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

[Stefan Beller <sbeller@google.com>]

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