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Exclusive: The Dual Actuator/Head Hard Drive

By - Source: Tom's Hardware US | B 54 comments
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Last week we reported about Seagate's latest Cheetah 15K.7, which is the company's latest enterprise level HDD, boasting a blistering spindle speed of 15,000 RPM and a decent capacity of 600 GB.

While talking about the drive, one commenter proposed a question: why don't hard drive manufacturers do something like add a second set of read/write heads to increase performance rather than turning up the spindle rate to dangerously high levels?

Good question! We went directly to Seagate for the answers.

A long time ago, there was a company called Conner Peripherals. Some Tom's Hardware readers may remember the company. Conner actually patented a hard drive design that utilized dual actuators, that is, a hard drive with two sets of read/write units. Later on when Seagate acquired Conner, the intellectual property transferred to Seagate.

Seagate's first drive with multiple read/write heads was actually a 10 MB drive. But instead of using two sets of actuators, each arm actually two read/write heads--one at the end, and one on the middle. The drive had two arms, one above and one below the platter, giving the single platter drive four read/write heads. The arm actually moved forward and backwards in a straight line--like today's optical drives--using a stepping servo instead of in a curved sweeping motion. One read/write head would be responsible for the inner half of the platter while the second read/write head would take care of the rest.

The 10 MB drive had a single platter with a track density of 550 tracks per inch. Each track being a full ring around the platter. (Right Photo: The 10 MB Seagate ST-412)

After one drive however, Seagate developed a 2 GB drive with dual actuators. The drive had an actuator mechanism at opposing corners which theoretically provided double the throughput and operated independently of each other. The drive had an RPM rate of 7200 RPM and had a track density of 3000 tracks per inch.

For comparison sake, the new Cheetah 15K.7 has a track density of 165,000 tracks per inch and Seagate's latest 2 TB drive has 236,000 tracks per inch.

As drive capacities continued to increase however, Seagate stopped producing dual-actuator drives for several reasons:

First and most importantly, it's cost prohibitive to produce such drives. When you factor in the extra components, increased material usage, and weight gains, the advantages are minimal said Seagate. Secondly, dual-actuator drives will be larger, produce more heat, and consume more power. Lastly and probably most important, is that current harddrives store detailed servo detail about the physical makeup of the platters. Due to fluctuations in heat, materials expand and contract differently at different locations on the disk. All this would have to be taken into account with a dual-actuator drive. Consequently, one actuator would actually perform differently than the other. In terms of logic, there would have to be two head controllers, or at least a controller that was capable of handling two actuators.

According to Seagate, such a drive would weigh approximately 20-percent heavier and take up more room in terms of length.

So while creating a dual-actuator drive today is technically possible, it's not a viable business solution.

Seagate also mentioned to us that decades ago when harddrives were too heavy to even lift, many drives actually had sweep-heads--literally. These heads had camel-hair brushes on them that would sweep the platters clean as they turn. Such drives were utilized in large-scale mainframe systems.

Will manufacturers like Seagate ever bring back hard drives with dual actuator heads? Unlikely, given that the focus is now on increasing capacities and SSDs.

Below we have photos of the original patent that Conner Peripherals filed for its dual-actuator hard drive technology:


Many thanks go to Seagate for its help on this article.

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Top Comments
  • 18 Hide
    IronRyan21 , July 17, 2009 8:08 PM
    Cool Article, Keep it up Toms!
  • 17 Hide
    nekatreven , July 17, 2009 8:11 PM
    So we just have to ask and the answer will be revealed?!?

    "Why do hotdog buns come in packs of 8 when hotdogs come in packs of 12?"

    (but really, nice article) :) 
  • 13 Hide
    nirvanabah , July 17, 2009 8:34 PM
    nekatrevenSo we just have to ask and the answer will be revealed?!?"Why do hotdog buns come in packs of 8 when hotdogs come in packs of 12?"



    To force OCD people to buy 2 packs of hot dogs and 3 packs of buns whenever they want a hotdog, duh!
Other Comments
    Display all 54 comments.
  • 10 Hide
    puddleglum , July 17, 2009 8:05 PM
    Because today instead of a Dual Actuator/Head setup, we use two HDDs in a RAID setup. :) 
  • 18 Hide
    IronRyan21 , July 17, 2009 8:08 PM
    Cool Article, Keep it up Toms!
  • 17 Hide
    nekatreven , July 17, 2009 8:11 PM
    So we just have to ask and the answer will be revealed?!?

    "Why do hotdog buns come in packs of 8 when hotdogs come in packs of 12?"

    (but really, nice article) :) 
  • 5 Hide
    joex444 , July 17, 2009 8:23 PM
    That first Seagate implementation isn't even what comes to mind. What I mean, is if you have one read head that does half the platter, and another for the other half then you can only increase the transfer speed if your data is completely randomly scattered. This would not help sequential reads whatsoever.

