From: Luke Kenneth Casson Leighton Date: Thu, 02 Sep 2010 18:07:07 GMT Subject: Re: git pack/unpack over bittorrent - works! Message-ID: In-Reply-To: <20100902155810.GB14508@sigill.intra.peff.net> On Thu, Sep 2, 2010 at 4:58 PM, Jeff King wrote: > pack-objects will reuse previously found deltas. So the deltas you have > in your existing packs matter. The deltas you have in your existing > packs depend on many things. At least: > >  1. Options you used when packing (e.g., --depth and --window). > >  2. Probably exactly _when_ you packed. You could find a good delta >     from A to B. Later, object C comes into existence, and would >     provide a better delta base for B. I don't think we will ever try A >     against C, unless --no-reuse-delta is set. > >     You have a different pack than somebody who packed after A, B, and >     C all existed. > >     In practice, this tends not to happen much because the best deltas >     are usually going backwards in time to a previous version. But it >     can happen. jeff, thanks for explaining (and to nicolas, i see, since beginning this) mrhmfffh. just been reading Documentation/technical/pack-heuristics.txt. so... options include: * writing an alternative "canonical" pack-object algorithm. i'm inclined to select "git format-patch"! :) but that would be the lazy way.... * taking the seeder's pack-objects as the "canonical" ones, regardless. cacheing of the results would, sadly, be virtually unavoidable, given the situation (multi-threading etc.) * throw away bittorrent entirely as a transport mechanism. * force-feed one peer to be "the" provider of a particular given pack. doesn't matter whom you contact to _obtain_ a pack from, as long as you solely and exclusively get the pack from that particular peer. * slight improvement / variation on the above: if two peers just coincidentally happen to create or have the same pack (as can be shown by having the same SHA-1, and/or by having the same data in their cache) then ta-daaa, you have a file-sharing network for that particular pack. i think.... i think i miiight be able, hmmm... i believe it would be possible to implement this last option by creating separate .torrents for packs (one each!). by splitting things down, so that pack objects are named as {ref}-{objref}-{SHA-1}.torrent and by providing a "top level" torrent which contains the refs/heads/* and the associate rev-list(s)... each set of rev-lists would have the SHA-1 of the pack-object that happened to be created (and shared) at that particular time, from that particular client: you then genuinely don't give a stuff about who has what, it's all the same, and... hmmm, i feel a modification to / deviation from the bittorrent protocol coming on :) l.