Re: [gentoo-user] Re: sata disk assignment mismatch...

[Dirk Heinrichs]

Am Sonntag 17 Januar 2010 11:30:00 schrieb Alan McKinnon:
> On Saturday 16 January 2010 20:28:53 Jarry wrote:
> > >> Now I installed one more sata-disk, attached to "sata4"
> > >> position on mobo. But this changed the way how other
> > >> disks are detected:
> > >>
> > >> Mobo: drive: system:
> > >> sata1 160GB /dev/sdb
> > >> sata2 160GB /dev/sdc
> > >> sata3 dvdrw (not_detected)
> > >> sata4 500GB /dev/sda
> >
> > ...
> >
> > > I don't know if that is normal behavior or a bug,
> >
> > In between I got a reply from other mailing list saying
> > "it is not a bug, it is a feature!". And the reason for
> > this feature is udev - it creates dev-files dynamically
> > and sata port-numbers do not play any role for order
> > in which hard-drives are detected and dev-file created.
> > Maybe some udev-expert here could explain in which
> > order udev writes device-files for hard-disks (maybe
> > serial number, or vendor name?)...
>
> Generally it's the order they are found in.
>
> udev gives you the ability to dynamically create only the nodes you need
> without having to worry if you've left something out of MAKEDEV. To do
>  this, the developer had to sacrifice your ability to predict what a device
>  name will be.
>
> You actually don't care what the name of a thing in /dev/ is, it really
> doesn't matter. The kernel knows what they are by looking at the major and
> minor numbers and the name only exists while that instance of udev is
>  running. To work with the device (eg mounting it), you should use some
>  other characteristic of the device, like it's serial number or volume
>  label. Which means things like /dev/sda3 should not appear in fstab.
>
> View it this way:
>
> You have a disk volume with a filesystem on it that you called "HOME", and
>  you want to mount that filesystem to /home. You should just do that
>  directly.
>
> The other way involves a completely useless extra step that the user doe
>  snot even need to know about: You have a filesystem on it called "HOME",
>  so you looked it up in some arcane table and found that it has the
>  arbitrary name of /dev/sda3, so you mount /dev/sda3 to /home. Hmmmmmm,
>  what's this extra step of looking something up somewhere? It serves no
>  useful purpose, gives no extra information and is completely redundant.
>
> If all you are doing is making filesystems available for use, and you find
>  you are getting involved with device names, then you are doing something
>  contrary to current kernel/udev/userspace practice.

To add some more alternatives to that list:

1) LVM. Logical volumes always get the same, user defined name, no matter what.
2) User defined udev rules to name your devices whatever you like (Google: 
writing udev rules).

Bye...

	Dirk