    Now who would want to run a 5.25" HDD? Maybe stick two sets of platters from 2.5" drives in there and see what happens. Technically its dual actuator... Implement a RAID controller onboard and see what happens.
  • 13 Hide
    nirvanabah , July 17, 2009 8:34 PM
    nekatrevenSo we just have to ask and the answer will be revealed?!?"Why do hotdog buns come in packs of 8 when hotdogs come in packs of 12?"



    To force OCD people to buy 2 packs of hot dogs and 3 packs of buns whenever they want a hotdog, duh!
  • 4 Hide
    B-Unit , July 17, 2009 8:51 PM
    Just eat your hotdogs on regular sliced bread, problem solved.
  • 2 Hide
    daneault , July 17, 2009 8:58 PM
    nekatreven"Why do hotdog buns come in packs of 8 when hotdogs come in packs of 12?"(but really, nice article)


    Not to derail the thread further, but the hotdog buns come in packs of 6 or 12 where i come from (QC, Canada).

    Nice article.
  • -9 Hide
    hemelskonijn , July 17, 2009 9:30 PM
    Make up your mind!

    Quote:
    Last week we reported about Seagate's latest Cheetah 15K.7, which is the company's latest enterprise level HDD, boasting a blistering spindle speed of 15,000 RPM and a decent capacity of 600 GB.


    The spec's say 15.700rpm and as any one can see so way the model indication why do you guys at toms keep calling it a 15,000rpm drive ?
  • -2 Hide
    Anonymous , July 17, 2009 9:51 PM
    I have another question. Why don't all heads read in parallel?. That would not reduce rotational delay but it could increase throughput.
  • 9 Hide
    tuannguyen , July 17, 2009 10:17 PM
    hemelskonijnMake up your mind! The spec's say 15.700rpm and as any one can see so way the model indication why do you guys at toms keep calling it a 15,000rpm drive ?


    Because Seagate says it's 15,000 RPM?

    http://www.seagate.com/docs/pdf/marketing/po_cheetah_15k_7.pdf

    And the ".7" in 15K.7 doesn't represent "15,700" RPM.

    / Tuan
  • 8 Hide
    DarkMantle , July 17, 2009 10:41 PM
    Good article, great images. Well done Tuan.
  • 4 Hide
    skykaptain , July 17, 2009 11:06 PM
    I was wondering this too. Why can't they then use laptop platters in desktop HHD's with dual arms? might work. I think they should continue to look into this. I'm sure there are some people that would pay more for this. Oh wel, I just need to wait till SSD's get cheap now :( 
  • 0 Hide
    brandonvi , July 17, 2009 11:09 PM
    this seems rather stupid honestly to me by the hard drive companys from what i understand a normal 7200rpm harddrive does somewhere around 75mb/s while the SATA 3.0 goes MUCH faster even if it cost $100 instead of $50 for a 500GB harddrive there are a lot of people that will pay that for 2x the speed and there are a lot that do by making raids i am right now planing to make a 4 drive raid in my next computer i have no need of more then even 500GB of total Harddrive space yet i am going to end up with 2TB of room instead buying 2 drives that were 2x as fast in a raid would of been somthing i would do with out a problem at all.
  • 1 Hide
    hemelskonijn , July 17, 2009 11:21 PM
    I stand corrected and thanks for the bit of extra information.

    ./me bows down
  • 7 Hide
    michaelahess , July 17, 2009 11:37 PM
    It would also raise failure rates......
  • 1 Hide
    flinxsl , July 17, 2009 11:42 PM
    a good rule to follow for when you get the idea "Hey a really good simple way to improve XX would be _______": somebody has already thought of it before and there is a good reason why it doesnt exist. usually the answer comes from following the money
  • 2 Hide
    computabug , July 17, 2009 11:56 PM
    Great article, Tuan! This is the stuff I read Toms for :)  and of course, the usual reviews and interviews... the interesting stuff on toms :D 
  • 1 Hide
    tuannguyen , July 17, 2009 11:58 PM
    Thanks for the feedback guys!
    More articles like these coming. :) 
    Hope everyone enjoys the weekend.

    / Tuan
  • 4 Hide
    Anonymous , July 18, 2009 12:16 AM
    I would say one dual head drive (eg: 500GB) would cost considerably less than 2 drives (2x250GB). It will run cooler,consume less, and will be about as fast as a raid setup of 2x250GB.

    It will also be smaller in size,and could be made size compatible with normal 3,5" drives easily.

    It would make sense for power efficient servers.
    I can't speak of heat issues, but if you would program the driver of the disk to not have the 2 arms read/write on the same ring or sector, that may reduce performance little, but it might increase data accuracy.
    Then again, if the ring has been preset, and would be a sector of about 500MB, the drive could be equipped with 256MB ram memory, that could buffer the data of the second arm (in case a linear file-read is preferred).

    Anyways, as I hear it from seagate, it's a technology that never made sense back in the days, but today it does,and is easy to create.
    I think it's perhaps a meager excuse why noone actually took time to further develop that theory or patent.
    Instead, they buy a patent and not use it? What good does that do?
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