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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. | ||