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 ==========================
 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.
 
 - https://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.

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