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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  • ataxy - Sunday, September 19, 2010 - link

    CrystalDiskMark 3.0 (C) 2007-2010 hiyohiyo
    Crystal Dew World : http://crystalmark.info/
    * MB/s = 1,000,000 byte/s [SATA/300 = 300,000,000 byte/s]

    Sequential Read : 97.361 MB/s
    Sequential Write : 95.143 MB/s
    Random Read 512KB : 43.965 MB/s
    Random Write 512KB : 60.802 MB/s
    Random Read 4KB (QD=1) : 0.686 MB/s [ 167.6 IOPS]
    Random Write 4KB (QD=1) : 1.350 MB/s [ 329.5 IOPS]
    Random Read 4KB (QD=32) : 1.675 MB/s [ 408.9 IOPS]
    Random Write 4KB (QD=32) : 1.517 MB/s [ 370.3 IOPS]

    Test : 1000 MB [E: 0.1% (0.7/465.8 GB)] (x5)
    Date : 2010/09/19 14:32:21
    OS : Windows 7 Ultimate Edition [6.1 Build 7600] (x86)
    Drive : WD 500GB black sata

    CrystalDiskMark 3.0 (C) 2007-2010 hiyohiyo
    Crystal Dew World : http://crystalmark.info/
    * MB/s = 1,000,000 byte/s [SATA/300 = 300,000,000 byte/s]

    Sequential Read : 58.064 MB/s
    Sequential Write : 57.884 MB/s
    Random Read 512KB : 18.819 MB/s
    Random Write 512KB : 26.504 MB/s
    Random Read 4KB (QD=1) : 0.382 MB/s [ 93.4 IOPS]
    Random Write 4KB (QD=1) : 1.098 MB/s [ 268.2 IOPS]
    Random Read 4KB (QD=32) : 0.675 MB/s [ 164.7 IOPS]
    Random Write 4KB (QD=32) : 1.040 MB/s [ 253.8 IOPS]

    Test : 1000 MB [D: 0.7% (1.1/149.0 GB)] (x5)
    Date : 2010/09/19 14:42:04
    OS : Windows 7 Ultimate Edition [6.1 Build 7600] (x86)
    Drive : WD 160GB pata

    CrystalDiskMark 3.0 (C) 2007-2010 hiyohiyo
    Crystal Dew World : http://crystalmark.info/
    * MB/s = 1,000,000 byte/s [SATA/300 = 300,000,000 byte/s]

    Sequential Read : 52.181 MB/s
    Sequential Write : 51.925 MB/s
    Random Read 512KB : 23.469 MB/s
    Random Write 512KB : 25.955 MB/s
    Random Read 4KB (QD=1) : 0.342 MB/s [ 83.5 IOPS]
    Random Write 4KB (QD=1) : 0.660 MB/s [ 161.2 IOPS]
    Random Read 4KB (QD=32) : 0.626 MB/s [ 152.7 IOPS]
    Random Write 4KB (QD=32) : 0.660 MB/s [ 161.1 IOPS]

    Test : 1000 MB [G: 82.5% (120.7/146.4 GB)] (x5)
    Date : 2010/09/19 14:52:55
    OS : Windows 7 Ultimate Edition [6.1 Build 7600] (x86)
    Drive : Hitachi 200GB sata

    CrystalDiskMark 3.0 (C) 2007-2010 hiyohiyo
    Crystal Dew World : http://crystalmark.info/
    * MB/s = 1,000,000 byte/s [SATA/300 = 300,000,000 byte/s]

    Sequential Read : 100.045 MB/s
    Sequential Write : 94.373 MB/s
    Random Read 512KB : 28.362 MB/s
    Random Write 512KB : 40.874 MB/s
    Random Read 4KB (QD=1) : 0.308 MB/s [ 75.2 IOPS]
    Random Write 4KB (QD=1) : 0.584 MB/s [ 142.7 IOPS]
    Random Read 4KB (QD=32) : 0.462 MB/s [ 112.9 IOPS]
    Random Write 4KB (QD=32) : 0.561 MB/s [ 137.0 IOPS]

