HAL-DISABLE-POLLING(1)					      General Commands Manual					    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/local/share/doc/hal/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 hardware is  malfunction-
       ing.  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 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 pre-
       vent the actual drive from reaching deep power states as well. As a result, more power is consumed and this affects battery life  for  lap-
       tops.

       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  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/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.redhat.com/bugzilla/show_bug.cgi?id=halpolling

AUTHOR

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

															    HAL-DISABLE-POLLING(1)