disk density – Silverton Consulting

Disk rulz, at least for now

Posted on October 16, 2017October 17, 2017 by Ray in Data density, Data retention, Disk storage, Executive leadership, Strategic Inflection Points, Visionary leadershp

Last week WDC announced their next generation technology for hard drives, MAMR or Microwave Assisted Magnetic Recording. This is in contrast to HAMR, Heat (laser) Assisted Magnetic Recording. Both techniques add energy so that data can be written as smaller bits on a track.

Disk density drivers

Current hard drive technology uses PMR or Perpendicular Magnetic Recording with or without SMR (Shingled Magnetic Recording) and TDMR (Two Dimensional Magnetic Recording), both of which we have discussed before in prior posts.

The problem with PMR-SMR-TDMR is that the max achievable disk density is starting to flat line and approaching the “WriteAbility limit” of the head-media combination.

That is even with TDMR, SMR and PMR heads, the highest density that can be achieved is ~1.1Tb/sq.in. The Writeability limit for the current PMR head-media technology is ~1.4Tb/sq.in. As a result most disk density increases over the past years has been accomplished by adding platters-heads to hard drives.

MAMR and HAMR both seem able to get disk drives to >4.0Tb/sq.in. densities by adding energy to the magnetic recording process, which allows the drive to record more data in the same (grain) area.

There are two factors which drive disk drive density (Tb/sq.in.): Bits per inch (BPI) and Tracks per inch (TPI). Both SMR and TDMR were techniques to add more TPI.

I believe MAMR and HAMR increase BPI beyond whats available today by writing data on smaller magnetic grain sizes (pitch in chart) and thus more bits in the same area. At 7nm grain sizes or below PMR becomes unstable, but HAMR and MAMR can record on grain sizes of 4.5nm which would equate to >4.5Tb/sq.in.

HAMR hurdles

It turns out that HAMR as it uses heat to add energy, heats the media drives to much higher temperatures than what’s normal for a disk drive, something like 400C-700C. Normal operating temperatures for disk drives is ~50C. HAMR heat levels will play havoc with drive reliability. The view from WDC is that HAMR has 100X worse reliability than MAMR.

In order to generate that much heat, HAMR needs a laser to expose the area to be written. Of course the laser has to be in the head to be effective. Having to add a laser and optics will increase the cost of the head, increase the steps to manufacture the head, and require new suppliers/sourcing organizations to supply the componentry.

HAMR also requires a different media substrate. Unclear why, but HAMR seems to require a glass substrate, the magnetic media (many layers) is deposited ontop of the glass substrate. This requires a new media manufacturing line, probably new suppliers and getting glass to disk drive (flatness-bumpiness, rotational integrity, vibrational integrity) specifications will take time.

Probably more than a half dozen more issues with having laser light inside a hard disk drive but suffice it to say that HAMR was going to be a very difficult transition to perform right and continue to provide today’s drive reliability levels.

MAMR merits

MAMR uses microwaves to add energy to the spot being recorded. The microwaves are generated by a Spin Torque Oscilator, (STO), which is a solid state device, compatible with CMOS fabrication techniques. This means that the MAMR head assembly (PMR & STO) can be fabricated on current head lines and within current head mechanisms.

MAMR doesn’t add heat to the recording area, it uses microwaves to add energy. As such, there’s no temperature change in MAMR recording which means the reliability of MAMR disk drives should be about the same as todays disk drives.

MAMR uses todays aluminum substrates. So, current media manufacturing lines and suppliers can be used and media specifications shouldn’t have to change much (?) to support MAMR.

MAMR has just about the same max recording density as HAMR, so there’s no other benefit to going to HAMR, if MAMR works as expected.

WDC’s technology timeline

WDC says they will have sample MAMR drives out next year and production drives out in 2019. They also predict an enterprise 40TB MAMR drive by 2025. They have high confidence in this schedule because MAMR’s compatabilitiy with current drive media and head manufacturing processes.

WDC discussed their IP position on HAMR and MAMR. They have 400+ issued HAMR patents with another 100+ pending and 75 issued MAMR patents with 46 more pending. Quantity doesn’t necessarily equate to quality, but their current IP position on both MAMR and HAMR looks solid.

