nova team mailing list archive

[Michael Gundlach <michael.gundlach@xxxxxxxxxxxxx>]

On Tue, Oct 5, 2010 at 2:31 PM, Ewan Mellor <ewan.mellor@xxxxxxxxxxxxx>wrote:

> Thanks Michael.  It would be great to see this in some doccomments in the
> code somewhere.
>

Yeah, I agree -- I first touched this code to rename ec2_id to internal_id,
and it's been a bit of a pain because there's a dearth of comments (and some
ambiguous naming, like "instance_id" referring to id in some places and to
ec2_id in other places.)  I plan to clean this code up as one of my post-FF
tasks.


> I agree with Jay's comments elsewhere in this thread -- it seems a better
> idea to use a UUID for your internal_id, rather than a long int.  That way,
> the ID is an extra 64 bits longer, so you can generate them randomly on
> independent nodes without worrying about collisions.
>

I think we discussed this in IRC -- 128 bits turns into a 26 byte EC2 ID vs
14 for a 64 bit int, and someone (Soren?) had a strong negative preference.

Actually, I just checked in code only using a 32 bit integer, and I'd like
some convincing that this isn't sufficient for the foreseeable future.  We
want to support 1 million instances, right?  Which is 1/4000 the keyspace,
so we have something like a 1 in 4000 chance of a collision once we hit a
million instances.  I know I'm not doing the statistics properly, since the
proper question is "what is the chance of at least one collision when
successively generating one million random 32-bit integers?", but it feels
like we can punt on larger values at least until Bexar.

Thoughts?

Michael


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