Re: How to coordinate a DVD burn program with udev ?

["Thomas Schmitt" <scdbackup@gmx.net>]

Hi,

Tom Gundersen wrote:
> Could you possibly use one of the /dev/disk/by-* links instead? As far
> as I understand, the rule generator is being deprecated.

Aside from the general problem, i can of course advise my users to use
whatever is reliably existent and can be unambigously documented.
The files in /dev/disk/by-id seem not to represent any of the two DVD drives.
In /dev/disk/by-path i see them as
  /dev/disk/by-path/pci-0000:00:11.0-scsi-2:0:0:0
  /dev/disk/by-path/pci-0000:00:14.1-scsi-0:0:0:0
But i understand that these addresses are prone to change at boot time, too.
And they would uglify any program message that contains a drive address.

Alternatively one can find the desired /dev/sr address by running
  xorriso -devices
I could even implement in libburn a functionality similar to udev
which identifies drives by unique persistent properties.

But that would not prevent other problems which can arise from
udev trying to inspect the busy CD drive.


> Sorry to not answer your main question, I'm not sure exactly what is
> happening... Maybe someone else can shed some light on it?

The vanishing links are only one example symptom of the general problem
that udev tries to perform SCSI commands on a CD drive that is already
aquired with open(O_EXCL) and might be brought into a state which does
not tolerate such interference.
Especially with media types CD-R, CD-RW and DVD-R this often results in
misburns.

Obviously there are race conditions deciding over success or failure
of the coexistence of udev and burn program. 
xorriso is hit more than the others, because it inspects the media
content additionally to the drive's MMC properties.
Nevertheless i can reproduce the problem with wodim.
It is a general one.


libburn and xorriso are part of a project that tries to improve the
neighborhood relations between Linux and burn programs.
Different to growisofs and wodim it is under maintenance (by me),
and different to the author of cdrecord i am willing to follow
instructions from the Linux developers.

But first i need to get such instructions when Linux changes its behavior.
The random permutation of /dev/sr addresses at boot time is such a change.
I understand the recommended way to cope with this device juggling
is to use the udev links.
Now what shall a user think of udev and/or xorriso if this recommended
solution goes up in smoke on first use ?


Is there a better place than this mailing list where i could ask for advise ?

Especially i wonder what the designers of kernel and udev have planned
for the use case of CD/DVD/BD burning.


Have a nice day :)

Thomas