RE: [RFC] Add support for downloading blobs on demand

["Ben Peart" <peartben@gmail.com>]

We actually pursued trying to make submodules work for some time and 
even built tooling around trying to work around some of the issues we 
ran into (not repo.py but along a similar line) before determining that 
we would be better served by having a single repo and solving the scale 
issues.  I don't want to rehash the arguments for/against a single repo 
- suffice it to say, we have opted for a single large repo. :)

Thanks,

Ben
> -----Original Message-----
> From: Stefan Beller [mailto:sbeller@google.com]
> Sent: Tuesday, January 17, 2017 5:24 PM
> To: Martin Fick <mfick@codeaurora.org>
> Cc: Ben Peart <peartben@gmail.com>; Shawn Pearce
> <spearce@spearce.org>; git <git@vger.kernel.org>;
> benpeart@microsoft.com
> Subject: Re: [RFC] Add support for downloading blobs on demand
> 
> On Tue, Jan 17, 2017 at 2:05 PM, Martin Fick <mfick@codeaurora.org>
> wrote:
> > On Tuesday, January 17, 2017 04:50:13 PM Ben Peart wrote:
> >> While large files can be a real problem, our biggest issue today is
> >> having a lot (millions!) of source files when any individual
> >> developer only needs a small percentage of them.  Git with 3+ million
> >> local files just doesn't perform well.
> >
> > Honestly, this sounds like a problem better dealt with by using git
> > subtree or git submodules, have you considered that?
> >
> > -Martin
> >
> 
> I cannot speak for subtrees as I have very little knowledge on them.
> But there you also have the problem that *someone* has to have a whole
> tree? (e.g. the build bot)
> 
> submodules however comes with a couple of things attached, both positive
> as well as negative points:
> 
> * it offers ACLs along the way. ($user may not be allowed to
>   clone all submodules, but only those related to the work)
> * The conceptual understanding of git just got a lot harder.
>   ("Yo dawg, I heard you like git, so I put git repos inside
>   other git repos"), it is not easy to come up with reasonable
>   defaults for all usecases, so the everyday user still has to
>   have some understanding of submodules.
> * typical cheap in-tree operations may become very expensive:
>   -> moving a file from one location to another (in another
>      submodule) adds overhead, no rename detection.
> * We are actively working on submodules, so there is
>   some momentum going already.
> * our experiments with Android show that e.g. fetching (even
>   if you have all of Android) becomes a lot faster for everyday
>   usage as only a few repositories change each day). This
>   comparision was against the repo tool, that we currently
>   use for Android. I do not know how it would compare against
>   single repo Git, as having such a large repository seemed
>   complicated.
> * the support for submodules in Git is already there, though
>   not polished. The positive side is to have already a good base,
>   the negative side is to have support current use cases.
> 
> Stefan