WDC believes that by 2020, ~90% of enterprise data will be stored on hard drives. However, this is predicated on achieving a continuing, 10X cost differential between disk drives and (QLC 3D) flash.

What comes after MAMR is subject of much speculation. I’ve written on one alternative which uses liquid Nitrogen temperatures with molecular magnets, I called CAMR (cold assisted magnetic recording) but it’s way to early to tell.

And we have yet to hear from the other big disk drive leader, Seagate. It will be interesting to hear whether they follow WDC’s lead to MAMR, stick with HAMR, or go off in a different direction.

Comments?

Photo Credit(s): WDC presentation

Shingled magnetic recording disks

Posted on October 4, 2012 by Ray in Block Storage, Data, data access, Data density, Disk storage, File Storage, Storage, Storage drive, Strategic Inflection Points, System effectiveness, System quality, Systems

A couple of weeks ago I attended a day of the SNIA Storage Developers Conference (SDC) where Garth Gibson of Carnegie Mellon University Parallel Data Lab (CMU PDL) and Panasas was giving a talk of what they are up to at CMU’s storage lab. His talk at the conference was on shingled magnetic recording (SMR) disks. We have discussed this topic before in our posts on Sequential only disks?! and in Disk trends revisited. SMR may require a re-thinking of how we currently access disk storage.

Recall that shingled magnetic recording uses a write head that overwrites multiple tracks at a time (see graphic above), with one track being properly written and the adjacent (inward) tracks being overwritten. As the head moves to the next track, that track can be properly written but more adjacent (inward) tracks are overwritten, etc. In this fashion data can be written sequentially, on overlapping write passes. In contrast, read heads can be much narrower and are able to read a single track.

In my post, I assumed that this would mean that the new shingled magnetic recording disks would need to be accessed sequentially not unlike tape. Such a change would need a massive rewrite to only write data sequentially. I had suggested this could potentially work if one were to add some SSD or other NVRAM to the device to help manage the mapping of the data to the disk. Possibly that plus a very sophisticated drive controller, not unlike SSD wear leveling today, could handle mapping a physically sequentially accessed disk to a virtually randomly accessed storage protocol.

Garth’s approach to the SMR dilemma

Garth and his team of researchers are taking another tack at the problem. In his view there are multiple groups of tracks on an SMR disk (zones or bands). Each band can be either written sequentially or randomly but all bands can be read randomly. One can break up the disk to include sections of multiple shingled bands, that are sequentially written and less, non-shingled bands that can be randomly written. Of course there would be a gap between the shingled bands in order not to overwrite adjacent bands. And there would also be gaps between the randomly written tracks in a non-shingled partition to allow for the wider track writing that occurs with the SMR write head.

His pitch at the conference dealt with some characteristics of such a multi-band disk device. Such as

How to determine the density for a device that has multiple bands of both shingled write data and randomly written data.
How big or small a shingled band should be in order to support “normal” small block and randomly accessed file IO.
How many randomly written tracks or what the capacity of the non-shingled bands would need to be to support “normal” file IO activity.

For maximum areal density one would want large shingled bands. There are other interesting considerations that were not as obvious but I won’t go into here.

SCSI protocol changes for SMR disks

The other, more interesting section of Garth’s talk was on recent proposed T10 and T13 changes to support SMR disks that supported shingled and non-shingled partitions and what needed to be done to support SMR devices.

The SCSI protocol changes being considered to support SMR devices include:

A new write cursor for shingled write bands that indicates the next LBA to be written. The write cursor starts out at a relative band address of 0 and as each LBA is written consecutively in the band it’s incremented by one.
A write cursor can be reset (to zero) indicating that the band has been erased
Each drive maintains the band map and current cursor position within each band and this can be requested by SCSI drivers to understand the configuration of the drive.

Probably other changes are required as well but these seem sufficient to flesh out the problem.

SMR device software support

Garth and his team implemented an SMR device, emulated in software using real random accessed devices. They then implemented an SMR device driver that used the proposed standards changes and finally, implemented a ShingledFS file system to use this emulated SMR disk to see how it would work. (See their report on Shingled Magnetic Recording for Big Data Applications for more information.)

