Re: [PATCH v2 1/8] packfile: prepare for the existence of '*.rev' files

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

Taylor Blau <me@ttaylorr.com> writes:

>> Thinking out loud a bit: a .rev file means we're spending an extra map
>> per pack (but not a descriptor, since we close after mmap). And like the
>> .idx files (but unlike .pack file maps), we don't keep track of these
>> and try to close them when under memory pressure. I think that's
>> probably OK in terms of bytes. It may mean running up against operating
>> system number-of-mmap limits more quickly ...
>> ...
>> >  	if (ends_with(name, ".idx"))
>> >  		return 3;
>> > -	return 4;
>> > +	if (ends_with(name, ".rev"))
>> > +		return 4;
>> > +	return 5;
>> >  }
>>
>> Probably not super important, but: should the .idx file still come last
>> here? Simultaneous readers won't start using the pack until the .idx
>> file is present. We'd probably prefer they see the whole thing
>> atomically, than see a .idx missing its .rev (they won't ever produce a
>> wrong answer, but they'll generate the in-core revindex on the fly when
>> they don't need to).

At some point, we may want to 

 - introduce .idx version 3 that is more extensible, so that the
   reverse info is included in one of its chunks;

 - make the .rev data for all packs stored as a chunk in .midx, so
   we can first check with .midx and not open any .rev files.

either of which would reduce the numberfrom 30k down to 10k ;-)