[PATCH 5 of 6 V2] obscache: instantiate the cache and keep it warm

[Augie Fackler]

> On May 24, 2017, at 13:39, Jun Wu <quark at fb.com> wrote:
> 
>> I seee, So if the obscache was included into the streaming clone, that 
>> would remove the impact to clone.
> 
> One of our goals is to avoid using revlog for scalability. Adding new
> dependence to revlog means we need more monkey patching to get rid of them
> later. So I think a config option could be a better choice if we do think
> obscache and its 20ms speedup is worthwhile.

I'm extremely sympathetic to wanting to avoid new deps on revlog, but in this case we're talking about a 500ms win on 'hg id' in the Mercurial repository, which is significant savings.

Realistically, any non-revlog storage engine is going to have to work out similar cache invalidation hooks anyway, isn't it?