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X11R7.4 - man page for hal-disable-polling (x11r4 section 1)

HAL-DISABLE-POLLING(1)		     General Commands Manual		   HAL-DISABLE-POLLING(1)

       hal-disable-polling - disable polling on drives with removable media

       hal-disable-polling [options]

       hal-disable-polling  can  be  used to to disable and enable media detection on drives with
       removable storage. For more information about both the big picture and specific HAL  prop-
       erties,	refer  to  the	HAL spec which can be found in /usr/local/share/doc/hal/spec/hal-
       spec.html depending on the distribution.

       The following options are supported:

       --udi  The UDI (Unique Device Identifier) of the device object.

	      The device file of the drive.

	      Enable polling instead of disabling it.

       --help Print out usage.

	      Print the version.

       This program requires super user privileges.

       If the requested operation was successful, this program will exit with exit code 0.

       Polling a storage drive is a necessary evil to detect when the  user  inserts  or  removes
       media.  Human  computer	interaction studies have shown that a broad class of users expect
       their system to react within a few seconds of this. Thus, the hald  daemon  polls  through
       the hald-addon-storage addon (one instance for each drive with removable media).

       The purpose of the hald-addon-storage addon is simply to open the special device file at a
       regular interval (either every 2 or every 16 seconds) to check for new media. This program
       tries  to  open	the  device  file  using the O_EXCL option which means that programs like
       cdrecord(1) that uses O_EXCL automatically prevents the hald-addon-storage  for	interfer-
       ring by continously opening the device file. In addition, if the drive is locked using HAL
       (see hal-lock(1)) the addon also stops polling.

       Unfortunately, polling a storage drive can have	adverse  side  effects	if  the  hardware
       and/or  device driver for the hardware is malfunctioning. Additionally, the operating sys-
       tem kernel itself may offer multiple interfaces for the same  device  (e.g.  /dev/sg0  and
       /dev/scd0)  so  even  O_EXCL  won't work. Also, polling a drive may decrease throughput in
       certain (odd and/or broken) configurations; for example, if two IDE drives shares the same
       host  (master/slave),  bus traffic and contention caused by polling e.g. the optical drive
       (slave) can reduce throughput to the hard disk (master) and/or interfere with  CD  burning
       on another optical drive (master). Finally, polling a drive incurs an overhead both in the
       host system (processes get woken up often, preventing the CPU to stay in a deep power sav-
       ing  states)  and it may prevent the actual drive from reaching deep power states as well.
       As a result, more power is consumed and this affects battery life for laptops.

       Despite the existence of support for asynchronous media change notification in recent  MMC
       (Multi-Media  Commands) specifications, virtually no optical drives are compliant with the
       specification. Fortunately newer SATA ATAPI hardware seems to support Asynchronous Notifi-
       cation  (AN)  and  at  this time of writing (March 2007) work is underway to make both the
       Linux operating system kernel and HAL take advantage of this.

       It is the position of the HAL team that polling should be avoided at all costs as long  as
       it  doesn't heavily impact the user experience in a negative way. This tool is provided as
       a stop-gap measure to use if a system is rendered useless due to bugs  in  drivers  and/or
       hardware that is provoked by HAL polling the drive. If such a bug is encountered it should
       be reported (see the BUGS section below) so it can be fixed - historically hald have trig-
       gered  a number of bugs in Linux storage drivers and related subsystems (such as USB) that
       have later been fixed.

       Please send bug	reports  to  either  the  distribution	or  the  HAL  mailing  list,  see
       http://lists.freedesktop.org/mailman/listinfo/hal on how to subscribe.

       hald(8),      lshal(1),	    hal-lock(1),      open(2),	   http://www.t10.org/scsi-3.htm,

       Written by David Zeuthen <david@fubar.dk> with a lot of help from many others.


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