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

At least... until hybrids inhale helium

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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.

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