January 25, 2010

5400RPM vs 7200RPM hard disks — should you care?

Filed under: Computer joy — dalvenjah @ 12:10 am

A recent twitter discussion via Doctor Karl led me to the question of whether it matters that while large capacity desktop drives spin anywhere between 7200RPM and 15,000RPM, most large capacity laptop drives spin at only 5400RPM.

For years, 5400RPM was the standard speed of a hard drive, back when 1GB and 2GB were huge. The speed demon server drives that came out shortly thereafter were 4GB and 9GB 7200RPM drives, and required active cooling so they didn’t melt from the heat. Everyone understood that 7200 RPM was faster, because it delivered your bits quicker.

Fast forward ten years, and you can get 1TB and 2TB hard drives spinning at 7200RPM, and smaller laptop drives at 500GB and 1TB spinning at 5400RPM. (You can also get 600GB drives spinning at 15,000 RPM, but that’s today’s bacon-cooker for servers.)

But is the 5400RPM laptop drive really slow?

I would argue it’s not. Areal Density increases as drive capacity goes up; and today’s 3.5 inch and 2.5 inch drives aren’t physically bigger than the same size drives of the past, meaning their areal density has gone way up (see some examples on Wikipedia’s Memory Storage Density page).

This means that when a 1TB disk makes one revolution (in 1/5400th of a second), it’s picking up much more data from the platter than, say, an 80GB disk spinning at 7200RPM is after a single revolution in 1/7200th of a second.

Granted, you’ll still get faster performance out of a 7200RPM drive than a same-sized 5400RPM drive, but it’s not the end of the world to pick the slower-rotating disk. It also won’t get as hot as the faster one.

Now keep in mind, too, that these speeds are talking about sequential or mostly-sequential data access, when you start at one point on the disk, and read continuously to a later point, like playing a record or a CD. Unless you’re playing audio or video, most data access isn’t like this. And that kind of performance — operations per second — hasn’t increased much over 20 years. But that’s another post.

8 Comments »

  1. Another thing to consider is the OS disk cache and (when appropriate) the drive cache.

    Comment by john — February 1, 2010 @ 10:37 pm

  2. Good article. I noticed that you said 1/5400 and 1/7200 per second is a revolution, but actually its per minute.

    Comment by Okay — April 24, 2010 @ 11:36 pm

  3. Thank you very much for your article. I was searching for information that would help one decide between a 640GB at 5400RPM and a 500GB at 7200RPM. Your clear explanation would lead one to conclude that the difference is minimal. (My calculations suggest that the 7200RPM would be slightly faster, but only slightly–the speed is increased by 33%; however, the size increase of 28% almost completely offsets the speed increase percentage.) The “slower” 640GB disc, then, will offer performance almost identical to the “faster” disc, plus providing higher capacity and cooler temps.

    Comment by Steve — August 8, 2010 @ 1:48 pm

  4. Nice write-up! I was wondering whether to get a WD 750GB 5400 RPM (8MB cache) or a 640GB 7200RPM (16MB cache) Samsung Spinpoint for my PS3. Per the calculations the 750 is 17% bigger offsetting the speed performance of the 7200rpm drive by 50%. I think I’ll go big!

    Comment by Mark — August 30, 2010 @ 10:27 am

  5. @ Okay

    While you are correct in saying that the drives spin at 5,400 RPM and 7,200RPM, this is equal to (in the case of the 7,200RPM Drive) 120 revolutions every 1 second, which is 12 revolutions every 0.1 seconds, which is 1.2 Revolutions every 0.01 seconds, which is 0.12 Revolutions every 0.001 seconds.

    To put it basically, it is (approx) One (1.2 Exact to one decimal place) revolution every One One Hundredth of a Second for a 7,200RPM Drive.

    The OP was talking in Revolutions per Fraction of a Second, not Revolutions Per Minute, however, he was incorrect in stating that the disk revolved once every 7,200th of a second.

    As before stated, every 100th of a second, the Disk revolves 1.2 times.

    Shaun

    Comment by Shaun — November 29, 2010 @ 12:55 am

  6. I Can Haz Cheezburger?

    Comment by hotmaul — November 30, 2010 @ 8:41 am

  7. @Steve: Despite what the article says, the 28% difference in size will offset the difference in rpm only as much as it is due to increased circumferential (denser tracks) or axial density (more platters.)

    For example, it is perfectly possible that the 28% difference comes solely from increased radial density, i.e. more tracks per side, in which case a single revolution will result in the same amount of data being read as in the 500G device.

    Comment by tranto — July 7, 2011 @ 12:51 am

  8. Data throughput is a product of density and rotational speed, yes. A 7200 RPM drive with the same density will read faster than a 5200rpm drive. This is important in a sequential data case (ie: streaming a movie, copying a file to an external drive)

    You’re missing the key point of rotational latency- the time it takes for data to end up under the head so it can be read. In a random access case (ie: read a large number of small files or different data areas), you’re spending more time waiting as it’s read the last item and you’re doing nothing while you wait for the platter to come around.

    So it depends on workload.

    Comment by Michael — February 8, 2014 @ 12:07 pm

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