Re: [RFC PATCH 0/3] Partial clone: promised blobs (formerly "missing blobs")

["Philip Oakley" <philipoakley@iee.org>]

From: "Jonathan Nieder" <jrnieder@gmail.com>
Sent: Monday, July 17, 2017 7:03 PM
> Hi Philip,
>
> Philip Oakley wrote:
>> From: "Jonathan Tan" <jonathantanmy@google.com>
>
>>> These patches are part of a set of patches implementing partial clone,
>>> as you can see here:
>>>
>>> https://github.com/jonathantanmy/git/tree/partialclone
> [...]
>> If I understand correctly, this method doesn't give any direct user
>> visibility of missing blobs in the file system. Is that correct?
>>
>> I was hoping that eventually the various 'on demand' approaches
>> would still allow users to continue to work as they go off-line such
>> that they can see directly (in the FS) where the missing blobs (and
>> trees) are located, so that they can continue to commit new work on
>> existing files.
>>
>> I had felt that some sort of 'gitlink' should be present (huma
>> readable) as a place holder for the missing blob/tree. e.g.
>> 'gitblob: 1234abcd' (showing the missing oid, jsut like sub-modules
>> can do - it's no different really.
>
> That's a reasonable thing to want, but it's a little different from
> the use cases that partial clone work so far has aimed to support.
> They are:
>
> A. Avoiding downloading all blobs (and likely trees as well) that are
>    not needed in the current operation (e.g. checkout).  This blends
>    well with the sparse checkout feature, which allows the current
>    checkout to be fairly small in a large repository.

True. In my case I was looking for a method that would allow a 'Narrow 
clone' such that the local repo would be smaller (have less content), but 
would feel as if all the usefull files/directories were available, and there 
would be place holders at the points where the trees were pruned, both in 
the object store, and in the user's work-tree.

As you say, in some ways its conceptually orthogonal to the original sparse 
checket (which has a full width object store / repo, and then omitted files 
from the checkout.
>
>    GVFS uses a trick that makes it a little easier to widen a sparse
>    checkout upon access of a directory.  But the same building blocks
>    should work fine with a sparse checkout that has been set up
>    explicitly.
>
> B. Avoiding downloading large blobs, except for those needed in the
>    current operation (e.g. checkout).
>
>    When not using sparse checkout, the main benefit out of the box is
>    avoiding downloading *historical versions* of large blobs.
>

> It sounds like you are looking for a sort of placeholder outside the
> sparse checkout area.

True.

> In a way, that's orthogonal to these patches:
> even if you have all relevant blobs, you may want to avoid inflating
> them to check them out and reading them to compare to the index (i.e.
> the usual benefits of sparse checkout).

In my concept, it should be possible to create the ('sparse'/narrow) index 
from the content of the local object store, without any network connection 
(though that content is determined by the prior fetch/clone;-). The proper 
git sparse checkout could proceed from there as being a further local 
restriction on what is omitted from the worktree.

Those missing from the narrow clone would still show as place holders with 
content ".gitnarrowtree 13a24b..<oid>" (so we know what the hash oid of the 
file/tree should be (so they can be moved/renamed etc!). The index would 
only know the content/structure as far as the place holders (just like 
sub-modules are a break point in the tracking, with identical caveats)


It would be interesting to know from Ben the level of sparseness/narrowness 
has been seen typically in the BigWin GVFS repo case.

>  In a sparse checkout, you
> still might like to be able to get a listing of files outside the
> sparse area (which you can get with "git ls-tree") and you may even
> want to be able to get such a listing with plain "ls" (as with your
> proposal).
>
> Thanks and hope that helps,
> Jonathan

Thanks, yes. It has help consolidate some of the parts of my concept that 
has been in the back of my mind for a while now.

Philip