STEC’s MLC enterprise SSD

So many choices by Robert S. Donovan
So Many Choices by Robert S. Donovan

I haven’t seen much of a specification on STEC’s new enterprise MLC SSD but it should be interesting.  So far everything I have seen seems to indicate that it’s a pure MLC drive with no SLC  NAND.  This is difficult for me to believe but could easily be cleared up by STEC or their specifications.  Most likely it’s a hybrid SLC-MLC drive similar, at least from the NAND technology perspective, to FusionIO’s SSD drive.

MLC write endurance issue

My difficulty with a pure MLC enterprise drive is the write endurance factor.  MLC NAND can only endure around 10,000 erase/program passes before it starts losing data.  With a hybrid SLC-MLC design one could have the heavy write data go to SLC NAND which has a 100,000 erase/program pass lifecycle and have the less heavy write data go to MLC.  Sort of like a storage subsystem “fast write” which writes to cache first and then destages to disk but in this case the destage may never happen if the data is written often enough.

The only flaw in this argument is that as the SSD drives get bigger (STEC’s drive is available supporting up to 800GB) this becomes less of an issue. Because with more raw storage the fact that a small portion of the data is very actively written gets swamped by the fact that there is plenty of storage to hold this data.  As such, when one NAND cell gets close to its lifetime another, younger cell can be used.  This process is called wear leveling. STEC’s current SLC Zeus drive already has sophisticated wear leveling to deal with this sort of problem with SLC SSDs and doing this for MLCs just means having larger tables to work with.

I guess at some point, with multi-TB per drives, the fact that MLC cannot sustain more than 10,000 erase/write passes becomes moot.  Because there just isn’t that much actively written data out there in an enterprise shop. When you amortize the portion of highly written data as a percentage of a drive, the more drive capacity, the smaller the active data percentages become. As such, as SSD drive capacities gets larger this becomes less of an issue.  I figure with 800GB drives, active data proportion might still be high enough to cause a problem but it might not be an issue at all.

Of course with MLC it’s also cheaper to over provision NAND storage to also help with write endurance. For an 800GB MLC SSD, you could easily add another 160GB (20% over provisioning) fairly cheaply. As such, over provisioning will also allow you to sustain an overall drive write endurance that is much higher than the individual NAND write endurance.

Another solution to the write endurance problem is to increase the power of ECC to handle write failures. This would probably take some additional engineering and may or may not be in the latest STEC MLC drive but it would make sense.

MLC performance

The other issue about MLC NAND is that it has slower read and erase/program cycle times.  Now these are still order’s of magnitude faster than standard disk but slower than SLC NAND.  For enterprise applications SLC SSDs are blistering fast and are often performance limited by the subsystem they are attached to. So, the fact that MLC SSDs are somewhat slower than SLC SSDs may not even be percieved by enterprise shops.

MLC performance is slower because it takes longer to read a cell with multiple bits in it than it takes with just one. MLC, in one technology I am aware of, encodes 2-bits in the voltage that is programmed in or read out from a cell, e.g., VoltageA = “00”, VoltageB=”01″, VoltageC=”10″, and VoltageD=”11″. This gets more complex with 3 or more bits per cell but the logic holds.  With multiple voltages, determining which voltage level is present is more complex for MLC and hence, takes longer to perform.

In the end I would expect STEC’s latest drive to be some sort of SLC-MLC hybrid but I could be wrong. It’s certainly possible that STEC have gone with just an MLC drive and beefed up the capacity, over provisioning, ECC, and wear leveling algorithms to handle its lack of write endurance

MLC takes over the world

But the major issue with using MLC in SSDs is that MLC technology is driving the NAND market. All those items in the photo above are most probably using MLC NAND, if not today then certainly tomorrow. As such, the consumer market will be driving MLC NAND manufacturing volumes way above anything the SLC market requires. Such volumes will ultimately make it unaffordable to manufacture/use any other type of NAND, namely SLC in most applications, including SSDs.

So sooner or later all SSDs will be using only MLC NAND technology. I guess the sooner we all learn to live with that the better for all of us.

Toshiba’s New MLC NAND Flash SSDs

Toshiba has recently announced a new series of SSD’s based on MLC NAND (Yahoo Biz story). This is only the latest in a series of MLC SSDs which Toshiba has released.

Historically, MLC (multi-level cell) NAND has supported higher capacity but has been slower and less reliable than SLC (single-level cell) NAND. The capacity points supplied for the new drive (64, 128, 256, & 512GB) reflect the higher density NAND. Toshiba’s performance numbers for new drives also look appealing but are probably overkill for most desktop/notebook/netbook users

Toshiba’s reliability specifications were not listed in the Yahoo story and probably would be hard to find elsewhere (I looked on the Toshiba America website and couldn’t locate any). However the duty cycle for a desktop/notebook data drive are not that severe. So the fact that MLC can only endure ~1/10th the writes that SLC can endure is probably not much of an issue.

