Hard disk shock protection
  ==========================
  
  Author: Elias Oltmanns <eo@nebensachen.de>
  Last modified: 2008-10-03
  
  
  0. Contents
  -----------
  
  1. Intro
  2. The interface
  3. References
  4. CREDITS
  
  
  1. Intro
  --------
  
  ATA/ATAPI-7 specifies the IDLE IMMEDIATE command with unload feature.
  Issuing this command should cause the drive to switch to idle mode and
  unload disk heads. This feature is being used in modern laptops in
  conjunction with accelerometers and appropriate software to implement
  a shock protection facility. The idea is to stop all I/O operations on
  the internal hard drive and park its heads on the ramp when critical
  situations are anticipated. The desire to have such a feature
  available on GNU/Linux systems has been the original motivation to
  implement a generic disk head parking interface in the Linux kernel.
  Please note, however, that other components have to be set up on your
  system in order to get disk shock protection working (see
  section 3. References below for pointers to more information about
  that).
  
  
  2. The interface
  ----------------
  
  For each ATA device, the kernel exports the file
  block/*/device/unload_heads in sysfs (here assumed to be mounted under
  /sys). Access to /sys/block/*/device/unload_heads is denied with
  -EOPNOTSUPP if the device does not support the unload feature.
  Otherwise, writing an integer value to this file will take the heads
  of the respective drive off the platter and block all I/O operations
  for the specified number of milliseconds. When the timeout expires and
  no further disk head park request has been issued in the meantime,
  normal operation will be resumed. The maximal value accepted for a
  timeout is 30000 milliseconds. Exceeding this limit will return
  -EOVERFLOW, but heads will be parked anyway and the timeout will be
  set to 30 seconds. However, you can always change a timeout to any
  value between 0 and 30000 by issuing a subsequent head park request
  before the timeout of the previous one has expired. In particular, the
  total timeout can exceed 30 seconds and, more importantly, you can
  cancel a previously set timeout and resume normal operation
  immediately by specifying a timeout of 0. Values below -2 are rejected
  with -EINVAL (see below for the special meaning of -1 and -2). If the
  timeout specified for a recent head park request has not yet expired,
  reading from /sys/block/*/device/unload_heads will report the number
  of milliseconds remaining until normal operation will be resumed;
  otherwise, reading the unload_heads attribute will return 0.
  
  For example, do the following in order to park the heads of drive
  /dev/sda and stop all I/O operations for five seconds:
  
  # echo 5000 > /sys/block/sda/device/unload_heads
  
  A simple
  
  # cat /sys/block/sda/device/unload_heads
  
  will show you how many milliseconds are left before normal operation
  will be resumed.
  
  A word of caution: The fact that the interface operates on a basis of
  milliseconds may raise expectations that cannot be satisfied in
  reality. In fact, the ATA specs clearly state that the time for an
  unload operation to complete is vendor specific. The hint in ATA-7
  that this will typically be within 500 milliseconds apparently has
  been dropped in ATA-8.
  
  There is a technical detail of this implementation that may cause some
  confusion and should be discussed here. When a head park request has
  been issued to a device successfully, all I/O operations on the
  controller port this device is attached to will be deferred. That is
  to say, any other device that may be connected to the same port will
  be affected too. The only exception is that a subsequent head unload
  request to that other device will be executed immediately. Further
  operations on that port will be deferred until the timeout specified
  for either device on the port has expired. As far as PATA (old style
  IDE) configurations are concerned, there can only be two devices
  attached to any single port. In SATA world we have port multipliers
  which means that a user-issued head parking request to one device may
  actually result in stopping I/O to a whole bunch of devices. However,
  since this feature is supposed to be used on laptops and does not seem
  to be very useful in any other environment, there will be mostly one
  device per port. Even if the CD/DVD writer happens to be connected to
  the same port as the hard drive, it generally *should* recover just
  fine from the occasional buffer under-run incurred by a head park
  request to the HD. Actually, when you are using an ide driver rather
  than its libata counterpart (i.e. your disk is called /dev/hda
  instead of /dev/sda), then parking the heads of one drive (drive X)
  will generally not affect the mode of operation of another drive
  (drive Y) on the same port as described above. It is only when a port
  reset is required to recover from an exception on drive Y that further
  I/O operations on that drive (and the reset itself) will be delayed
  until drive X is no longer in the parked state.
  
  Finally, there are some hard drives that only comply with an earlier
  version of the ATA standard than ATA-7, but do support the unload
  feature nonetheless. Unfortunately, there is no safe way Linux can
  detect these devices, so you won't be able to write to the
  unload_heads attribute. If you know that your device really does
  support the unload feature (for instance, because the vendor of your
  laptop or the hard drive itself told you so), then you can tell the
  kernel to enable the usage of this feature for that drive by writing
  the special value -1 to the unload_heads attribute:
  
  # echo -1 > /sys/block/sda/device/unload_heads
  
  will enable the feature for /dev/sda, and giving -2 instead of -1 will
  disable it again.
  
  
  3. References
  -------------
  
  There are several laptops from different vendors featuring shock
  protection capabilities. As manufacturers have refused to support open
  source development of the required software components so far, Linux
  support for shock protection varies considerably between different
  hardware implementations. Ideally, this section should contain a list
  of pointers at different projects aiming at an implementation of shock
  protection on different systems. Unfortunately, I only know of a
  single project which, although still considered experimental, is fit
  for use. Please feel free to add projects that have been the victims
  of my ignorance.
  
  - http://www.thinkwiki.org/wiki/HDAPS
    See this page for information about Linux support of the hard disk
    active protection system as implemented in IBM/Lenovo Thinkpads.
  
  
  4. CREDITS
  ----------
  
  This implementation of disk head parking has been inspired by a patch
  originally published by Jon Escombe <lists@dresco.co.uk>. My efforts
  to develop an implementation of this feature that is fit to be merged
  into mainline have been aided by various kernel developers, in
  particular by Tejun Heo and Bartlomiej Zolnierkiewicz.