grub: Root Identifcation Heuristics
6.2 Root Identifcation Heuristics
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If the target operating system uses the Linux kernel, 'grub-mkconfig'
attempts to identify the root file system via a heuristic algoirthm.
This algorithm selects the identification method of the root file system
by considering three factors. The first is if an initrd for the target
operating system is also present. The second is
'GRUB_DISABLE_LINUX_UUID' and if set to 'true', prevents 'grub-mkconfig'
from identifying the root file system by its UUID. The third is
'GRUB_DISABLE_LINUX_PARTUUID' and if set to 'true', prevents
'grub-mkconfig' from identifying the root file system via the UUID of
its enclosing partition. If the variables are assigned any other value,
that value is considered equivalent to 'false'. The variables are also
considered to be set to 'false' if they are not set.
When booting, the Linux kernel will delegate the task of mounting the
root filesystem to the initrd. Most initrd images determine the root
file system by checking the Linux kernel's command-line for the 'root'
key and use its value as the identification method of the root file
system. To improve the reliability of booting, most initrd images also
allow the root file system to be identified by its UUID. Because of this
behavior, the 'grub-mkconfig' command will set 'root' to 'root=UUID=...'
to provide the initrd with the filesystem UUID of the root file system.
If no initrd is detected or 'GRUB_DISABLE_LINUX_UUID' is set to
'true' then 'grub-command' will identify the root filesystem by setting
the kernel command-line variable 'root' to 'root=PARTUUID=...' unless
'GRUB_DISABLE_LINUX_PARTUUID' is also set to 'true'. If
'GRUB_DISABLE_LINUX_PARTUUID' is also set to 'true', 'grub-command' will
identify by its Linux device name.
The following table summarizes the behavior of the 'grub-mkconfig'
command.
Initrd GRUB_DISABLE_LINUX_PARTUUID GRUB_DISABLE_LINUX_UUID Linux Root
detected Set To Set To ID Method
--------------------------------------------------------------------------------
false false false part UUID
false false true part UUID
false true false dev name
false true true dev name
true false false fs UUID
true false true part UUID
true true false fs UUID
true true true dev name
Remember, 'GRUB_DISABLE_LINUX_PARTUUID' and 'GRUB_DISABLE_LINUX_UUID'
are also considered to be set to 'false' when they are unset.
Info Catalog
grub: Simple configuration
grub: Configuration
grub: Shell-like scripting