1Hard disk shock protection 2========================== 3 4Author: Elias Oltmanns <eo@nebensachen.de> 5Last modified: 2008-10-03 6 7 80. Contents 9----------- 10 111. Intro 122. The interface 133. References 144. CREDITS 15 16 171. Intro 18-------- 19 20ATA/ATAPI-7 specifies the IDLE IMMEDIATE command with unload feature. 21Issuing this command should cause the drive to switch to idle mode and 22unload disk heads. This feature is being used in modern laptops in 23conjunction with accelerometers and appropriate software to implement 24a shock protection facility. The idea is to stop all I/O operations on 25the internal hard drive and park its heads on the ramp when critical 26situations are anticipated. The desire to have such a feature 27available on GNU/Linux systems has been the original motivation to 28implement a generic disk head parking interface in the Linux kernel. 29Please note, however, that other components have to be set up on your 30system in order to get disk shock protection working (see 31section 3. References below for pointers to more information about 32that). 33 34 352. The interface 36---------------- 37 38For each ATA device, the kernel exports the file 39block/*/device/unload_heads in sysfs (here assumed to be mounted under 40/sys). Access to /sys/block/*/device/unload_heads is denied with 41-EOPNOTSUPP if the device does not support the unload feature. 42Otherwise, writing an integer value to this file will take the heads 43of the respective drive off the platter and block all I/O operations 44for the specified number of milliseconds. When the timeout expires and 45no further disk head park request has been issued in the meantime, 46normal operation will be resumed. The maximal value accepted for a 47timeout is 30000 milliseconds. Exceeding this limit will return 48-EOVERFLOW, but heads will be parked anyway and the timeout will be 49set to 30 seconds. However, you can always change a timeout to any 50value between 0 and 30000 by issuing a subsequent head park request 51before the timeout of the previous one has expired. In particular, the 52total timeout can exceed 30 seconds and, more importantly, you can 53cancel a previously set timeout and resume normal operation 54immediately by specifying a timeout of 0. Values below -2 are rejected 55with -EINVAL (see below for the special meaning of -1 and -2). If the 56timeout specified for a recent head park request has not yet expired, 57reading from /sys/block/*/device/unload_heads will report the number 58of milliseconds remaining until normal operation will be resumed; 59otherwise, reading the unload_heads attribute will return 0. 60 61For example, do the following in order to park the heads of drive 62/dev/sda and stop all I/O operations for five seconds: 63 64# echo 5000 > /sys/block/sda/device/unload_heads 65 66A simple 67 68# cat /sys/block/sda/device/unload_heads 69 70will show you how many milliseconds are left before normal operation 71will be resumed. 72 73A word of caution: The fact that the interface operates on a basis of 74milliseconds may raise expectations that cannot be satisfied in 75reality. In fact, the ATA specs clearly state that the time for an 76unload operation to complete is vendor specific. The hint in ATA-7 77that this will typically be within 500 milliseconds apparently has 78been dropped in ATA-8. 79 80There is a technical detail of this implementation that may cause some 81confusion and should be discussed here. When a head park request has 82been issued to a device successfully, all I/O operations on the 83controller port this device is attached to will be deferred. That is 84to say, any other device that may be connected to the same port will 85be affected too. The only exception is that a subsequent head unload 86request to that other device will be executed immediately. Further 87operations on that port will be deferred until the timeout specified 88for either device on the port has expired. As far as PATA (old style 89IDE) configurations are concerned, there can only be two devices 90attached to any single port. In SATA world we have port multipliers 91which means that a user-issued head parking request to one device may 92actually result in stopping I/O to a whole bunch of devices. However, 93since this feature is supposed to be used on laptops and does not seem 94to be very useful in any other environment, there will be mostly one 95device per port. Even if the CD/DVD writer happens to be connected to 96the same port as the hard drive, it generally *should* recover just 97fine from the occasional buffer under-run incurred by a head park 98request to the HD. Actually, when you are using an ide driver rather 99than its libata counterpart (i.e. your disk is called /dev/hda 100instead of /dev/sda), then parking the heads of one drive (drive X) 101will generally not affect the mode of operation of another drive 102(drive Y) on the same port as described above. It is only when a port 103reset is required to recover from an exception on drive Y that further 104I/O operations on that drive (and the reset itself) will be delayed 105until drive X is no longer in the parked state. 106 107Finally, there are some hard drives that only comply with an earlier 108version of the ATA standard than ATA-7, but do support the unload 109feature nonetheless. Unfortunately, there is no safe way Linux can 110detect these devices, so you won't be able to write to the 111unload_heads attribute. If you know that your device really does 112support the unload feature (for instance, because the vendor of your 113laptop or the hard drive itself told you so), then you can tell the 114kernel to enable the usage of this feature for that drive by writing 115the special value -1 to the unload_heads attribute: 116 117# echo -1 > /sys/block/sda/device/unload_heads 118 119will enable the feature for /dev/sda, and giving -2 instead of -1 will 120disable it again. 121 122 1233. References 124------------- 125 126There are several laptops from different vendors featuring shock 127protection capabilities. As manufacturers have refused to support open 128source development of the required software components so far, Linux 129support for shock protection varies considerably between different 130hardware implementations. Ideally, this section should contain a list 131of pointers at different projects aiming at an implementation of shock 132protection on different systems. Unfortunately, I only know of a 133single project which, although still considered experimental, is fit 134for use. Please feel free to add projects that have been the victims 135of my ignorance. 136 137- http://www.thinkwiki.org/wiki/HDAPS 138 See this page for information about Linux support of the hard disk 139 active protection system as implemented in IBM/Lenovo Thinkpads. 140 141 1424. CREDITS 143---------- 144 145This implementation of disk head parking has been inspired by a patch 146originally published by Jon Escombe <lists@dresco.co.uk>. My efforts 147to develop an implementation of this feature that is fit to be merged 148into mainline have been aided by various kernel developers, in 149particular by Tejun Heo and Bartlomiej Zolnierkiewicz. 150