3D NAND, how high can it go?

450_x_492_3d_nand_32_layer_stackI was at the Flash Memory Summit a couple of weeks ago and a presenter (from Hynix, I think) got up and talked about how 3D NAND was going to be the way forward for all NAND technology. I always thought we were talking about a handful of layers. But on the slide he had what looked to be a skyscraper block with 20-40 layers of NAND.

Currently shipping 3D NAND

It seems all the major NAND fabs are shipping 30+ layer 3D NAND. Samsung last year said they were shipping 32-layer 3D (V-)NANDToshiba announced earlier this year that they had 48-layer 3D NANDHynix is shipping 36-layer 3D NAND.  Micron-Intel is also shipping 32-layer 3D NAND. Am I missing anyone?

Samsung also said that they will be shipping a 32GB, 48-layer V-NAND chip later this year. Apparently, Samsung is also working on 64-layer V-NAND in their labs and are getting good results.  In an article on Samsung’s website they mentioned the possibility of 100 layers of NAND in a 3D stack.

The other NAND fabs are also probably looking at adding layers to their 3D NAND but aren’t talking as much about it. i5QVjaOmlEZHmjM34GrH3NFORjU9A-xAk_JUvkzS8Os

Earlier this year on a GreyBeards on Storage Podcast we talked with Jim Handy, Director at Objective Analysis on what was going on in NAND fabrication. Talking with Jim was fascinating but one thing he said was that with 3D NAND, building a hole with the right depth, width and straight enough was a key challenge. At the time I was thinking a couple of layers deep. Boy was I wrong.

How high/deep can 3D NAND go?

On the podcast, Jim said he thought that 3D NAND would run out of gas around 2023. Given current press releases, it seems NAND fabs are adding ~16 layers a year to their 3D-NAND.

So if 32 to 48 layers is todays 3D-NAND and we can keep adding 16 layers/year through 2023 that’s 8 years *16 layers or an additional 128 layers  to the 32  to 48 layers currently shipping. With that rate we should get to 160 to 176 layer 3D NAND chips. And if 48 layers is 32GB then we maybe we could see  ~+100GB  3D NAND chips.

This of course means that there is no loss in capacity as we increase layers. Also that the industry can continue to add 16 layers/year to 3D-NAND chips.

I suppose there’s one other proviso, that nothing else comes along that is less expensive to fabricate while still providing ever increasing capacity of lightening fast, non-volatile storage (see a recent post on 3D XPoint NVM technology).

Photo Credit(s):

  1. Micron’s press release on 3D NAND, (c) 2015 Micron
  2. Toshiba’s press release as reported by AnandTech, (c) 2015 Toshiba

Toshiba studies laptop write rates confirming SSD longevity

Toshiba's New 2.5" SSD from SSD.Toshiba.com
Toshiba's New 2.5in SSD from SSD.Toshiba.com

Today Toshiba announced a new series of SSD drives based on their 32NM MLC NAND technology. The new technology is interesting but what caught my eye was another part of their website, i.e., their SSD FAQs. We have talked about MLC NAND technology before and have discussed its inherent reliability limitations, but this is the first time I have seen some company discuss their reliability estimates so publicly. This was documented more in an IDC white paperon their site but the summary on the FAQ web page speaks to most of it.

Toshiba’s answer to the MLC write endurance question all revolves around how much data a laptop user writes per day which their study makes clear . Essentially, Toshiba assumes MLC NAND write endurance is 1,400 write/erase cycles and for their 64GB drive a user would have to write, on average, 22GB/day for 5 years before they would exceed the manufacturers warranty based on write endurance cycles alone.

Let’s see:

  • 5 years is ~1825 days
  • 22GB/day over 5 years would be over 40,000GB of data written
  • If we divide this by the 1400 MLC W/E cycle limits given above, that gives us something like 28.7 NAND pages could fail and yet still support write reliability.

Not sure what Toshiba’s MLC SSD supports for page size but it’s not unusual for SSDs to ship an additional 20% of capacity to over provision for write endurance and ECC. Given that 20% of 64GB is ~12.8GB, and it has to at least sustain ~28.7 NAND page failures, this puts Toshiba’s MLC NAND page at something like 512MB or ~4Gb which makes sense.

MLC vs, SLC write endurance from SSD.Toshiba.com
MLC vs, SLC write endurance from SSD.Toshiba.com

The not so surprising thing about this analysis is that as drive capacity goes up, write endurance concerns diminish because the amount of data that needs to be written daily goes up linearly with the capacity of the SSD. Toshiba’s latest drive announcements offer 64/128/256GB MLC SSDs for the mobile market.

Toshiba studies mobile users write activity

To come at their SSD reliability estimate from another direction, Toshiba’s laptop usage modeling study of over 237 mobile users showed the “typical” laptop user wrote an average of 2.4GB/day (with auto-save&hibernate on) and a “heavy” labtop user wrote 9.2GB/day under similar specifications. Now averages are well and good but to really put this into perspective one needs to know the workload variability. Nonetheless, their published results do put a rational upper bound on how much data typical laptop users write during a year that can then be used to compute (MLC) SSD drive reliability.

I must applaud Toshiba for publishing some of their mobile user study information to help us all better understand SSD reliability for this environment. It would have been better to see the complete study including all the statistics, when it was done, how users were selected, and it would have been really nice to see this study done by a standard’s body (say SNIA) rather than a manufacturer, but these are all personal nits.

Now, I can’t wait to see a study on write activity for the “heavy” enterprise data center environment, …

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.