Howard Marks

Network Computing Blogger


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Thursday, July 25, 2013
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Is A Hybrid Disk Drive In Your Future?

Seagate this week announced an updated version of its Momentus XT hybrid disk drive. By combining flash and a spinning disk, hybrids promise most of the performance of an SSD at the cost per gigabyte of a hard drive. The new version comes pretty close to delivering on that promise.

The first generation of hybrid disks from Samsung and Seagate relied on the operating system to allocate data between their flash and spinning disk, and were, as a result, spectacularly unsuccessful--no one wanted to invest in hardware that relied on special features of Windows Vista to do anything. Today’s Momentus XTs rely on their own intelligence to populate their flash memory as a read cache. The second generation Momentus XT has been a lot more successful, with Seagate shipping more than a million drives, including the one in the laptop I’m using to write this blog post.

To build the latest Momentus XT, Seagate not only boosted disk capacity 50% to 750 Gbytes, and flash capacity 100% to 8 Gbytes, but also added new levels of intelligence to the caching algorithms the drive uses to allocate disk blocks to the flash memory. Like most of the high-capacity drives introduced in the past year or so, it also uses 4-Kbyte blocks and delivers data through a 6G-bps SATA interface. All this comes at an manufacturer's retail price of $249, which isn’t bad considering the upward pressure Thai flooding is having on the disk market.

The biggest change is that the latest drive explicitly speeds system boot. When you install the OS on a system using a 750Gbyte Momentus XT, it segments its cache, storing operating system and other boot data in a dedicated section of cache. From that point on you brag that your system boots as fast as an SSD-equipped machine.

Hybrid drives make the most sense in laptops where space and power are severely limited resources. While some of my friends have pulled the optical drive out of their laptops so they can have an SSD for speed and a HDD for space, I run in rather geeky circles and most laptop users don’t want to mess with such an operation. Even my fellow geeks seem to agree, as Seagate tells me that half of Momentus XT sales have been through retail channels like Best Buy and CDW, where 2.5-inch drives are usually over 90% OEM sales to laptop vendors.

The bigger question is whether hybrid disk drives will move out of the laptop ghetto into more general applications. My friend Larry Freeman of NetApp thinks that what we now call hybrid disk drives will become the industry standard during the next few years and that we'll see them in desktops and disk arrays. I’m not so sure.

On the desktop front, Intel and Marvell have built SSD caching into the SATA controllers they sell to motherboard vendors, simplifying the process of using a stand-alone hard disk and SSD as a single accelerated volume. On the other hand, some workstation vendors have been using Momentus XTs in their high-end systems, as yet another disk drive option is easier to manage in their processes than a combined SSD/HDD option.

In the disk array, I think I’d much rather have the intelligence that decides what data to place on SSD and what data to place on HDD in the controller than in the individual drives. If I have 20 to 50 disks in an array and 160 Gbytes of flash, the controller can place the hottest data in flash. If that same flash was distributed across 20 drives, the flash would only hold the hottest data for that drive. If a given drive didn’t hold part of the hottest LUN, its flash wouldn’t be best used.

Need speed and capacity in your laptop. The Momentus XT is for you.

Disclaimer: Seagate sent me two of the older 500-Gbyte Momentus XT drives and a new 750-Gbyte drive gratis.


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