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On Saturday 16 January 2010 20:28:53 Jarry wrote: |
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> >> Now I installed one more sata-disk, attached to "sata4" |
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> >> position on mobo. But this changed the way how other |
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> >> disks are detected: |
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> >> |
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> >> Mobo: drive: system: |
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> >> sata1 160GB /dev/sdb |
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> >> sata2 160GB /dev/sdc |
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> >> sata3 dvdrw (not_detected) |
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> >> sata4 500GB /dev/sda |
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> |
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> ... |
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> |
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> > I don't know if that is normal behavior or a bug, |
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> |
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> In between I got a reply from other mailing list saying |
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> "it is not a bug, it is a feature!". And the reason for |
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> this feature is udev - it creates dev-files dynamically |
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> and sata port-numbers do not play any role for order |
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> in which hard-drives are detected and dev-file created. |
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> Maybe some udev-expert here could explain in which |
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> order udev writes device-files for hard-disks (maybe |
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> serial number, or vendor name?)... |
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> |
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Generally it's the order they are found in. |
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udev gives you the ability to dynamically create only the nodes you need |
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without having to worry if you've left something out of MAKEDEV. To do this, |
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the developer had to sacrifice your ability to predict what a device name will |
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be. |
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You actually don't care what the name of a thing in /dev/ is, it really |
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doesn't matter. The kernel knows what they are by looking at the major and |
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minor numbers and the name only exists while that instance of udev is running. |
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To work with the device (eg mounting it), you should use some other |
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characteristic of the device, like it's serial number or volume label. Which |
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means things like /dev/sda3 should not appear in fstab. |
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View it this way: |
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You have a disk volume with a filesystem on it that you called "HOME", and you |
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want to mount that filesystem to /home. You should just do that directly. |
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The other way involves a completely useless extra step that the user doe snot |
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even need to know about: You have a filesystem on it called "HOME", so you |
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looked it up in some arcane table and found that it has the arbitrary name of |
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/dev/sda3, so you mount /dev/sda3 to /home. Hmmmmmm, what's this extra step of |
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looking something up somewhere? It serves no useful purpose, gives no extra |
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information and is completely redundant. |
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If all you are doing is making filesystems available for use, and you find you |
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are getting involved with device names, then you are doing something contrary |
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to current kernel/udev/userspace practice. |
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-- |
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alan dot mckinnon at gmail dot com |
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