As of Ubuntu 5.10, the boot process relies on a kernel parameter to provide the device path for the root filesystem. This mechanism is subject to failure in the following scenarios:

  1. The device containing the root filesystem moves to a different bus, driver, port, or otherwise is recognized by Linux as a "different" device
  2. The device containing the root filesystem is relocated from one system to another (especially applicable to removable devices such as USB storage media)
  3. The hardware detection process within Linux or early userspace changes in incompatible ways (such as changing the order in which devices on a bus are detected)

We should provide a mechanism which is not subject to these problems, while maintaining robustness in the common case (a device which doesn't move).

Use cases

Scott has this insane habit of swapping around devices but he is too lazy to fix mount points and boot bits around.


Make so that Scott doesn't need to change any bit at all.


Use UUID to uniquely identify root partition and fall back to old behavior if UUID are not available.

UUID in this case is the "Unique Identifier" generated at filesystem creation time.


  • d-i requires changes to provide the new /dev/disks/by-uuid (see UdevRoadmap) when possible as new root device.

  • No other changes other than udev 0.71 are required (see UdevRoadmap).


  • base-installer and bootloader-installers.

Data preservation and migration

  • None

Outstanding issues

  • Not all filesystem exports a UUID by default (hence "when possible" in the implementation) otherwise we roll back to the old format root=/dev/hdXX.

Random comments/questions/inputs

  • What implications does this have for mass duplication of systems? In Ghost the filesystem is recreated on the target drive (and thus, I assume, a new UUID). How might this compare to mounting via the label instead? -- JayCamp

UUID needs to be unique on the same machine. LABEL are more prone to clashing and this idea has been discarded at BOF -- FabioMassimoDiNitto

XXX: So moving the hard drive to another machine will break? (i. e. root=/dev/XXX is required)?

jbailey Replies: This is more confusing than it needs to be. In any case where the UUID is actually unique, you may move it between machines. If at any time there is a conflict between them, your system will break in interesting ways. Luckily, "Universal Unique IDentifier" have a tendancy of being universally unique. =)

  • RAID volumes actually have two UUIDs -- one for the RAID, and one for the filesystem on them. We should either support both, or synchronize them.

This is no problem at all. The root UUID is required at installer time and encoded in the bootloader as root=/dev/discs/by-uuid so there is no problem at all to get the right one.


ProbeForRootFilesystem (last edited 2008-08-06 16:24:45 by localhost)