    Test : 1000 MB [C: 9.8% (45.4/465.8 GB)] (x5)
    Date : 2010/09/19 15:00:50
    OS : Windows 7 Ultimate Edition [6.1 Build 7600] (x86)
    Drive : Seagate Momentus xt 500GB Hybrid sata

    HD Tune pro 4.01

    HD Tune Pro: WDC WD5001AALS-00L3B File Benchmark

    Drive E:

    File Size: 64 MB

    Block size Read speed
    0.5 KB 2830 KB/s
    1 KB 5540 KB/s
    2 KB 10456 KB/s
    4 KB 17874 KB/s
    8 KB 35154 KB/s
    16 KB 53983 KB/s
    32 KB 69579 KB/s
    64 KB 81769 KB/s
    128 KB 89444 KB/s
    256 KB 89529 KB/s
    512 KB 95159 KB/s
    1024 KB 92953 KB/s
    2048 KB 89519 KB/s
    4096 KB 89284 KB/s
    8192 KB 90795 KB/s

    Block size Write speed
    0.5 KB 2924 KB/s
    1 KB 6357 KB/s
    2 KB 10032 KB/s
    4 KB 19255 KB/s
    8 KB 30796 KB/s
    16 KB 45387 KB/s
    32 KB 57495 KB/s
    64 KB 71824 KB/s
    128 KB 76834 KB/s
    256 KB 77428 KB/s
    512 KB 86689 KB/s
    1024 KB 79702 KB/s
    2048 KB 84710 KB/s
    4096 KB 88730 KB/s
    8192 KB 86945 KB/s

    HD Tune Pro: WDC WD1600JB-22GVA0 File Benchmark

    Drive D:

    File Size: 64 MB

    Block size Read speed
    0.5 KB 7064 KB/s
    1 KB 13345 KB/s
    2 KB 22214 KB/s
    4 KB 37595 KB/s
    8 KB 53365 KB/s
    16 KB 58037 KB/s
    32 KB 57311 KB/s
    64 KB 57415 KB/s
    128 KB 57270 KB/s
    256 KB 57429 KB/s
    512 KB 57313 KB/s
    1024 KB 57010 KB/s
    2048 KB 56553 KB/s
    4096 KB 57260 KB/s
    8192 KB 57085 KB/s

    Block size Write speed
    0.5 KB 6287 KB/s
    1 KB 11314 KB/s
    2 KB 19527 KB/s
    4 KB 37841 KB/s
    8 KB 47869 KB/s
    16 KB 60798 KB/s
    32 KB 60732 KB/s
    64 KB 60779 KB/s
    128 KB 61654 KB/s
    256 KB 61031 KB/s
    512 KB 61790 KB/s
    1024 KB 61536 KB/s
    2048 KB 61438 KB/s
    4096 KB 61347 KB/s
    8192 KB 52047 KB/s

    HD Tune Pro: Hitachi HTS722020K9S File Benchmark

    Drive G:

    File Size: 64 MB

    Block size Read speed
    0.5 KB 2493 KB/s
    1 KB 4753 KB/s
    2 KB 8566 KB/s
    4 KB 16159 KB/s
    8 KB 28805 KB/s
    16 KB 40326 KB/s
    32 KB 48187 KB/s
    64 KB 48074 KB/s
    128 KB 48357 KB/s
    256 KB 47654 KB/s
    512 KB 47100 KB/s
    1024 KB 46436 KB/s
    2048 KB 45869 KB/s
    4096 KB 46734 KB/s
    8192 KB 46829 KB/s

    Block size Write speed
    0.5 KB 1844 KB/s
    1 KB 3659 KB/s
    2 KB 7149 KB/s
    4 KB 13897 KB/s
    8 KB 25630 KB/s
    16 KB 43394 KB/s
    32 KB 50669 KB/s
    64 KB 47477 KB/s
    128 KB 50672 KB/s
    256 KB 48710 KB/s
    512 KB 49707 KB/s
    1024 KB 49625 KB/s
    2048 KB 48736 KB/s
    4096 KB 48497 KB/s
    8192 KB 46016 KB/s

    HD Tune Pro: ST95005620AS File Benchmark

    Drive C:

