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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.
-
-- 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.