The CMU team implemented a log structured file system for the ShingledFS that only wrote data to the emulated SMR disk shingled partition sequentially, except for mapping and meta-data information which was written and updated randomly in a non-shingled partition.

You may recall that a log structured file system is essentially written as a sequential stream of data (not unlike a log). But there is additional mapping required that indicates where file data is located in the log which allows for randomly accessing the file data.

In their report and at the conference, Garth presented some benchmark results for a big data application called Terasort (essentially Teragen, Terasort and Teravalidate) which seems to use Hadoop to sort a large body of data. Not sure I can replicate this information here but suffice it to say at the moment the emulated SMR device with ShingledFS did not beat a base EXT3 or FUSE using the same hardware for these applications.

Now the CMU project wAs done by a bunch of smart researchers but it’s still relatively new and not necessarily that optimized. Thus, there’s probably some room for improvement in the ShingledFS and maybe even the emulated SMR device and/or the SMR device driver.

At the moment Garth and his team seem to believe that SMR devices are certainly feasible and would take only modest changes to the SCSI protocols to support such devices. As for file system support there is plenty of history surrounding log structured file systems so these are certainly doable but would require probably extensive development to implemented in various OS to support an SMR device. The device driver changes don’t seem to be as significant.

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It certainly looks like there’s going to be SMR devices in our future. It’s just a question whether they will be ever as widely supported as the randomly accessed disk device we know and love today. Possibly, this could all be behind a storage subsystem that makes the technology available as networked storage capacity and over time maybe SMR devices could be implemented in more standard OS device drivers and file systems. Nevertheless, to keep capacity and areal density on their current growth trajectory, SMR disks are coming, it’s just a matter of time.

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Disk density hits new record, 1Tb/sqin with HAMR

Posted on March 22, 2012 by Ray in Disk storage, Storage density, Strategic Inflection Points

Seagate has achieved 1Tb/sqin recording (source: http://www.gizmag.com)

Well I thought 36TB on my Mac was going to be enough. Then along comes Seagate with this weeks announcement of reaching 1Tb/sqin (1 Trillion bits per square inch) using their new HAMR (heat assisted magnetic recording) technology.

Current LFF drive technology runs at about 620Gb/sqin providing a 3.5″ drive capacity of around 3TB or about 500Gb/sqin for 2.5″ drives supporting ~750GB. The new 1Tb/sqin drives will easily double these capacities.

But the exciting part is that with the new HAMR or TAR (thermally assisted recording) heads and media, the long term potential is even brighter. This new technology should be capable of 5 to 10Tb/sqin which means 3.5″ drives of 30 to 60TB and 2.5″ drives of 10 t0 20TB.

HAMR explained

HAMR uses both lasers and magnetic heads to record data in even smaller spaces than current PMR (perpendicular magnetic recording) or vertical recording heads do today. You may recall that PMR was introduced in 2006 and now, just 6 years later we are already seeing the next generation head and media technologies in labs.

Denser disks requires smaller bits and with smaller bits disk technology runs into three problems readability, writeability and stability, AKA the magnetic recording trilemma. Smaller bits require better stability, but better stability makes it much harder to write or change a bits magnetic orientation. Enter the laser in HAMR, with laser heating the bits can become much more maleable. These warmed bits can be more easily written bypassing the stability-writeability problem, at least for now.

However, just as in any big technology transition there are other competing ideas with the potential to win out. One possibility we have discussed previously is shingled writes using bit patterned media (see my Sequential only disk post) but this requires a rethinking/re-architecting of disk storage. As such, at best it’s an offshoot of today’s disk technology and at worst, it’s a slight detour on the overall technology roadmap.

Of course PMR is not going away any time soon. Other vendors (and proboblf Seagate) will continue to push PMR technology as far as it can go. After all, it’s a proven technology, inside millions of spinning disks today. But, according to Seagate, it can achieve 1Tb/sqin but go no further.

