The return of devfs [LWN.net]

The return of devfs

Posted May 7, 2009 9:53 UTC (Thu) by epa (subscriber, #39769)
In reply to: The return of devfs by nix
Parent article: The return of devfs

In some respects, all that matters is that the name is *consistent*: e.g. local fixed disks, which are often referenced in files such as /etc/fstab.

Often nowadays fstab refers to devices by their disk label, because the device names (sda1, sdb1, etc) might jump around depending on what's plugged in where. Which makes you wonder why the disk label names have to be checked in some magic way, rather than just being exported by the kernel under /dev/disklabel/xxx.

The return of devfs

Posted Oct 5, 2009 7:47 UTC (Mon) by cmccabe (guest, #60281) [Link] (5 responses)

> Which makes you wonder why the disk label names have to be checked in some
> magic way, rather than just being exported by the kernel under
> /dev/disklabel/xxx.

If you're running a non-ancient system with udev, you can find all the disks by label under:

/dev/disk/by-label/

Actually, come to think of it, I'm not sure why the LABEL=foo hack was ever implemented. There must be a reason but I don't know it.

Colin

The return of devfs

Posted Oct 6, 2009 0:07 UTC (Tue) by nix (subscriber, #2304) [Link] (4 responses)

Firstly, LABEL= predates udev. Secondly, LABEL= works even before udev has
initialized. Thirdly, it works on systems without udev. (udev and mount
don't yet use the same probing library, but this is changing, I
understand.)

The return of devfs

Posted Oct 18, 2009 22:43 UTC (Sun) by cmccabe (guest, #60281) [Link]

Ah, that makes sense. Thanks.

The return of devfs

Posted Apr 2, 2010 11:15 UTC (Fri) by skitching (guest, #36856) [Link] (2 responses)

Hi nix,

You mention in your comment that "root=LABEL=" work even when there is no udev.

Does this also apply to "root=UUID="?

And would you be able to give me a hint about where to find that code? All the relevant rootfs mounting bits appear to be in init/do_mounts.c, but I can't find anything related to handling LABEL or UUID. Function name_to_dev_t maps things like "root=08:05", "root=0x0805", "root=/dev/sda5" to a (major,minor) but I can't see UUID/LABEL handling anywhere.

Thanks, Simon

The return of devfs

Posted Apr 2, 2010 12:09 UTC (Fri) by hppnq (guest, #14462) [Link]

I'm sure nix will have more useful information, but you could take a look at libblkid in the meantime. This is used by mount to find devices identified by label or uuid (check the util-linux source).

The return of devfs

Posted Apr 3, 2010 0:50 UTC (Sat) by nix (subscriber, #2304) [Link]

I never mentioned root=. For everything *other* than root filesystem
mounting, the code to look at is libblkid, which supports both LABEL= and
UUID=. That's what mount(8) uses.

Raw non-initramfs/initrd kernel root= parameterwise, all that is supported
in /dev/{fake device name}, root={major:minor}, and
root={major*256+minor}. This is of course yet another reason to use an
initramfs, which can easily use normal mount and/or blkid directly to
support whatever permutations of LABEL=, UUID= or whatever you prefer.