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