Original URL: http://www.theregister.co.uk/2013/02/12/hybrid_sshds/

Only way to stop the iPad: Flash-disk mutant SPEED FREAKS

At least... until hybrids inhale helium

By Chris Mellor

Posted in Storage, 12th February 2013 12:24 GMT

With the rise of the iPad and the growing desire among consumers and enterprise for sexy and speedy gadgets, it seemed that flash gits were holding all the cards.

The hard drive industry was holding its breath, wondering how to replicate the expensive experience. But what about hybrid disk drives, the ones that have a lump of flash cache inside their casing? Are they just the thing to spread tablets and Ultrabooks into the mass market? It's now increasingly likely that hybrid drives combining flash speed and disk drive capacity and affordability will take a massive mass-market bite out of the computing pie, leaving a reduced wedge for pricy all-flash tablets and notebooks and just a small sliver for lumbering and slow disk-only notebooks and desktops.

When Apple introduced its all-flash MacBook Air notebook a few years ago it prompted a great deal of heartache inside disk drive suppliers and Intel who both feared that ARM-powered, flash-using ultra-thin Air-style notebooks would decimate the Wintel disk-drive notebook business. The result was Intel's massive ultrathin notebook design initiative.

A Momentus event

Seagate, seeing which way this wind was blowing, also saw that a flash cache could be used to hold the most sensitive files from a user wait time point of view - the operating system and applications - and provide 90 per cent or so of flash speed at boot, app load and shutdown times, along with with basic disk affordability.

So Seagate started the hybrid disk drive ball rolling, with its Momentus XT in May 2010. It was a 2.5-inch hard drive, spinning at 7,200rpm, not a slow drive, holding 250GB, 320GB or 500GB of data on the two disk platters, and coming with a 4GB cache of single level cell NAND flash - the good, fast stuff ... not the cheaper, slower and shorter life multi-level cell flash.

This class of drive is called a solid state hard drive (SSHD) to differentiate it from the bog-standard HDD. Seagate claimed 350,000 had been sold by February 2011. It was encouraged enough to bring out a second generation in November 2011, with disk capacity increased to 750GB and the maximum flash capacity uprated to 8GB on the 750GB model, while it remained at 4GB on the 500GB model.

Momentus XT gen 2

Seagate Momentus XT

Seagate supplies "Adaptive Memory" with these hybrid drives, a brand name for a set of algorithms to keep the data that requires fast loading into the host computer's memory, in the NAND cache. That would be the operating system to speed boot time and applications to shorten application load times. It's not a great deal of flash capacity though: just over 1 per cent of overall capacity for the 750GB XT, and 0.8 per cent for the 500GB model. You can't store much data in that.

Latterly the rest of the hard disk drive industry, meaning Toshiba and Western Digital, are climbing aboard the hybrid disk drive magic roundabout. WD announced its SSHD intentions in September 2012, and Toshiba the same month, with a 1TB drive featuring an 8GB flash cache - the same as WD's intended 1TB SSHD.

Of course Apple launched a so-called Fusion hybrid drive option for its iMac desktop PCs in October 2012, but these consist of separate 128GB flash storage and a 1TB hard disk drive, not a single unit. The flash storage is not a cache but a tier, so users see 1.12TB of capacity. Also the flash capacity is 13 per cent of the 1TB disk space. These two things make it quite different from the hybrid SSHDs discussed here, and we'll set Apple's Fusion drive off to one side and discuss it no more here.

Where, beyond portables, could SSHDs be used in the spectrum of disk and flash storage applications?

Storage arrays

Could storage arrays find a use for hybrid drives? The pattern of data access in storage arrays suggests not. They store incoming files or blocks of data in NAS or SAN-style arrangements respectively, and these tend to be spread across multiple drives so that read and write access times are both reduced by having more than one drive involved.

A hot file or a hot group of blocks, "hot" meaning a high access rate to that data, is best given faster access by having the caching take place outside the disk drives and in the storage array controller. Alternatively, if you know which files and/or which blocks are generally always going to need fast access then they can be stored on solid state drives (SSDs) in the array and this, obviously, obviates the need for flash caching at the individual drive level.

There seems to be no need for hybrid flash/disk drives in a storage array because, as we might say, the unit of storage is counted in multiple drives and isn't a single drive. It is only where the unit of storage is the single drive that on-board caching becomes useful.

Hybrid drive scenarios

A hybrid drive, with its combination of NAND flash caching and disk drive capacity, is useful in single-drive products: where an all-flash storage scheme is seen as attractive from a speed point of view but costly, and a disk-based alternative is seen as desirable from a capacity viewpoint, but slow.

That means desktop PCs, notebook computers and tablets. We'll rule out digital video recorders and set-top boxes because, although they are typically single-drive products, they depend on streaming speed rather than random data access and are not as sensitive to boot time or application load time duration frustrations.

Notebook computers and tablets have tighter design constraints than desktop PCs because they are battery powered and have limited internal room. They can't use 3.5-inch disks, there not being the room available, and 2.5-inch drives, which fit the space constraint if thin enough, hold less data than a 3.5-incher. So the flash vs disk comparison is less favourable to disk on capacity and cost/GB grounds.

