1HAL-DISABLE-POLLING(1) General Commands Manual HAL-DISABLE-POLLING(1)
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6 hal-disable-polling - disable polling on drives with removable media
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9 hal-disable-polling [options]
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13 hal-disable-polling can be used to to disable and enable media detec‐
14 tion on drives with removable storage. For more information about both
15 the big picture and specific HAL properties, refer to the HAL spec
16 which can be found in /usr/share/doc/hal-0.5.9/spec/hal-spec.html
17 depending on the distribution.
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21 The following options are supported:
22 --udi The UDI (Unique Device Identifier) of the device object.
24 --device
26 The device file of the drive.
27 --enable-polling
29 Enable polling instead of disabling it.
30 --help Print out usage.
32 --version
34 Print the version.
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38 This program requires super user privileges.
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42 If the requested operation was successful, this program will exit with
43 exit code 0.
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47 Polling a storage drive is a necessary evil to detect when the user
48 inserts or removes media. Human computer interaction studies have shown
49 that a broad class of users expect their system to react within a few
50 seconds of this. Thus, the hald daemon polls through the hald-addon-
51 storage addon (one instance for each drive with removable media).
52 The purpose of the hald-addon-storage addon is simply to open the spe‐
54 cial device file at a regular interval (either every 2 or every 16 sec‐
55 onds) to check for new media. This program tries to open the device
56 file using the O_EXCL option which means that programs like cdrecord(1)
57 that uses O_EXCL automatically prevents the hald-addon-storage for
58 interferring by continously opening the device file. In addition, if
59 the drive is locked using HAL (see hal-lock(1)) the addon also stops
60 polling.
61 Unfortunately, polling a storage drive can have adverse side effects if
63 the hardware and/or device driver for the hardware is malfunctioning.
64 Additionally, the operating system kernel itself may offer multiple
65 interfaces for the same device (e.g. /dev/sg0 and /dev/scd0) so even
66 O_EXCL won't work. Also, polling a drive may decrease throughput in
67 certain (odd and/or broken) configurations; for example, if two IDE
68 drives shares the same host (master/slave), bus traffic and contention
69 caused by polling e.g. the optical drive (slave) can reduce throughput
70 to the hard disk (master) and/or interfere with CD burning on another
71 optical drive (master). Finally, polling a drive incurs an overhead
72 both in the host system (processes get woken up often, preventing the
73 CPU to stay in a deep power saving states) and it may prevent the
74 actual drive from reaching deep power states as well. As a result, more
75 power is consumed and this affects battery life for laptops.
76 Despite the existence of support for asynchronous media change notifi‐
78 cation in recent MMC (Multi-Media Commands) specifications, virtually
79 no optical drives are compliant with the specification. Fortunately
80 newer SATA ATAPI hardware seems to support Asynchronous Notification
81 (AN) and at this time of writing (March 2007) work is underway to make
82 both the Linux operating system kernel and HAL take advantage of this.
83 It is the position of the HAL team that polling should be avoided at
85 all costs as long as it doesn't heavily impact the user experience in a
86 negative way. This tool is provided as a stop-gap measure to use if a
87 system is rendered useless due to bugs in drivers and/or hardware that
88 is provoked by HAL polling the drive. If such a bug is encountered it
89 should be reported (see the BUGS section below) so it can be fixed -
90 historically hald have triggered a number of bugs in Linux storage
91 drivers and related subsystems (such as USB) that have later been
92 fixed.
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96 Please send bug reports to either the distribution or the HAL mailing
97 list, see http://lists.freedesktop.org/mailman/listinfo/hal on how to
98 subscribe.
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102 hald(8), lshal(1), hal-lock(1), open(2), http://www.t10.org/scsi-3.htm,
103 https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=halpolling
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107 Written by David Zeuthen <david@fubar.dk> with a lot of help from many
108 others.
109 HAL-DISABLE-POLLING(1)