Re: reftable [v4]: new ref storage format

[Dave Borowitz <dborowitz@google.com>]

On Tue, Aug 1, 2017 at 10:38 PM, Shawn Pearce <spearce@spearce.org> wrote:
>> Peff and I discussed off-list whether the lookup-by-SHA-1 feature is
>> so important in the first place. Currently, all references must be
>> scanned for the advertisement anyway,
>
> Not really. You can hide refs and allow-tip-sha1 so clients can fetch
> a ref even if it wasn't in the advertisement. We really want to use
> that wire protocol capability with Gerrit Code Review to hide the
> refs/changes/ namespace from the advertisement, but allow clients to
> fetch any of those refs if they send its current SHA-1 in a want line
> anyway.
>
> So a server could scan only the refs/{heads,tags}/ prefixes for the
> advertisement, and then leverage the lookup-by-SHA1 to verify other
> SHA-1s sent by the client.
>
>> so avoiding a second scan to vet
>> SHA-1s received from the client is at best going to reduce the effort
>> by a constant factor. Do you have numbers showing that this
>> optimization is worth it?
>
> No, but I don't think I need to do much to prove it. My 866k ref
> example advertisement right now is >62 MiB. If we do what I'm
> suggesting in the paragraphs above, the advertisement is ~51 KiB.

That being said, our bias towards minimizing the number of ref scans
is rooted in our experience where scanning 866k refs takes 5 seconds
to get the response from the storage backend into the git server.
Cutting ref scans from 2 to 1 (or 1 to 0) is a big deal in that case.
But that 5s number is based on our current, slow storage, not on
reftable. If migrating to reftable turns each 5s scan into a 400ms
scan, we might be able to live with that, even if we don't have fast
lookup by SHA-1.

>> OTOH a mythical protocol v2 might reduce the need to scan the
>> references for advertisement, so maybe this optimization will be more
>> helpful in the future?

I haven't been following the status of the proposal, but I was
assuming a client-speaks-first protocol would also imply the client
asking for refnames, not SHA-1s, in which case lookup by SHA-1 is no
longer relevant.