SSD market dynamics

Will Hybrid drives conquer enterprise storage?

Toyota Hybrid Synergy Drive Decal: RAC Future Car Challenge by Dominic's pics (cc) (from Flickr)

I saw where Seagate announced the next generation of their Momentus XT Hybrid (SSD & Disk) drive this week. We haven’t discussed Hybrid drives much on this blog but it has become a viable product family.

I am not planning on describing the new drive specs here as there was an excellent review by Greg Schulz at StorageIOblog.

However, the question some in the storage industry have had is can Hybrid drives supplant data center storage. I believe the answer to that is no and I will tell you why.

Hybrid drive secrets

The secret to Seagate’s Hybrid drive lies in its FAST technology. It provides a sort of automated disk caching that moves frequently accessed OS or boot data to NAND/SSD providing quicker access times.

Storage subsystem caching logic has been around in storage subsystems for decade’s now, ever since the IBM 3880 Mod 11&13 storage control systems came out last century. However, these algorithms have gotten much more sophisticated over time and today can make a significant difference in storage system performance. This can be easily witnessed by the wide variance in storage system performance on a per disk drive basis (e.g., see my post on Latest SPC-2 results – chart of the month).

Enterprise storage use of Hybrid drives?

The problem with using Hybrid drives in enterprise storage is that caching algorithms are based on some predictability of access/reference patterns. When you have a Hybrid drive directly connected to a server or a PC it can view a significant portion of server IO (at least to the boot/OS volume) but more importantly, that boot/OS data is statically allocated, i.e., doesn’t move around all that much. This means that one PC session looks pretty much like the next PC session and as such, the hybrid drive can learn an awful lot about the next IO session just by remembering the last one.

However, enterprise storage IO changes significantly from one storage session (day?) to another. Not only are the end-user generated database transactions moving around the data, but the data itself is much more dynamically allocated, i.e., moves around a lot.

Backend data movement is especially true for automated storage tiering used in subsystems that contain both SSDs and disk drives. But it’s also true in systems that map data placement using log structured file systems. NetApp Write Anywhere File Layout (WAFL) being a prominent user of this approach but other storage systems do this as well.

In addition, any fixed, permanent mapping of a user data block to a physical disk location is becoming less useful over time as advanced storage features make dynamic or virtualized mapping a necessity. Just consider snapshots based on copy-on-write technology, all it takes is a write to have a snapshot block be moved to a different location.

Nonetheless, the main problem is that all the smarts about what is happening to data on backend storage primarily lies at the controller level not at the drive level. This not only applies to data mapping but also end-user/application data access, as cache hits are never even seen by a drive. As such, Hybrid drives alone don’t make much sense in enterprise storage.

Maybe, if they were intricately tied to the subsystem

I guess one way this could all work better is if the Hybrid drive caching logic were somehow controlled by the storage subsystem. In this way, the controller could provide hints as to which disk blocks to move into NAND. Perhaps this is a way to distribute storage tiering activity to the backend devices, without the subsystem having to do any of the heavy lifting, i.e., the hybrid drives would do all the data movement under the guidance of the controller.

I don’t think this likely because it would take industry standardization to define any new “hint” commands and they would be specific to Hybrid drives. Barring standards, it’s an interface between one storage vendor and one drive vendor. Probably ok if you made both storage subsystem and hybrid drives but there aren’t any vendor’s left that does both drives and the storage controllers.

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So, given the state of enterprise storage today and its continuing proclivity to move data around accross its backend storage, I believe Hybrid drives won’t be used in enterprise storage anytime soon.

Comments?

Posted on March 18, 2011April 10, 2012 by Ray in Market dynamics, Storage, storage economics, Strategic Inflection Points

Toshiba's 2.5" SSD (from SSD.Toshiba.com)

Had a talk the other week with an storage executive about SSD and NAND cost trends. It seemed that everyone thought that $/GB for SSD was going to overtake (be less costly) than enterprise class disk sometime in 2013. But it appeared that NAND costs weren’t coming down as fast as anticipated and now this was going to take longer than expected.

A couple of other things are going on in the enterprise disk market that are also having an effect on the relative advantage of SSDs over disks. Probably, most concerning to SSD market is enterprise storage’s new penchant for sub-LUN tiering.

Automated sub-LUN storage tiering

The major storage vendors all currently support some form of automated storage tiering for SSD storage (NetApp’s Flash Cache does this differently but the impact on NAND storage requirements is arguably similar). Presumably, such tiering should take better advantage of any amount of SSD/NAND storage available to a storage system.

Prior to automated sub-LUN storage tiering, one had to move a whole LUN to SSDs to take advantage of its speed. However, I/O requests or access are not necessarily at the same intensity for all blocks of a LUN. So one would typically end up with an SSD LUN with a relatively few blocks being heavily accessed while the vast majority of its blocks would not be being hit that much. We paid the high price of SSD LUNs gladly to get the high performance for those few blocks that really needed it.

