touched.


  2. Robert Norman
    August 3, 2011 at 5:54 pm

    I think on of the big issues with disk size has to do with the usable area on a platter. As the bit densities increase it becomes harder and harder to account for variations do to size. Servo tracking over very tight tolerances and the read-back path are hard to optimize. The coating thickness varies from inner and out radius, the angular velocity changes so bit size varies. The flying height changes with the angular velocity. These type of parameters with wide design variations tends to make the system unreliable. By keeping cylinders grouped in a very compact area tight design tolerances can be kept. The heat and power to spin a big platter vs a small one is significant. Cost enters with motor size and power drivers. I’m not a Disk Engineer, but I think this is some of the reasons for the evolution.

    Interesting idea of SSD to tape without Disk involved. Essential to this is a good tape library that is small and easily supported. I haven’t kept up on the evaluation of Tape, but the big IBM and Storage Technology types are not attractive for smaller business environments. I need to go do some catching up to see where that Technology has migrated.

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