However users of all three single drive devices - PCs, notebooks, and tablets - hate waiting while a disk dribbles operating systems and applications into memory at start-up time, and takes a seeming age to shut the systems down. Hybrid drives would fix that issue and that gives us three target devices for them: the desktop, the notebook and tablet.

Hybrid SSHD scenarios

Click on table for larger version.

Money: It's the BOM

NAND costs money and disk drive buyers, especially OEMs, want to buy drives at as low a cost as possible. The Bill of Material (BOM) cost has to meet this need. Adding dollars with NAND caches needs justifying; the vendors have to be convinced there is a market. That encourages the disk drive manufacturers to be parsimonious with the amount of flash they add. There has to be as big a difference between the cost/GB of all-flash storage and hybrid flash/disk storage as possible, and the smaller the amount of flash in the hybrid drive the greater this difference is.

Hence Seagate's Momentus XT has 0.8 to 1 per cent of its disk capacity duplicated in flash. That means, for example, Excel and Word app load times will be at flash speed but spreadsheet and Word documents will be loaded from disk and saved to disk at disk speeds. The bigger these files get, the slower their access will be be. But them's the rules: hybrid drives are a halfway house between all-flash and all-disk - you get some flash speed added to your disk capacity for a little more than all-disk money.

Content consumption vs creation

This won't matter so much for tablets where data consumption takes precedence over content creation. It will be a benefit to be able to store many, many more videos and photos and music tracks for your money than you would with an all-flash tablet. Compare, for example, a 124GB flash tablet with a 512GB hybrid flask/disk tablet and you get three times more space for content with the hybrid.

The ratio of content creation to consumption will incline more to creation as we change from table to notebook, whether ultrathin or not, and again as we change from notebook to desktop. The way desktop designs are changing towards all-in-one screen and system units means a desktop these days is practically a notebook mounted vertically behind a large screen, albeit a notebook with more real estate inside the thin enclosure. This trend encourages the use of flash caching to provide speed instead of having two disk drives, where that was the case.

Extrapolating into the future

Two trends appear likely to change the coming hybrid drive scenarios we've painted. One is three-layer-cell flash (TLC), and the other is helium gas-filled drives.

Current hybrid drives use fast and expensive 1-bit or single layer cell (SLC) flash. All-flash drives typically use less expensive 2-bit or multi-layer cell (MLC) flash. As it stores two bit values per cell, it holds twice as much data in the same space as SLC flash at the cost of slower access speed and a shorter working life. TLC flash has a much shorter working life and a slower access speed but costs less than MLC flash. Controller technology, like STEC's CellCare, can extend TLC endurance to acceptable levels meaning that an all-TLC flash drive will be much more affordable than an all-MLC flash drive.

The price premium you have to pay for a TLC flash tablet or notebook will be significantly less than for an MLC one, making the price difference with a hybrid tablet or notebook less and so prompting users to pay the extra money needed to get all-flash speed.

We don't see this happening with desktops though. Helium-filled drives, with helium gas offering less resistance than air to spinning disk platters and moving heads, enables a 3.5-inch drive to have more platters in the same space, six or seven instead of four or five. This means today's 4TB drive could be tomorrow's 6TB drive using the same recording technology level - or the 8TB drive which emerges later in the future using an even more advanced recording technology.

The cost/GB differential between an all-flash drive and a hybrid, helium-filled drive will remain wide, even if the comparison is with a TLC flash drive, and so, we believe, hybrid disk drives will persist in desktop PCs, evolving to hybrid helium-filled drives.

This pattern may occur with notebook computers too if they use multi-platter 2.5-inch drives. Ultrathin notebooks will probably be constrained to single platter drives because of dimensional constraints. Thicker notebooks could use 2- or 3-platter drives and these could be helium-filled drives cramming 3 platters into today's 2-platter form factor, and so increasing capacity by 50 per cent. That could preserve a meaningful cost/GB difference between all-flash notebooks and hybrid flash/disk notebooks and so preserve the notebook 2-5-inch disk drive market which will be threatened by all-TLC flash drives, assuming they are built.

Crossovers appeal

Hybrid disk drives look to have a viable role, certainly in the 2.5-inch form factor, and probably in the 3.5-inch form factor. It appears to us that desktop hybrid helium-filled 3.5-inch drives can survive a transition from MLC to TLC flash drives but TLC drives will tend to eject 2.5-inch disk drives, hybrid or not, from tablets and ultrathin notebooks, not so from thicker notebooks where multi-platter 2.5-inch drives can be used.

We could draw a parallel with cars. A few years ago owners could choose to buy a car for comfort and convenience but forego 4-wheel drive off-road capability and also van capacity levels. But off-road vehicles were clunky and complex and vans were simply not comfortable or well-equipped.

Now we have sports-utility vehicles and people carriers that marry saloon car comfort and convenience with some 4-wheel drive off-road capability on the one hand or with luxurious well-equipped van capacity on the other. Such vehicles are very popular. On that basis hybrid disk drives could also become very popular indeed. ®