When will disks become extinct?

A head assembly on a Seagate disk drive by Robert Scoble (cc) (from flickr)
A head assembly on a Seagate disk drive by Robert Scoble (cc) (from flickr)

Yesterday, it was announced that Hitachi General Storage Technologies (HGST) is being sold to Western Digital for $4.3B and after that there was much discussion in the tweeterverse about the end of enterprise disk as we know it.  Also, last week I was at a dinner at an analyst meeting with Hitachi, where the conversation turned to when disks will no longer be available. This discussion was between Mr. Takashi Oeda of Hitachi RSD, Mr. John Webster of Evaluator group and myself.

Why SSDs will replace disks

John was of the opinion that disks would stop being economically viable in about 5 years time and will no longer be shipping in volume, mainly due to energy costs.  Oeda-san said that Hitachi had predicted that NAND pricing on a $/GB basis would cross over (become less expensive than) 15Krpm disk pricing sometime around 2013.  Later he said that NAND pricing had not come down as fast as projected and that it was going to take longer than anticipated.  Note that Oeda-san mentioned density price cross over for only 15Krpm disk not 7200rpm disk.  In all honesty, he said SATA disk would take longer, but he did not predict when

I think both arguments are flawed:

  • Energy costs for disk drives drop on a Watts/GB basis every time disk density increases. So the energy it takes to run a 600GB drive today will likely be able to run a 1.2TB drive tomorrow.  I don’t think energy costs are going to be the main factor to drives disks out of the enterprise.
  • Density costs for NAND storage are certainly declining but cost/GB is not the only factor in technology adoption. Disk storage has cost more than tape capacity since the ’50s, yet they continue to coexist in the enterprise. I contend that disks will remain viable for at least the next 15-20 years over SSDs, primarily because disks have unique functional advantages which are vital to enterprise storage.

Most analysts would say I am wrong, but I disagree. I believe disks will continue to play an important role in the storage hierarchy of future enterprise data centers.

NAND/SSD flaws from an enterprise storage perspective

All costs aside, NAND based SSDs have serious disadvantages when it comes to:

  • Data retention – the problem with NAND data cells is that they can only be written so many times before they fail.  And as NAND cells become smaller, this rate seems to be going the wrong way, i.e,  today’s NAND technology can support 100K writes before failure but tomorrow’s NAND technology may only support 15K writes before failure.  This is not a beneficial trend if one is going to depend on NAND technology for the storage of tomorrow.
  • Sequential access – although NAND SSDs perform much better than disk when it comes to random reads and less so, random writes, the performance advantage of sequential access is not that dramatic.  NAND sequential access can be sped up by deploying multiple parallel channels but it starts looking like internal forms of wide striping across multiple disk drives.
  • Unbalanced performance – with NAND technology, reads operate quicker than writes. Sometimes 10X faster.  Such unbalanced performance can make dealing with this technology more difficult and less advantageous than disk drives of today with much more balanced performance.

None of these problems will halt SSD use in the enterprise. They can all be dealt with through more complexity in the SSD or in the storage controller managing the SSDs, e.g., wear leveling to try to prolong data retention, multi-data channels for sequential access, etc. But all this additional complexity increases SSD cost, and time to market.

SSD vendors would respond with yes it’s more complex, but such complexity is a one time charge, mostly a one time delay, and once done, incremental costs are minimal. And when you come down to it, today’s disk drives are not that simple either with defect skipping, fault handling, etc.

So why won’t disk drives go away soon.  I think other major concern in NAND/SSD ascendancy is the fact that the bulk NAND market is moving away from SLC (single level cell or bit/cell) NAND to MLC (multi-level cell) NAND due to it’s cost advantage.  When SLC NAND is no longer the main technology being manufactured, it’s price will not drop as fast and it’s availability will become more limited.

Some vendors also counter this trend by incorporating MLC technology into enterprise SSDs. However, all the problems discussed earlier become an order of magnitude more severe with MLC NAND. For example, rather than 100K write operations to failure with SLC NAND today, it’s more like 10K write operations to failure on current MLC NAND.  The fact that you get 2 to 3 times more storage per cell with MLC doesn’t help that much when one gets 10X less writes per cell. And the next generation of MLC is 10X worse, maybe getting on the order of 1000 writes/cell prior to failure.  Similar issues occur for write performance, MLC writes are much slower than SLC writes.

So yes, raw NAND may become cheaper than 15Krpm Disks on a $/GB basis someday but the complexity to deal with such technology is also going up at an alarming rate.

Why disks will persist

Now something similar can be said for disk density, what with the transition to thermally assisted recording heads/media and the rise of bit-patterned media.  All of which are making disk drives more complex with each generation that comes out.  So what allows disks to persist long after $/GB is cheaper for NAND than disk:

  • Current infrastructure supports disk technology well in enterprise storage. Disks have been around so long, that storage controllers and server applications have all been designed around them.  This legacy provides an advantage that will be difficult and time consuming to overcome. All this will delay NAND/SSD adoption in the enterprise for some time, at least until this infrastructural bias towards disk is neutralized.
  • Disk technology is not standing still.  It’s essentially a race to see who will win the next generations storage.  There is enough of an eco-system around disk that will keep pushing media, heads and mechanisms ever forward into higher densities, better throughput, and more economical storage.

However, any infrastructural advantage can be overcome in time.  What will make this go away even quicker is the existance of a significant advantage over current disk technology in one or more dimensions. Cheaper and faster storage can make this a reality.

Moreover, as for the ecosystem discussion, arguably the NAND ecosystem is even larger than disk.  I don’t have the figures but if one includes SSD drive producers as well as NAND semiconductor manufacturers the amount of capital investment in R&D is at least the size of disk technology if not orders of magnitude larger.

Disks will go extinct someday

So will disks become extinct, yes someday undoubtedly, but when is harder to nail down. Earlier in my career there was talk of super-paramagnetic effect that would limit how much data could be stored on a disk. Advances in heads and media moved that limit out of the way. However, there will come a time where it becomes impossible (or more likely too expensive) to increase magnetic recording density.

I was at a meeting a few years back where a magnetic head researcher predicted that such an end point to disk density increase would come in 25 years time for disk and 30 years for tape.  When this occurs disk density increase will stand still and then it’s a certainty that some other technology will take over.  Because as we all know data storage requirements will never stop increasing.

I think the other major unknown is other, non-NAND semiconductor storage technologies still under research.  They have the potential for  unlimited data retention, balanced performance and sequential performance orders of magnitude faster than disk and can become a much more functional equivalent of disk storage.  Such technologies are not commercially available today in sufficient densities and cost to even threaten NAND let alone disk devices.

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So when do disks go extinct.  I would say in 15 to 20 years time we may see the last disks in enterprise storage.  That would give disks an almost an 80 year dominance over storage technology.

But in any event I don’t see disks going away anytime soon in enterprise storage.

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