karl@??? writes:

> And that works when the root filesystem is on a device with fixed
> major/ minor number [...] It doesn't work for devices with dynamic
> device numbers.

That used to be true, but it's improved quite a bit in recent
years. Since 2.6.37 (2011), the kernel lets you specify a partition in a
GPT partition table by UUID:

root=PARTUUID=39a734b6-4068-4ee7-8f6f-b6248588af37

And since 3.8 (2013) you can specify a partition in a regular MS-DOS
partition table by disk ID and partition number:

root=PARTUUID=e50b843a-01

This is really handy when you're using dynamically-numbered devices and
don't otherwise need an initramfs (e.g. root on a USB stick, in
combination with rootwait).

See name_to_dev_t in init/do_mounts.c for the full details:
http://elixir.free-electrons.com/linux/v4.11.7/source/init/do_mounts.c#L182

-- 
Adam Sampson <ats@???>                         <http://offog.org/>