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
>     more.
>     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.


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