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Based on kernel version 3.16. Page generated on 2014-08-06 21:40 EST.

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