    File Size: 64 MB

    Block size Read speed
    0.5 KB 3822 KB/s
    1 KB 7154 KB/s
    2 KB 11238 KB/s
    4 KB 22509 KB/s
    8 KB 39420 KB/s
    16 KB 52214 KB/s
    32 KB 41775 KB/s
    64 KB 80403 KB/s
    128 KB 79209 KB/s
    256 KB 76877 KB/s
    512 KB 77466 KB/s
    1024 KB 71046 KB/s
    2048 KB 85175 KB/s
    4096 KB 79494 KB/s
    8192 KB 80592 KB/s

    Block size Write speed
    0.5 KB 1592 KB/s
    1 KB 2918 KB/s
    2 KB 6250 KB/s
    4 KB 11365 KB/s
    8 KB 29331 KB/s
    16 KB 47430 KB/s
    32 KB 61350 KB/s
    64 KB 71344 KB/s
    128 KB 82653 KB/s
    256 KB 81207 KB/s
    512 KB 81749 KB/s
    1024 KB 83883 KB/s
    2048 KB 79688 KB/s
    4096 KB 82106 KB/s
    8192 KB 82661 KB/s

    HD tune pro 4.01 extra test

    HD Tune Pro: WDC WD5001AALS-00L3B Extra Tests

    Test capacity: full

    Random seek 81 IOPS 12.4 ms 0.039 MB/s
    Butterfly seek 70 IOPS 14.3 ms 0.034 MB/s
    Random seek / size 64 KB 79 IOPS 12.6 ms 1.219 MB/s
    Random seek / size 8 MB 14 IOPS 71.1 ms 57.051 MB/s
    Sequential read outer 1456 IOPS 0.7 ms 91.002 MB/s
    Sequential read middle 1257 IOPS 0.8 ms 78.568 MB/s
    Sequential read inner 723 IOPS 1.4 ms 45.176 MB/s
    Burst rate 1575 IOPS 0.6 ms 98.444 MB/s

    HD Tune Pro: ST95005620AS Extra Tests

    Test capacity: full

    Random seek 52 IOPS 19.2 ms 0.025 MB/s
    Butterfly seek 46 IOPS 21.8 ms 0.022 MB/s
    Random seek / size 64 KB 54 IOPS 18.4 ms 0.837 MB/s
    Random seek / size 8 MB 13 IOPS 75.5 ms 53.697 MB/s
    Sequential read outer 1600 IOPS 0.6 ms 99.975 MB/s
    Sequential read middle 1429 IOPS 0.7 ms 89.309 MB/s
    Sequential read inner 848 IOPS 1.2 ms 52.994 MB/s
    Burst rate 1569 IOPS 0.6 ms 98.040 MB/s

    HD Tune Pro: WDC WD5001AALS-00L3B Random Access

    Test capacity: full

    Read test

    Transfer size operations / sec avg. access time avg. speed
    512 bytes 79 IOPS 12 ms 0.039 MB/s
    4 KB 80 IOPS 12 ms 0.314 MB/s
    64 KB 73 IOPS 13 ms 4.583 MB/s
    1 MB 37 IOPS 26 ms 37.286 MB/s
    Random 49 IOPS 20 ms 25.362 MB/s

    HD Tune Pro: ST95005620AS Random Access

    Test capacity: full

    Read test

    Transfer size operations / sec avg. access time avg. speed
    512 bytes 50 IOPS 19 ms 0.025 MB/s
    4 KB 53 IOPS 18 ms 0.207 MB/s
    64 KB 52 IOPS 19 ms 3.273 MB/s
    1 MB 32 IOPS 31 ms 32.107 MB/s
    Random 40 IOPS 24 ms 20.671 MB/s

    in all honesty the drive did boost my boot time but it certainly aint no wonder
  • amad2892 - Sunday, October 31, 2010 - link

    Does anyone know if this Seagate Momentus XT hybrid drive work in desktops with an adapter?
  • ataxy - Friday, November 19, 2010 - link

    no need for an adapter, plug it the same way you would plug a sata drive
  • cdxpat - Monday, November 1, 2010 - link

    With much larger memory will come much larger windows swap files. I wonder how Seagate is managing that. I would have prefered an implemention that allowed for some configuration of what is cached.

