Re: reftable [v4]: new ref storage format

[Junio C Hamano <gitster@pobox.com>]

Shawn Pearce <spearce@spearce.org> writes:

> On Wed, Aug 2, 2017 at 2:28 AM, Jeff King <peff@peff.net> wrote:
>> On Tue, Aug 01, 2017 at 07:38:37PM -0700, Shawn Pearce wrote:
>>
>>> > OBJS blocks can also be
>>> > unbounded in size if very many references point at the same object,
>>> > thought that is perhaps only a theoretical problem.
>>>
>>> Gah, I missed that in reftable. The block id pointer list could cause
>>> a single object id to exceed what fits in a block, and that will cause
>>> the writer to fail unless its caller sets the block size larger. I
>>> basically assumed this overflow condition is very unlikely, as its not
>>> common to have a huge number of refs pointing to the same object.
>>
>> It's actually quite common for us, as we have big shared-object repos
>> that contain a copy of the refs of all of their child repos (for
>> reachability during packing, etc). So tags, where the value is the same
>> in each fork, you have one ref per fork pointing to it.
>>
>> Just peeking at torvalds/linux, we have some objects with ~35K refs
>> pointing to them (e.g., the v2.6.11 tag).
>
> Oy. I'll bet that every occurrence winds up in its own block due to
> the layout of the namespace, and so the obj block list needs 35k
> varint pointers. That requires a larger block size if it has any
> chance of fitting into the reftable format.
>
> Another option is disable the obj table for these shared-object repos.
> Its an optional part of the format and can be omitted if the reader
> isn't likely to need to lookup by SHA-1, or is willing to pay the
> brute force cost of scanning every ref.

I am wondering if we need the reverse look-up for a regular
repository that allows "fetch anything at the tip".  It only needs
"I got this request for an object name--does it sit at the tip of
any ref?  Yes/No".  It does not need to know exactly which ref
points at the asked object.

Yes, I know Gerrit has its own ACL that limits the visibility of
refs so the question becomes "does it sit at the tip of any ref that
is visible to the current user?".  An Yes/No answer for "any ref?"
may still give you a short-cut when rejecting, but you'd then need
to scan to give a positive answer without a full reverse mapping.

For the use of "consolidated object store for all forks", I do not
think it makes sense to even have "Does it sit at a tip, Yes/No"
information.  Even when Linus's repository and a fork share the same
object store, I do not think anybody expects to be able to fetch a
commit at the tip of a branch in the fork but not in Linus's
repository.