Why disks can't read - or write

Why disks can't read - or write

Summary: We worry about a disk drive's "click of death" but the fact is that unrecoverable read errors are much more common. These errors are what remains after sophisticated signal processing fails to recover the data. What causes these errors? Many things.

TOPICS: Hardware, Storage

No bit left behind We worry about a disk drive's "click of death" but the fact is that unrecoverable read errors are much more common. Ever look at a pretty external disk and wonder "what could go wrong?" If you must know. . . .

Why do we have errors? Most SATA drives have an unrecoverable read error (URE) or bit error rate (BER) of 1 in 1014, or about 1 block in every 12 TB. These errors are sometimes latent - we don't know about them until we can't read the data.

But these errors are what remains after very sophisticated servo and signal processing technology tries to recover the data. What causes these errors? Many things.

Hot head Disks are permanently formatted at the factory with information that tells the read/write head where it is. These embedded servo tracks tell the r/w electronics where the head is - until they don't.

Like every other bit on a disk, servo tracks can be damaged or corrupted - and your formatting software or RAID array can't replace them. A grain of dust under the head can create a momentary flash of heat - thermal asperity in engineering lingo - that wipes out a few hundred bits. Or something harder and chunkier, say a chip of metal media plating, can scratch out those bits.

Either way, vital positioning information is lost - and so is your data.

Round and round we go and where we stop . . . Disk drive tracks are not perfectly circular. At 120 revolutions per second - @7200 rpm - it is a roller-coaster ride for the heads.

The head positioning system knows this and adapts, like we adapt to the motion of an escalator. But as disks shake from noise or vibration, or bearings in motors or actuators wear, the heads can't adapt as fast or as reliably. They start to lose the ability to lock onto a track long enough to read your data.

Which translates to losing your data.

Consumer SATA drives like most of us use will retry dozens, maybe more than 100 times before giving up. But RAID drives and costly enterprise drives will quit after a few tries and declare the drive failed, the idea being that mission-critical systems need the best performance.

So when your consumer drive can't find the data, it really can't find the data.

Other read problems can be due to electrostatic discharge - which is hard to prove - and damage to the drive electronics. So keep your fingers off the drive's circuit board!

Scribble, scribble, scribble Sometimes reads aren't readable because the write, not the read, failed. Read-after-write checks slow drives down too much to be practical in most applications.

Media damage - scratches, pits, irregularities - can corrupt data. Hard particles scratch, while soft ones like aluminum, will smear across the surface, making magnetization more difficult.

Since the outer tracks move faster than the inner tracks, the speed difference may help account for the observed difference in outer track errors. Another possible cause: disk platter lubricant build up due to centrifugal force which forces the head away from the media.

The Storage Bits take Disk drives are incredible machines. 30 years ago a 500 MB drive was the size of a washing machine and cost $50,000. Now you can get a 1 TB drive that fits in shirt pocket: 2000x the capacity; 25x more reliable; and 75x the data rate. All for 1/200th the price.

But as we put more data on a smaller machine, the impact of data loss grows. Users who rely on them should appreciate their limitations so they can protect themselves and their data.

Comments welcome, of course.

Topics: Hardware, Storage

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  • Many people rock and sway their notebooks

    Thanks for article. I notice that many people routinely rock and sway their notebook computers while the hard disk is running. Any comments about this? I have always felt uncomfortable about doing that sort of thing, especially on a bumpy subway ride, so I recently switched to SSD.
    Benjie Dog
    • RE: Why disks can't read - or write

      @Benjie Dog
      Notebook drives are designed for notebook environments and their smaller size makes them much more shock resistant than 3.5" drives. The real issue is sudden, sharp acceleration, not the swaying of a subway car.

      R Harris
  • Write after read

    I wonder, with such a huge amount of ram on PCs these days, (e.g. you save your 500k document from a machine that has 4GB of RAM), why not que up checks and do it during idle.
    • RE: Why disks can't read - or write

      I've posed that question to notebook designers because with a battery the chance that the DRAM will lose power is so much less. Spin the drive down and leave it down until needed. They aren't ready to go there yet which, IMHO, is overly conservative.

      Desktops are a different story.

      R Harris
  • They are reliable

    I had an old SCSI hard drive from 1996 that still worked just fine till recently.
    It just died one day with no forewarning. I did not have anything important on it.
    • RE: Why disks can't read - or write

      A lightbulb burned for 41 years. Do you consider lightbulbs reliable?

      R Harris
  • Thank-you for three things

    Thanks, Robin Harris, for doing three things:

    1. Writing a helpful article
    2. Personally answering our questions and comments
    3. Getting rid of that annoying and intrusive ad trying to sell us junk
    Benjie Dog
  • RE: Why disks can't read - or write

    My 1 month old 1TB hard drive recently developed a few bad sectors.. yet I have 40's, 80's, 160's, 250's and another 1TB that have been running fine since I bought them. It's the luck of the draw... no biggie, send the drive back to the manufacture and have it replaced. :-)
  • RE: Why disks can't read - or write

    I think in time we will have drives with a built in R.A.I.D type functionality which, while lowering the data capacity, will certainly reduce the odds of lost data. If the drive is clever enough it can have a built warning feature once a certain percentage of space has been written off as "bad".
  • Your figures are, as usual, bunk.

    If read errors were as common as you maintain, then computers would be crashing every few hours from read errors in the swap file. It ain't happening. Reality trumps ivory tower theory every time.
  • RE: Why disks can't read - or write

    hdd have had failure warnings built in for some time it is called "smart" self monitoring and reporting the drive regulary checks its self and updates a log inside the drive this log can be read my software that reads the log and then reports to the user or sets off an alarm once the details in the log gets out side of a predifined limit, many mother boards support smart but again no one ever turns it on or makes use of it if they did their drives would warn them of impeding drive failure and permanat data loss.
  • Very good to remember

    that physics functions even when everything seems to operate naturally - it's all a technical wonder and it is not easy to get today's standard level of reliability! Already that old 500MB disk unit ("washing machine") was a wonder!
  • 500mb??? I thought 30 years ago it was only 5mb!

    OK, maybe they weren't the size of a washing machine, but they were about the same size as an under-counter ice maker.

    And if you looked at them sideways, they failed.