On Wed, Sep 28, 2016 at 04:30:47PM -0700, Junio C Hamano wrote:
> As Peff said, responding in a thread started by Linus's suggestion
> to raise the default abbreviation to 12 hexdigits:
> I actually think "12" might be sane for a long time. That's 48 bits of
> sha1, so we'd expect a 50% change of a _single_ collision at 2^24, or 16
> million. The biggest repository I know about (in number of objects) is
> the one holding all of the objects for all of the forks of
> torvalds/linux on GitHub. It's at about 15 million objects.
> Which _seems_ close, but remember that's the size where we expect to see
> a single collision. They don't become common until much later (I didn't
> compute an exact number, but Linus's 16x sounds about right). I know
> that the growth of the kernel isn't really linear, but I think the need
> to bump to "13" might not just be decades, but possibly a century or
> So 12 seems reasonable, and the only downside for it (or for "13", for
> that matter) is a few extra bytes. I dunno, maybe people will really
> hate that, but I have a feeling these are mostly cut-and-pasted anyway.
I am not sure my quote is a good rationale for this bump. It was meant
to be a rationale that "12" is big enough, but the "I dunno" at the end
kind of glosses over the downsides.