A few years ago it seemed as if Hybrid Hard Drives were the future. Yet after a bunch of announcements and hope today we find ourselves in a world with two distinct markets: HDDs and SSDs. If you're willing to pay the price premium and limit maximum capacity, today's SSDs are very fast and if you choose well, reliable.

For a desktop PC this isn't a tough choice to make. I've been advocating a setup where you have a SSD for your OS + applications and a separate RAID-1 array of 1TB or larger drives for all of your music, movies and photos.

Notebook users don't usually have a ton of drive bays and thus only have room for a single drive. It's not a lost cause though, if your notebook is your only machine you can get away with an internal SSD + external storage whether in the form of a NAS or just something you attach via USB when you're at your desk.

For the very portable users that don't want to lug around another hard drive, or for those who refuse to pay the high dollar per GB rates that SSDs command, there hasn't really been an option other than mechanical storage.

Today Seagate is attempting to change that with its latest Hybrid HDD: the Momentus XT.

More 918 Spyder than Prius

Simply put a Hybrid HDD is a mechanical drive with some NAND flash on it that is automatically used by the drive to store data for quicker access. A hybrid drive really just attempts to do what my setup of two drives (SSD + HDD) does manually: put small, frequently used data on NAND flash and put larger, less frequently used data on platters.

In theory you get the best of both worlds, the overall capacity of a HDD and (most of the time) the performance of an SSD.

Seagate's Momentus XT starts with a standard 7200PM 2.5" Momentus drive and adds a 32MB buffer, the largest on any 2.5" Momentus drive. Seagate then makes it a hybrid by adding a single 4GB SLC (!) NAND chip on the drive's PCB. Connect a controller to manage what goes into the NAND and we're in business.

Seagate Momentus XT Drive Pricing (MSRP)
  250GB 320GB 500GB
Seagate Momentus XT $113 $132 $156
Seagate Momentus 7200.4 $55 $55 $85
Seagate Momentus 5400.6 $50 $55 $65

The size of the NAND was a shocker to me when I first heard it. I honestly expected something much larger. In the Momentus XT however, the SLC NAND acts exclusively as a read cache - writes never touch the NAND. The drive looks at access patterns over time (most likely via a history table of LBAs and their frequency of access) and pulls some data into the NAND. If a read request comes in for an LBA that is present in the NAND, it's serviced out of the 4GB chip. If the LBA isn't present in the NAND, the data comes from the platters.

If a read request can be serviced out of the NAND, the drive can be spun down which should save power. In practice it's rare that a sequence of reads can entirely be serviced out of NAND. What usually happens is you get a little bit of data out of the NAND and then the drive has to spin up to give you the rest. This can be a bit annoying because you get a drive spinup event in the middle of a data access rather than just before it.

The data in the NAND remains persistent across power cycles, however not formats or defrags. You still have to defragment the drive, but doing so resets the drive's learning back to zero. Defragmenting less often is the only real solution.

It's not a huge problem because the drive learns pretty quickly. By the second time you do anything the Momentus XT is usually a lot faster at the task assuming Seagate's algorithms pull any of the data you're accessing into its on-board NAND.

The chart below shows a comparison of a Western Digital VelociRaptor 600GB, a SandForce SF-1200 based SSD and the Momentus XT in boot times. I simply timed how long it took to boot into Windows 7 from the point the OS began loading to the time I got a cursor on the desktop. After a completed boot I shut down the machine and tried again.

While the first boot takes pretty long on both hard drives, by the second boot the Momentus XT is already noticeably faster than the VelociRaptor. Seagate appears to focus mostly on small, frequently used files and aggressively pulls them into the NAND.

The chart also illustrates a very important point. The SSD's performance is consistent even compared to the first run, while the Momentus XT needs to run through a workload once before it's optimized. This applies to more than just boot time, application loads or any sort of disk access.

Over a short period of time the Momentus XT should get many of the small files you use regularly into its NAND cache but the drive is best optimized for repeatable usage patterns. If you always use the same few applications in the same way the Momentus XT will work very well. I found that it's not very difficult to get data evicted from the NAND cache if you throw a random set of applications/workloads at the drive.

Why a Read Cache?

I've written a lot about the struggles that SSD controllers must deal with to manage writing and re-writing NAND. It's not surprising that Seagate opted to use the NAND on the Momentus XT as a read-only cache. Seagate controls what data gets written to the NAND, which makes block management much simpler. There's never a situation where the drive doesn't know what it needs to keep track of. And by using SLC instead of MLC NAND, Seagate doesn't even have to worry about aggressive wear leveling either. So it's about simplicity, not the perfect hybrid design.

Seagate claims that the 4GB size offered the best balance of price/performance, but I suspect that with more aggressive caching algorithms Seagate could benefit from a larger cache. With more NAND Seagate could also prefetch data into the cache. And eventually, if there is to be a future for hybrid drives, Seagate will have to enable a NAND write cache.

Once you start caching writes as well then you effectively make the jump into the SSD realm in terms of complexity. The read-only design is pretty simple but until we see Seagate enter the consumer SSD space I doubt we'll see a more aggressive hybrid drive.

Note that you shouldn't expect to get the same performance out of the Momentus XT's single NAND device as you would an SSD. Remember that modern SSDs have anywhere between 4 and 10 channels of NAND accessed in parallel to reach their very high transfer rates. A single NAND device isn't going to end up anywhere near as fast. At best the Momentus XT should be able to read from the cache at 20 - 40MB/s depending on the data being accessed and the type of NAND Seagate is using.

The Test - Real World First
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  • 7Enigma - Tuesday, May 25, 2010 - link

    " With So much RAM we could cache those Random Read Write Directly to RAM. Why would we need a Hybrid HDD?"

