What is hybrid storage? The emergence of a new technology

03 March 2015 by Sam Wiltshire


For the past decade or so, solid state drives (SSDs) have been the golden child of the storage world. These devices are basically hard disk replacements that use the same flash memory found in USB drives, mobile phones and SD cards, and they offer a long list of benefits against their electromechanical counterparts: they have no moving parts, are more resistant to impact and other forms of physical stress, and are blazing fast.

The humble hard disk drive (HDD), meanwhile, hasn't progressed much in recent years. Yes, HDD storage capacities have increased pretty consistently - nowadays, massive three and four-terabyte drives are eminently affordable, and even eight and ten-terabyte behemoths are weaving their way into the enterprise market. Performance-wise, however, HDDs have topped out at 15,000 revolutions per minute and make more noise, create more heat, consume more power and are less durable than their flash-based counterparts.

So, why haven't we all ditched our HDDs and switched to SSDs instead? Simple - in terms of cost per gigabyte, SSDs are far more expensive. As of January 2015, a one-terabyte HDD can be picked up for under £50, whereas an equivalent SSD hovers around the £300 mark.

Since 2007, however, a number of storage device manufacturers -starting with Seagate and Samsung - have attempted to tackle the price differential between HDDs and SSDs by introducing a third option that, in theory, delivers the best of both worlds. This is hybrid storage, which marries the speed of an SSD with the cost-effectiveness of an electromechanical HDD.

How a solid state hybrid drive works

The principle behind hybrid storage is literally to combine flash-based memory with the rotating electromagnetic platters of a traditional hard disk. A solid state hybrid drive (SSHD) will therefore comprise a few gigabytes of flash capacity plus a larger HDD, with the idea being that the most frequently-accessed or performance-critical data - so-called 'hot data' - can be cached on the SSD and retrieved more quickly than if it were stored on the platters themselves. It's basically the same as installing both an HDD and an SSD in your machine, which is known as a dual drive hybrid solution, except there's no need to manually optimise performance by moving files and applications to the appropriate device - all of the hard work is handled automatically.

All of the SSHDs on the market today operate in one of two different ways. The first is called self-optimised mode, or self-pinning mode, and describes devices that decide what constitutes hot and cold data themselves. To the host machine, the drive appears no different to a traditional internal storage device.

Other SSHDs run in host-optimised mode, or host-pinning mode. In this instance, the host machine's operating system and device drivers - or, potentially, some other software - make decisions concerning hot and cold data, sending these instructions to the drive via a regular SATA interface.

The advantages of hybrid storage...

As you might expect, the key advantage of using a hybrid storage device is that you gain faster access to your most important data, without paying significantly more than you might for a traditional hard disk. SSHDs are pricier than HDDs, yes, but they're not nearly as expensive as buying SSDs of equivalent capacity.

Besides this, several other benefits associated with flash memory also apply to SSHDs. A hybrid storage solution is said to consume less power and generate less heat than a purely electromechanical drive, for example, and this reduced stress ostensibly contributes to a longer lifespan and better reliability.

... And the disadvantages

For obvious reasons, though, some of the advantages of using an SSD aren't applicable when that flash memory is bolted onto an HDD. Naturally, retrieval of data from the hard disk component is as slow as it would be from a traditional HDD, while benefits like resistance to physical stress are also lost when flash accounts for only a small portion of the device's total storage capacity. Some users might also miss the silent operation of a fully-fledged SSD.

It's also worth thinking about the data recovery implications of hybrid storage. If you're switching to SSHDs from SSDs, and you've started to depend on the latter's durability, then hybrid storage devices' aforementioned susceptibility to mechanical damage might come as a nasty surprise. To date, no SSD user has ever ended up with a data recovery bill to pay following a mechanical failure.

As for the prospect of losing data from the solid state portion of the SSHD, this is thankfully unlikely as it's normally used as a cache or to store operating system and program files. If, however, you do need to recover information from a failed SSD module, this presents its own set of unique challenges such as proprietary methods of data organisation.

All in all, hybrid storage has plenty to offer to consumers and business users seeking the speed of an SSD at the same time as the attractive cost per gigabyte of an HDD. Nonetheless, they need to be aware of the unique risk factors that come with this new form of storage.