Bug#595790: [Pkg-zfsonlinux-devel] Bug#595790: hostid: useless unless fixed

[Florian Weimer <fw@deneb.enyo.de>]

* Petter Reinholdtsen:

> [Florian Weimer]
>> That's not very different from /etc/machine-id, isn't it?
>
> Ah, thank you very much for bringing this systemd setting to my
> attention.  I was not aware of it.
>
> I agree that it seem very similar in purpose and implementation.  Will
> it be available on non-linux Debian architectures too?

It might be possible to port over this part, yes.

>>> We need to figure out how to transform the UUID to a 32 bit integer,
>>> of course.
>>
>> And I think this is the crux of the problem.  Whatever we do, with
>> today's cluster sizes it's just not reliably unique.
>
> Well, for the set of machines we have available at work (ca. 3000) it
> would be sufficiently unique.

I simulated 100,000 random assigns of 32-bit host IDs to 3,000 hosts,
and got collisions in 104 cases.

For 5,000 hosts, I got 286, and for 10,000, 1,112 (again in 100,000
runs).  I was lazy, it shouldn't be too hard to calculate expected
values accurately.

So a 32-bit value without central coordination is pretty much a time
bomb.

> For most sites it would make the return value from gethostid()
> unique.

The IP address of a host could be better than that.  I doubt it is
possible to imrpove upon the glibc implementation.

>> DMI data seems risky because it depends on firmware, and there are so
>> many firmware bugs out there.
>
> I did not quite understand what you mean here.  Do you mean the DMI
> value in your experience isn't unique?

I wouldn't count on them being unique.  Most such ID fields are
definitely not, and there are groups out there who strongly oppose
device IDs.