Re: Borrowing objects from nearby repositories

[Andrew Keller <andrew@kellerfarm.com>]

On Mar 25, 2014, at 6:17 PM, Junio C Hamano <gitster@pobox.com> wrote:

> Junio C Hamano <gitster@pobox.com> writes:
> 
>> Ævar Arnfjörð Bjarmason <avarab@gmail.com> writes:
>> 
>>>> 1) Introduce '--borrow' to `git-fetch`.  This would behave similarly
>>> to '--reference', except that it operates on a temporary basis, and
>>> does not assume that the reference repository will exist after the
>>> operation completes, so any used objects are copied into the local
>>> objects database.  In theory, this mechanism would be distinct from
>>> --reference', so if both are used, some objects would be copied, and
>>> some objects would be accessible via a reference repository referenced
>>> by the alternates file.
>>> 
>>> Isn't this the same as git clone --reference <path> --no-hardlinks
>>> <url> ?
>>> 
>>> Also without --no-hardlinks we're not assuming that the other repo
>>> doesn't go away (you could rm-rf it), just that the files won't be
>>> *modified*, which Git won't do, but you could manually do with other
>>> tools, so the default is to hardlink.
>> 
>> I think that the standard practice with the existing toolset is to
>> clone with reference and then repack.  That is:
>> 
>>    $ git clone --reference <borrowee> git://over/there mine
>>    $ cd mine
>>    $ git repack -a -d
>> 
>> And then you can try this:
>> 
>>    $ mv .git/objects/info/alternates .git/objects/info/alternates.disabled
>>    $ git fsck
>> 
>> to make sure that you are no longer borrowing anything from the
>> borrowee.  Once you are satisfied, you can remove the saved-away
>> alternates.disabled file.
> 
> Oh, I forgot to say that I am not opposed if somebody wants to teach
> "git clone" a new option to copy its objects from two places,
> (hopefully) the majority from near-by reference repository and the
> remainder over the network, without permanently relying on the
> former via the alternates mechanism.  The implementation of such a
> feature could even literally be "clone with reference first and then
> repack" at least initially but even in the final version.

That was actually one of my first ideas - adding some sort of '--auto-repack' option to git-clone.  It's a relatively small change, and would work.  However, keeping in mind my end goal of automating the feature to the point where you could run simply 'git clone <url>', an '--auto-repack' option is more difficult to undo.  You would need a new parameter to disable the automatic adding of reference repositories, and a new parameter to undo '--auto-repack', and you'd have to remember to actually undo both of those settings.

In contrast, if the new feature was '--borrow', and the evolution of the feature was a global configuration 'fetch.autoBorrow', then to turn it off temporarily, one only needs a single new parameter '--no-auto-borrow'.  I think this is a cleaner approach than the former, although much more work.

Thanks,
 - Andrew Keller