    I think what is confusing you is the differences between RAM and Flash. RAM stores data while constantly consuming power. When the power is cut the data is lost. Flash retains the data when power is cut. It is also much slower than RAM in terms of read/write performance (RAM drives have been a geeks dream for a couple years now but the costs are still quite prohibitive. It seemed like we were getting closer to being possible for the average tech geek without a trust fund when RAM prices were so low last year but due to the recent price hikes it's again out of reach).

    So to answer your question IMO this drive is designed for a laptop computer where you only have a single HD bay and power consumption is a significant issue. In a laptop situation it is not practical to use RAM as a cache since you'd be draining the battery even when the computer is off.
  • janus-cassandra - Monday, May 24, 2010 - link

    Anand I think you are missing important advantages of and SSD in a laptop, (1) lower power consumption than that of a traditional drive with spinning platters and more importantly (2) decreased susceptibility to catastrophic data loss due to a hard drive crash when one's computer is dropped or undergoes any other rapid acceleration and deceleration. As this last consideration happens all to often with laptops, it seems to me that an SSD drive should be standard issue in most, if not all laptops. A hybrid drive will not provide this protection.
  • johndoe74 - Tuesday, May 25, 2010 - link

    you guys should have included a 500 GB 7200 rpm drive in there as well (or instead of the 5400 rpm) for the sake of comparison. a 5400 rpm drive is kind of a dinosaur these days
  • Klinky1984 - Tuesday, May 25, 2010 - link

    Hybrid drives are probably one of the most pointless technologies of the last give years. I don't see how they will ever amount to much. The only way they will is if the cache becomes massive, about the size of what SSDs are now and has a smarter caching algorithm. I can do the same thing that they are doing with their hybrid drive with windows already. Just plop in 8GB of RAM & set windows to prefer system caching. Bam, whatever ram you're not using is used as a cache for reads & that memory can always be reclaimed by programs needing to use it.

    Hybrid drives only have the negatives of SSD(cost) with the negatives of HDDs(speed, reliability, noise, power consumption), without much of the benefit. Whoever thought "hey, put some flash on it and then we'll let the HDD spin down!', great idea when the hard drive takes probably about 0.5 seconds in good scenario to boot back up, but some could take a couple seconds. Not a good user experience. Boot times look nice, but who spends their time rebooting their computer all day? Is 30 extra seconds booting going to be worth the added cost?

    Maybe Seagate should focus efforts on a "dual drive" where you get a 32GB SSD & a 500GB HDD combined into a a single 2.5" form factor. That would let people use the 32GB as their OS drive & the 500GB for their large files, etc.
  • mpx999 - Tuesday, May 25, 2010 - link

    Actually best of high-end disk arrays of today use Flash as a cache. For example NetApp uses module called PAM II which is up to 512MB of SLC used a cache


    Oracle storage systems, especially 7000 series, also use Flash Caches known as L2ARC i ZIL.


    So flash cache is a proven technology! They just need to go above 4GB to get better results.
  • hadifa - Tuesday, May 25, 2010 - link

    Normal HDD write speed, and very good read speed the second time for small files.

    For that purpose, this drive is very good and the 4GB is not that bad because only small files are cached, though I would have hoped for at least double that. There is no mention of how small the file needs to be in order to be cached, or maybe I misunderstood the article.

    This drive faces two challenges:
    1- Just one NAND chip so no parallel read so limited read speed of about 30-40 GB p/s
    2- The NAND is used only for reads, so the write speed is not accelerated.

    I'm not going to fuss a lot about the write, but I hope they make a version with say 2*4GB chips and a controller to raid the reads, I would be happy to shell some extra hard earned money for that.
  • Klinky1984 - Tuesday, May 25, 2010 - link

    It's not a "Smart Hybrid" like what was being proposed earlier, with Windows Vista/7 being able to identify it as being a hybrid drive and optimize your experience by placing the most used files into the flash cache. It's as "Dumb Hybrid" essentially the flash is just another layer on top of the normal in-memory buffer. Play a large video file a couple times and all your program files are flushed from the cache, just due to the amount of data going back and forth and since the cache is "dumb" it doesn't know if the sectors it's caching are part of an important document or some image from Temporary Internet Files.

    It may not be bad if you have small needs, but I think a lot of enthusiasts would be disappointed with it.
  • zodiacfml - Tuesday, May 25, 2010 - link

    I agree with all his comments.
    This is a good hybrid HDD, but to laptops only since the price premium is $50 to $90 which is quite expensive. For the same price or little more for the desktop, we can buy an SSD and HDD with a lot more performance and capacity.

    For his recommendation, this is still a good drive for notebook users looking for more capacity and performance since SSD's, though perfect for the application, is too expensive and lack capacity. Don't doubt the reliability since writes and storage are done on the platters, same reliability with mechanical HDD.

    Lastly, I think the 4GB size of SLC NAND flash is chosen as the least (expensive SLC) amount of memory for the most often used files or blocks (typical usage - OS, common Apps). Adding more or making it twice bigger will only benefit users with more and varied apps. It can be done with dual channel but the performance improvement will not be as great. Simply, Seagate just wants more margins and less trouble.
  • Chloiber - Tuesday, May 25, 2010 - link

    4GB is more than enough!
    On a regular desktop system, you can store ALL files up to 256k on 4GB...and for bigger files, a HDD isn't much slower than a single NAND module.
  • ABR - Tuesday, May 25, 2010 - link

    I hate to burst peoples' bubbles here but how many of the files slowing down boot are going to stay in the 4GB cache? Assuming you spend more of your time running your computer and working with other files that get loaded and unloaded more often than core startup components (somewhat questionable with Windows, I realize ;), then the only time you'll see boot speedup is in artificial tests. Of course, that's the way you should want it...

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