SNIA is working on SSD (or SSS as SNIA calls it, see SNIA SSSI forum website) reliability but have yet to publish anything externally. Unsure whether they will break out MLC vs SLC drives but it’s certainly worthy of discussion.

But the advantage of MLC NAND SSDs is that they should be 2 to 4X cheaper than SLC SSDs, depending on the number (2, 3 or 4) of bits/cell, and as such, more affordable. This advantage can be reduced by the need to over-provision the device and add more parallelism in order to improve MLC reliability and performance. But both of these facilities are becoming more commonplace and so should be relatively straight forward to support in an SSD.

The question remains, given the reliability differences, when and if MLC NAND will ever become reliable enough for enterprise class SSDs. Although many vendors make MLC NAND SSDs for the notebook/desktop market (Intel, SanDISK, Samsung, etc.), FusionIO is probably one of the few using a combination of SLC and MLC NAND for enterprise class storage (see FusionIO press release). Although calling the FusionIO device an SSD is probably a misnomer. And what FusionIO does to moderate MLC endurance issues is not clear but buffering write data to SLC NAND must certainly play some part.

Testing storage systems – Intel’s SSD fix

Intel’s latest (35nm NAND) SSD shipments were halted today because a problem was identified when modifying BIOS passwords (see IT PRO story). At least they gave a timeframe for a fix – a couple of weeks.

The real question is can products be tested sufficiently these days to insure they work in the field. Many companies today will ship product to end-user beta testers to work out the bugs before the product reaches the field. But beta-testing has got to be complemented with active product testing and validation. As such, unless you plan to get 100s or perhaps 1000s of beta testers you could have a serious problem with field deployment.

And therein lies the problem, software products are relatively cheap and easy to beta test, just set up a download site and have at it. But with hardware products beta testing actually involves sending product to end-users which costs quite a bit more $’s to support. So I understand why Intel might be having problems with field deployment.

So if you can’t beta test hardware products as easily as software – then you have to have a much more effective test process. Functional testing and validation is more of an art than a science and can cost significant $’s and more importantly, time. All of which brings us back to some form of beta testing.

Perhaps Intel could use their own employees as beta testers rotating new hardware products from one organization to another, over time to get some variability in the use of a new product. Many companies use their new product hardware extensively in their own data centers to validate functionality prior to shipment. In the case of Intel’s SSD drives these could be placed in the in-numberable servers/desktops that Intel no-doubt has throughout it’s corporation.

One can argue whether beta testing takes longer than extensive functional testing. However given today’s diverse deployments, I believe beta testing can be a more cost effective process when done well.

Intel is probably trying to figure out just what went wrong in their overall testing process today. I am sure, given their history, they will do better next time.

Tape v Disk v SSD v RAM

There was a time not long ago when the title of this post wouldn’t have included SSD. But, with the history of the last couple of years, SSD has earned its right to be included.

A couple of years back I was at a Rocky Mountain Magnetics Seminar (see IEEE magnetics societies) and a disk drive technologist stated that Disks have about another 25 years of technology roadmap ahead of them. During this time they will continue to increase density, throughput and other performance metrics. After 25 years of this they will run up against some theoretical limits which will halt further density progress.

At the same seminar, the presenter said that Tape was lagging Disk technology by about 5-10 years or so. As such, tape should continue to advance for another 5-10 years after disk stops improving at which time tape would also stop increasing density.

Does all this mean the end of tape and disk? I think not. Paper stopped advancing in density theoretically about 2 to 3000 years ago (the papyrus scroll was the ultimate in paper “rotating media”). If we move up to the codex or book form- which in my view is a form factor advance – this took place around 400AD (see history of scroll and codex). Paperback, another form factor advance, took place in the early 20th century (see paperback history).

Turning now to write performance, moveable type was a significant paper (write) performance improvement and started in the mid 15th century. The printing press would go on to improve (paper write) performance for the next six centuries (see printing press history) and continues today.

All this indicates that some data technology, whose density was capped over 2000 years ago, can continue to advance and support valuable activity in today’s world and for the foreseeable future. “Will disk and tape go away” is the wrong question, the right question is “can disk or tape, after SSDs reach price equivalence on a $/GB basis, still be useful to the world”?

I think yes, but that depends on a number of factors as to how the relative SSD-Disk-Tape technologies advance. Assuming someday all these technologies support equivalent Tb/SqIn or spatial density and

  • SSD’s retain their relative advantage in random access speed,
  • Tape it’s advantage in sequential throughput, volumetric density, and long media life, and
  • Disk it’s all around, combined sequential and random access advantage

It seems likely that each can sustain some niche in the data center/home office of tomorrow, although probably not where they are today.

One can see trends being enacted in the enterprise data centers today that are altering the relative positioning of SSDs, disks and tape. Tape is now being relegated to long term, archive storage, Disk is moving to medium-term, secondary storage and SSDs is replacing top tier, primary storage.

More thoughts on this in future posts.