• 01/30/2013
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SSD/Hard Disk Hybrids Bridge Storage Divide

Solid-state is creeping into every nook of the storage ecosystem. The next frontier could be the rebirth of hybrids. Case in point is a startup that combines SSDs and traditional disks to maximize both performance and capacity.

But building MicroTiering software into a hardware adapter doesn't just offer maximal flexibility in choice and mix of drives; it also enables the tiering action to happen outside and independent of the guest OS or hypervisor. As Mill points out, this means MicroTiered storage, much like a RAID stripe, is presented to the host as a standard disk volume that can be carved up and spread across all the guest VMs. But unlike RAIDs, MicroTiers don't spread blocks equally among available devices, but intelligently place active data on SSD.

And in contrast to server-based caching, MicroTiers not only aggregate solid-state and disk storage, treating SSDs as a persistent device, but also don't require any system-level software to manage data movement between the two.

Mills claims that operating below the hypervisor mitigates the infamous I/O blender effect, in which sequential I/O streams from guest OSs, when merged and serviced by the hypervisor, turn into a mishmash of random I/O to the physical storage devices. Mills says the MicroTier will always serve the data with the highest activity out of flash

Hybrid storage also simplifies design and deployment of VM environments using live migration because, unlike server-side caches, the guests VMs need not be specially configured to take advantage of flash storage, while the self-tuning MicroTier means those workloads with the greatest I/O demands will automatically get a greater share of the flash tier.

Enmotus' hybrid adapter uses RAID-like redundancy for both the SSD and HDD layers. MicroTier storage pools are fully virtualized so that new devices are automatically incorporated into the pool, a feature Mill notes makes it easy to increase the SSD/HDD ratio should for new applications with a larger memory footprint. Its software also detects failing or end-of-life SSDs, automatically moving existing data to other devices and reincorporating replacement drives into the SSD pool.

Solid-state/disk hybrids promise the best of both worlds, speed and size and after years of false starts, as storage consultant Greg Schulz of StorageIO puts it, "the teething problems are being solved." Products like the Seagate Momentus XT and Apple Fusion Drive demonstrate the feasibility of a commercially successful hybrid product in the consumer market, but newcomers like Enmotus hope to do the same for enterprise storage.

As Mills put it in a presentation to last year's Flash Memory Summit, "Ease of use and transparent integration of SSDs are more important than raw IOPs for the next two to three." Translated: The 80/20 rule will apply to storage. A product that delivers most of the performance improvements of an all SSD or solid-state caching system at a fraction of the cost and implementation complexity should be a winner. The thesis will get a test once Enmotus starts actually selling products later this year.

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