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Old 10-22-2007, 09:13 PM   #1
tlemons
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Registered: Jun 2004
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udev consistent across distributions?


Hi

I'm developing an application that runs on several Linux distributions. This application uses tape devices. It's important that I use a persistent name for each tape device, a name that will remain the same even when other tape devices are added to the system. This needs to be true even when new tape devices are added earlier in the device scan path than existing tape devices, and so will receive lower system-assigned /dev/[n]stN device files.

I know, I know: this is why udev was invented. And, on SLES 9 SP3, this seems to work great: the /dev/tape/by-id/ directoy is populated automatically (using udev rules that come with the system) with device-persistent device files names.

But RHEL 5, for instance, does not create /dev/tape/by-id. And I'm hearing that other Linux distributions do not function in the same way as SLES 9.

So, is there any commonality across distributions?

Also, I searched for (but didn't find) a udev mailing list or forum. Can you suggest a place where udev 'people' hang out?

Thanks!
tl
 
Old 10-23-2007, 01:15 PM   #2
raskin
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Registered: Sep 2005
Location: France
Distribution: approximately NixOS (http://nixos.org)
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From my not-always-positive experience with udev I can say the following:
1) /dev/tape/by-id/ is a good example of a configuration option that s used in some distributions, but not in all - not even in all major ones. So you cannot rely on presence of a similar feature.
2) udev consistency is not always attainable. Seeing how udev can react on a kernel upgrade (or you can say, what parts of /sys/ kernel team changes every so often) doesn't make me believe that you can bet data consistency on understanding now how udev will work one year later.
3) You can ask user to add the following lines </dev/tape/by-id/ configuration from SuSE, GPL notice> into udev rules. You'll at least notice if it disappears anyway.
4) You can try to use sysfs to decide what device do you need (choosing one tape device out of a few using stat is easy when you know major&minor device numbers). Again, you'll know when it breaks.
 
  


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