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diff --git a/Documentation/laptops/disk-shock-protection.txt b/Documentation/laptops/disk-shock-protection.txt new file mode 100644 index 000000000..0e6ba2663 --- /dev/null +++ b/Documentation/laptops/disk-shock-protection.txt @@ -0,0 +1,149 @@ +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. |