However, with sub-LUN tiering or NAND caching, one no longer has to move all the blocks of a LUN into NAND storage to gain its benefits. One can now just have the system identify those select blocks which need high performance and move those blocks and those blocks only to NAND storage. The net impact of sub-LUN tiering or NAND caching is that one should require less overall NAND storage to obtain the same performance as one had previously with SSDs alone.

On the other hand, some would say that making the performance advantages of NAND be available at a lower overall cost might actually increase the overall amount of NAND shipments. Also with automated sub-LUN tiering in place, this removes all the complexity needed previously to identify which LUNs needed higher performance. Reducing such complexity should increase SSD or NAND market penetration.

Nonetheless, I feel that given todays price differential of SSDs over enterprise disk, the people buying SSDs today have a very defined need for speed and would have paid the price anyways for SSD storage. Anything we do to make satisfying that need with less SSD or NAND storage should reduce the amount of SSDs shipped today.

But getting back to that price crossover point, as the relative price of NAND on $/GB comes down, having an easy way to take advantage of its better performance should increase its market adoption, even faster than price would do alone.

Comments?

When will disks become extinct?

Posted on March 8, 2011April 10, 2012 by Ray in Data retention, Market dynamics, Storage, Storage architecture, Storage density, storage economics, Storage longevity, Storage performance, Strategic Inflection Points, System effectiveness, Systems

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.

Comments?

Are SSDs an invasive species?

Posted on January 22, 2010April 10, 2012 by Ray in Market dynamics, Storage, storage economics

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

I was reading about pythons becoming an invasive species in the Florida Everglades and that brought to mind SSDs. The current ecological niche in data storage has rotating media as the most prolific predator with tape going on the endangered species list in many locales.

So where does SSD enter into the picture. We have written before on SSD shipments start to take off but that was looking at the numbers from another direction. Given recent announcements it appears that in the enterprise, SSDs seem to be taking over the place formerly held by 15Krpm disk devices. These were formerly the highest performers and most costly storage around. But today, SSDs, as a class of storage, are easily the most costly storage and have the highest performance currently available.

The data

Seagate announced yesterday that they had shipped almost 50M disk drives last quarter up 8% from the prior quarter or ~96M drives over the past 6 months. Now Seagate is not the only enterprise disk provider (Hitachi, Western Digital and others also supply this market) but they probably have the lion’s share. Nonetheless, Seagate did mention that the last quarter was supply constrained and believed that the total addressible market was 160-165M disk drives. That puts Seagate’s market share (in unit volume) at ~31% and at that rate the last 6 months total disk drive production should have been ~312M units.

In contrast, IDC reports that SSD shipments last year totaled 11m units. In both the disk and SSD cases we are not just talking enterprise class devices, the numbers include PC storage as well. If we divide this number in half we have a comparable number of 5.5M SSDs for the last 6 months, giving SSDs less than a 2% market share (in units).

Back to the ecosystem. In the enterprise, there are 15Krpm disks, 10Krpm disks and 7.2Krpm rotating media disks. As speed goes down, capacity goes up. In Seagate’s last annual report they stated that approximately 10% of the drives they manufactured were shipped to the enterprise. Given that rate, of the 312M drives, maybe 31M were enterprise class (this probably overstates the number but usable as an upper bound).

As for SSDs, in the IDC report cited above, they mentioned two primary markets the PC and enterprise markets for SSD penetration. In that same Seagate annual report, they said their desktop and mobile markets were around 80% of disk drives shipped. If we use that proportion for SSDs that would say that of the 5.5M units shipped last half year, 4.4 were in the PC space and 1.1M were for the enterprise. Given that, it would state that the enterprise class SSDs represent ~3.4% of the enterprise class disk drives shipped. This is over 10X more than my prior estimate of SSDs being (<0.2%) of enterprise disk drives. Reality probably lies somewhere between these two estimates.

I wrote a research report a while back which predicted that SSDs would never take off in the enterprise, I was certainly wrong then. If these numbers are correct, capturing 10% of the enterprise disk market in little under 2 years can only mean that high-end, 15Krpm drives are losing ground faster than anticipated. Which brings up the analogy of the invasive species. SSDs seem to be winning a significant beach head in the enterprise market.

In the mean time, drive vendors are fighting back by moving from the 3.5″ to 2.5″ form factor, offering both 15K and 10K rpm drives. This probably means that the 15Krpm 3.5″ drive’s days are numbered.

I made another prediction almost a decade ago that 2.5″ drives would take over the enterprise around 2005 – wrong again, but only by about 5 years or so. I got to stop making predictions, …