Re: RFC: Design and code of partial clones (now, missing commits and trees OK) (part 3)

From: Jonathan Tan <jonathantanmy@google.com>
Date: Sep 21, 2017, 23:04 UTC
Message-ID: <20170921160416.1c4c6e2c@twelve2.svl.corp.google.com>
In-Reply-To: <af717446-95bf-c5a5-cd27-aaf20531db0f@jeffhostetler.com>

On Thu, 21 Sep 2017 14:00:40 -0400 Jeff Hostetler <git@jeffhostetler.com> wrote:

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> (part 3)
> 
> Additional overall comments on:
> https://github.com/jonathantanmy/git/commits/partialclone2
> 
> {} WRT the code in is_promised() [1]
> 
> [1]
> https://github.com/jonathantanmy/git/commit/7a9c2d9b6e2fce293817b595dee29a7eede0dddd#diff-5d5d5dc185ef37dc30bb7d9a7ae0c4e8R1960
> 
>     {} it looked like it was adding ALL promisor- and
> promised-objects to the "promised" OIDSET, rather than just
> promised-objects, but I could be mistaken.

As far as I can tell, it is just adding the promised objects (some of which may also be promisor objects). If you're saying that you expected it to add the promisor objects as well, that might be a reasonable expectation...I'm thinking of doing that.

>     {} Is this iterating over ALL promisor-packfiles?

Yes.

>     {} It looked like this was being used by fsck and rev-list.  I
> have concerns about how big this OIDSET will get and how it will
> scale, since if we start with a partial-clone all packfiles will be
>        promisor-packfiles.

It's true that scaling is an issue. I'm not sure if omitting the oidset will solve anything, though - as it is, Git maintains an object hash and adds to it quite liberally.

One thing that might help is some sort of flushing of objects in promisor packfiles from the local repository - that way, the oidset won't be so large.

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> 
>     {} When iterating thru a tree object, you add everything that it
>        references (everything in that folder).  This adds all of the
>        child OIDs -- without regard to whether they are new to this
>        version of the tree object. (Granted, this is hard to compute.)

The oidset will deduplicate OIDs.

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>        My concern is that this will add too many objects to the
> OIDSET. That is, a new tree object (because of a recent change to
> something in that folder) will also have the OIDs of the other
> *unchanged* files which may be present in an earlier non-provisor
> packfile from an earlier commit.
> 
>        I worry that this will grow the OIDSET to essentially include
>        everything.  And possibly defeating its own purpose.  I could
> be wrong here, but that's my concern.

Same answer as above (about flushing of objects in promisor packfiles).

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> {} I'm not opposed to the .promisor file concept, but I have concerns
>     that in practice all packfiles after a partial clone will be
>     promisor-packfiles and therefore short-cut during fsck, so fsck
>     still won't gain anything.
> 
>     It would help if there are also non-promisor packfiles present,
>     but only for objects referenced by non-promisor packfiles.
> 
>     But then I also have to wonder whether we can even support
> non-promisor packfiles after starting with a partial clone -- because
> of the properties of received thin-packs on a non-partial fetch.

Same reply as to your other e-mail - locally created objects are not in promisor packfiles. (Or were you thinking of a situation where locally created objects are immediately uploaded to the promisor remote, thus making them promisor objects too?)