So when can I get HAMR disks

There was no mention in the press release as to when HAMR disks would be made available to the general public, but typically the drive industry has been doubling densities every 18 to 24 months. Assuming they continue this trend across a head/media technology transition like HAMR, we should have those 6GB hard disk drives sometime around 2014, if not sooner.

HAMR technology will likely make it’s first appearance in 72oorpm drives. Bigger capacities seem to always first come out in slower performing disks (see my Disk trends, revisited post)

HAMR performance wasn’t discussed in the Seagate press release, but with 2Mb per linear track inch and 15Krpm disk drives, the transfer rates would seem to need to be on the order of at least 850MB/sec at the OD (outer diameter) for read data transfers.

How quickly HAMR heads can write data is another matter. The fact that the laser heats the media before the magnetic head can write it seems to call for a magnetic-plus-optical head contraption where the laser is in front of the magnetics (see picture above).

How long it takes to heat the media to enable magnetization is one critical question in write performance. But this could potential be mitigated by the strength of the laser pulse and how far the laser has to be in front of the recording head.

With all this talk of writing, there hasn’t been lots of discussion on read heads. I guess everyone’s assuming the current PMR read heads will do the trick, with a significant speed up of course, to handle the higher linear densities.

What’s next?

As for what comes after HAMR, checkout another post I did on using lasers to magnetize (write) data (see Magnetic storage using lasers alone). The advantage of this new “laser-only” technology was a significant speed up in transfer speeds. It seems to me that HAMR could easily be an intermediate step on the path to laser-only recording having both laser optics and magnetic recording/reading heads in one assembly.

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Lets see 6TB in 2014, 12TB in 2016 and 24TB in 2018, maybe I won’t need that WD Thunderbolt drive string as quickly as I thought.

Comments?

Disk drive density multiplying by 6X

Posted on October 18, 2011 by Ray in Disk storage, Storage, Storage density, Strategic Inflection Points, System effectiveness

Sodium Chloride by amandabhslater (cc) (From Flickr)

In a news story out of Singapore Institute of Materials Research and Engineering (IMRE), Dr. Joel Yang has demonstrated 6X the current density on disk platter media, or up to 3.3 Terabits /square inch (Tb/sqin). And it all happens due to salt (sodium chloride) crystals.

I have previously discussed some of the problems encountered by the disk industry going to the next technology transition trying to continue current density trends. At the time, the then best solution was to use bit-patterned media (BPM) and shingled writes discussed in my Sequential Only Disk!? and Disk trends, revisited posts. However, this may have been premature.

Just add salt

It turns out that by adding salt to the lithographic process used to disperse magnetic particles onto disk platters for BPM, the particles are more regularly spaced. In contrast, todays process used in current disk media manufacturing, causes the particles to be randomly spaced.

More regular magnetic particle spacing on media provides two immediate benefits for disk density:

More particles can be packed in the same area. With increased magnetic particles located in a square inch of media, more data can be recorded.
Bigger particles can be used for recording data. With larger grains, data can be recorded using a single structure rather than using multiple, smaller particles, increasing density yet again.

Combining these two attributes increases disk platter capacities by a factor of 6 without having to alter read-write head technology. The IMRE team demonstrated 1.9Tb/sqin recording capacity but fabricated media with particles at levels that could provide 3.3Tb/sqin. Currently, the disk industry is demonstrating 0.5Tb/sqin.

Other changes needed

I suppose other changes will also be needed to accommodate the increased capacity, not the least of which is speeding up the read-write channels to support 6X more bits being accessed per revolution. Probably other items need to be changed as well, but these all come with increased disk density.

Before this technique came along the next density levels was turning out to be a significant issue. But now that salt is in use, we can all rest easy knowing that disk capacity trends can continue to increase with todays recording head technology.

Using the recent 4TB 7200RPM hard drives (see my Disk capacity growing out-of-sight post), but moving to salt and BPM, the industry could potentially create a 24TB 7200RPM drive or for the high performance 600GB 15KRPM drives, 3.6TB high performance disks! Gosh, not to long ago 24TB of storage was a good size storage system for SMB shops, with this technology, it’s just a single disk drive.

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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.

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