Re: Where do I find the definitive man page for mdadm?

["Thomas Schmitt" <scdbackup@gmx.net>]

Hi,

Greg Wooledge wrote:
> > > ├─sda1   8:1    0   260M  0 part /boot/efi  4C30-7972

i wrote:
> > The universe must be small where this FAT UUID is unique.

David Wright wrote:
> I think I can live with odds of 1:4294967296.

Don't forget the birthday paradox. For getting a first collision you
need roughly as many random tries as the square root of the space size.
  https://en.wikipedia.org/wiki/Birthday_problem#Square_approximation

I.e. a hash of 32 bit is good for roughly 2 exp 16 = 65336 tries before
you have to expect two of them to match.

Ok. I confess that having ten thousands of partitions is unlikely on a
single computer. But mathematically a 32 bit number is far from being a
Universally Unique IDentifier.


> What's more important
> is that ID_PART_ENTRY_TYPE=c12a7328-f81f-11d2-ba4b-00a0c93ec93b (or
> 0xef on MBRs), which of course is anything but unique.

Yeah. The long one is on about any computer which boots by EFI from a
disk that's partitioned by GPT. Partition type UUIDs are not random but
rather intended to be the same for any partition of the same type. Even
the MBR partition type 0xef is supposed not to be used for anything but
EFI System Partitions.
(We have 0xef partitions in the Debian installation ISOs. But many old
EFI partitions on disk have MBR partition types like 0x06. Microsoft is
always a bit stronger than public specs.)


Have a nice day :)

Thomas