Re: RefTree: Alternate ref backend

[Michael Haggerty <mhagger@alum.mit.edu>]

On 12/17/2015 10:02 PM, Shawn Pearce wrote:
> I started playing around with the idea of storing references directly
> in Git. Exploiting the GITLINK tree entry, we can associate a name to
> any SHA-1.
> 
> By storing all references in a single tree, atomic transactions are
> possible. Its a simple compare-and-swap of a single 40 byte SHA-1.
> This of course leads to a bootstrapping problem, where do we store the
> 40 byte SHA-1? For this example its just $GIT_DIR/refs/txn/committed
> as a classical loose reference.

I like this general idea a lot, even while recognizing some practical
problems that other people have pointed out. I especially like the idea
of having truly atomic multi-reference updates.

I'm curious why you decided to store all of the references in a single
list, similar to the packed-refs file. This design means that the whole
object has to be rewritten whenever any reference is updated [1].
Certainly, storing the references in a single tree *object* is not a
requirement for having atomic transitions.

I would have expected a design where the layout of the references in
trees mimics the layout of loose references in the filesystem; e.g., one
tree object for "refs/", one for "refs/heads/" one for "refs/remotes/"
etc. This design would reduce the amount of rewriting that is needed
when one or a few references are updated.

Another reason that I find a hierarchical layout intriguing would be
that one could imagine using the SHA-1s of reference namespace subtrees
to speed up the negotiation phase of "git fetch". In the common case
that I use the local namespace "refs/remotes/origin" to track an
upstream repo, the SHA-1 of my "refs/remotes/origin" tree would usually
represent a complete description of the state of the upstream references
at the time that I last fetched. My client could tell the server

    have-tree $SHA1

, where $SHA1 is the hash of the tree representing
"refs/remotes/origin/". If the server keeps a reflog as you have
described (but hierarchically), then the server could look up $SHA1 and
immediately know the full set of references, and therefore of objects,
that I fetched last time. More generally, the negotiation could proceed
down the reference namespace tree and stop whenever commonality is found.

There are a lot of "if"s in that last paragraph, and maybe it's not
workable. For example, if I'm not pruning on fetch, then my reference
tree won't be identical to one that was ever present on the server and
this technique wouldn't necessarily help. But if, for example, we change
the default to pruning, or perhaps record some extra reftree SHA-1's,
then in most cases I would expect that this trick could reduce the
effort of negotiation to negligible in most cases, and reduce the time
of the whole fetch to negligible in the case that the clone is already
up-to-date.

Michael

[1] At GitHub, we store public repositories in networks with a shared
object store. The central repository in each network can have 10M+
references. So for us, rewriting that many references for every
reference update would be unworkable.

-- 
Michael Haggerty
mhagger@alum.mit.edu