    Example cache only files that have not changed in the last hour since constantly updated files will ruin any caching algorithm .

    Or cache by file type eg .exe, .dll . ini but not .doc
  • Mordanti - Tuesday, January 25, 2011 - link

    Bought drive in Nov 2010, started having issues with my Gateway 6864 FX not waking from sleep mode in Win7-64bit after 2 weeks. As it was past the 30 day NCIX return limit, I finally dealt with Seagate for a warranty exchange. Replacement drive worked fine for a week, then did EXACT same thing... only NOW my laptop refuses to boot or save my BIOS settings. CMOS clearing, different SATA drives and tech support visits to no effect; I now have a dead drive AND a dead laptop. Thanks for nothing, Seagate... not to mention the 60+ hours I've spent scouring forums, calling tech support and swapping drives. All because Seagate couldn't design a drive with APM that knew what it was doing.

    While it WAS working, it woke from Sleep mode almost instantly and did indeed cut boot times by half. For general laptop use( no heavy video / encoding )it made things snappy.

    Bottom line: DO NOT BUY this hybrid drive unless you are prepared for the consequences. Cutting-edge tech sometimes means you bleed, as is my case. I'm out $$ for the drive, and $$ for my was-perfectly-fine laptop that's now a brick and Seagate has no intention of doing anything except sending me ANOTHER one of these computer-killers in a box.

    No thanks. I'll buy an SSD and keep backing up my data, like a wary consumer should.
  • jb510 - Saturday, March 5, 2011 - link

    I'm curious why you are so confident that the problem originated with your Momentus XT and not with your laptop itself? While I suppose a drive could cause the problems you describe my instinct would be the exact oposite that the laptop fried the drive.

    Personally I've had one of these for about a month and another for a few days (different laptops) and absolutely love this drive.
  • jb510 - Saturday, March 5, 2011 - link

    Anand or anyone... I'm thinking about getting a OCZ V3 SSD (when available) for my new MBP arriving next week. The MBP is arriving with a 750GB 5400rpm drive which I'd thought about moving into drive caddy to replace the ODD. However, I'm wondering how much benefit there would be putting one of my 500GB Momentus XT's in the ODD bay instead of the 750GB HDD?

    I've seen the amazing speed advantages using the Momentus XT as my boot drive (only drive) in my current laptops, but am wondering about it's performance when used as data drive. I imagine it'd be similar or even better, but thought I'd ask in case anyone knew better...

    Finally, Anand, one thing that seems to be missing from all but the first test is how the performance changes over time as data ges cached by the HHDD (Hybrid Hard Disk Drive) and what you did or didn't do to standardize that performance.
  • Hrel - Wednesday, July 6, 2011 - link

    I read this long ago, probably a similar comment on here. But why even bother comparing to a 5400rpm hdd? The momentus XT runs at 7200rpm. I want to see it compared to a regular 32MB cache 7200rpm 2.5" hdd. My friends has a 7200rpm 2.5" drive in his Compal NBLB2 and it loads games and stuff REALLY fast. I have a striped RAID on my desktop and his laptop loads up League of Legends RIGHT after my desktop. My desktop was built in 07, but still. 2 7200rpm 3.5" drives in stripped RAID should blow his away... it doesn't.
  • cam94z28 - Monday, November 21, 2011 - link

    I think a lot of what we're seeing here, is simply a more modern 7200rpm 2.5" drive that just happens to hang with an older raptor. FWIR, the NAND speeds don't even really come into play during an instant performance test. It pulls frequently used data, and smaller files during normal desktop use. Over a few days to a week, you would see a performance increase in boot times, email, web browser startup, etc.. if these are the common apps you tend to use. Running CrystalDiskMark, HdTune, or HdTach only one time, would be mostly physical disk performance.
  • rajesh91 - Wednesday, March 14, 2012 - link

    I am not experienced in such things and hence seeking some help here. I want 500GB plus storage space in my MBP-2009 and SSD are way too pricy for that. This solution seems really good, but can some one tell if I can use this in my MacBook Pro (running the latest update of Lion)?

    Any help is appreciated! Thank you...

    Note: In their site they have a new 700GB version. Can I use that instead?

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