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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 remov-
       able 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  hardware 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 through-
       put  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  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  Com-
       mands) specifications, 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.freedesk-
       top.org/mailman/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)