Seagate fires the first round in the modern hybrid drive war

Momentus XT brings hybrid array performance to the laptop

By James E. Bagley
Senior Analyst, Storage Strategies NOW
May 2010

With all of the advancements in flash memory technology and hot data caching algorithms, it was only a matter of time until a major hard disk drive (HDD) manufacturer brought a drive to market that incorporates the best of both worlds. Seagate has announced shipments of its Momentus XT hybrid drive that combines rotating media at 7200RPM with 4GB of single level cell (SLC) flash memory. Seagate’s Adaptive Memory technology analyzes frequently read data areas and copies these blocks to flash acting as purely a read cache. Over time, and depending on the usage patterns, this type of algorithm has been proven in hybrid arrays supplied by many high end products, including big dollar 3PAR storage networks. What hasn’t been available is hybrid technology in a 250GB drive that costs $115.

Who needs a hybrid drive?
Single bay laptops are the perfect application for a hybrid drive. Hot data can be accessed without spinning up the hard drive, saving on battery capacity and providing instant access to most users’ data that is read frequently. And in drive capacities of 250GB, 320GB and 500GB, there is plenty of room to store the big files, like pictures, music and movies that many people love to carry around on their laptops. These big files, under the adaptive memory caching algorithm, will stay on the hard drive. But metadata, boot blocks, frequently used applications and indices will automatically stick in the flash memory. In the many analyses of hybrid storage arrays, the ratio of solid state memory to HDD can be very small and still reap performance gains of more than an order of magnitude.  At 4GB for 250GB to 500GB, this strikes me as a little low, but the first product reviews that analyze laptop workloads indicate that it is sufficient for dramatic performance increases.  Couple that with longer run time between battery recharge cycles, spending $160 on a 500GB drive in your laptop could be Nirvana for many users. For me, I still haven’t used more than 50GB of my 250GB drive (I do run disk cleanup about once a month) and the keys will fall off my keyboard before I fill the drive. But then, I watch movies on my big HDTV and don’t consider MPP music as part of my net-worth. In any event, this is a drive I would gladly fork out the $115 to get.

Conservative, value-based engineering
Interestingly, Seagate opted for the more expensive SLC flash memory for this low cost drive. To me, this seems a prudent approach. SLC has faster access and better longevity than multi-level cell (MLC) flash, which stuffs two or more bits into each physical area (cell). Using a single 4GB module, cost factors do not multiply like pure solid state drives (SSD) that need dozens of modules to achieve a 250GB capacity. In another conservative design decision, Seagate opted for a 3Gbit/sec. Serial ATA interface, as opposed to the latest 6Gbit/sec. chips. Since the laptop market is targeted, few could take advantage of a 6Gbit/sec. design anyway.

Anand Shimpi has done a quick review of the Momentus XT and while I agree with him that a larger flash memory will improve performance, it will be at an additional cost at a rate of about $4 per GB. Just to go to 8GB, a $115 drive becomes $130, and now people start looking back to $75 drives in the trade-off.  Anand’s review can be found at: http://www.anandtech.com/show/3734/seagates-momentus-xt-review-finally-a-good-hybrid-hdd

SSG-NOW Assessment

By targeting the laptop market, Seagate has clearly hit a sweet spot for the hybrid drive market. Adaptive Memory caching algorithms have been proven by high end systems manufacturers like 3PAR and in DAS solutions like the MaxIQ from Adaptec. With a conservative value-based design, the Momentus XT should make its mark in both the aftermarket and OEM applications. In just the first round of what we expect to be an expanding war for hybrid drives aimed at the mobile client, it will be interesting to see if the foundry makers will be diving in. Toshiba is well positioned for this since its acquisition of Fujitsu’s HDD business.