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. -Peff