HAL-DISABLE-POLLING(1)                                                                                    HAL-DISABLE-POLLING(1)



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

SYNOPSIS
       hal-disable-polling [options]


DESCRIPTION
       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 properties,  refer  to  the  HAL  spec  which  can  be  found  in
       /usr/share/doc/hal-0.5.14/spec/hal-spec.html depending on the distribution.


OPTIONS
       The following options are supported:

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

       --device
              The device file of the drive.

       --enable-polling
              Enable polling instead of disabling it.

       --help Print out usage.

       --version
              Print the version.


NOTES
       This program requires super user privileges.


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


HISTORY
       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 interferring 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 hard-
       ware is malfunctioning. Additionally, the operating system 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 inter-
       fere with CD burning on another optical drive (master). Finally, polling a drive incurs an overhead both in the host sys-
       tem  (processes  get  woken  up  often,  preventing the CPU to stay in a deep power saving 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) specifi-
       cations, virtually no optical drives are compliant with the specification. Fortunately newer SATA ATAPI hardware seems to
       support  Asynchronous  Notification (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 triggered a number of bugs in Linux
       storage drivers and related subsystems (such as USB) that have later been fixed.


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


SEE ALSO
       hald(8),       lshal(1),       hal-lock(1),       open(2),      http://www.t10.org/scsi-3.htm,      https://bugzilla.red-
       hat.com/bugzilla/show_bug.cgi?id=halpolling


AUTHOR
       Written by David Zeuthen <davidATfubar.dk> with a lot of help from many others.




                                                                                                          HAL-DISABLE-POLLING(1)