hal-disable-polling - disable polling on drives with removable media
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 <email@example.com> with a lot of help from many others.