Re: [PATCH] git exproll: steps to tackle gc aggression

Jeff King [off-list ref] writes:

... The reason we do not store thin-packs on disk is that you
run into problems with cycles in the delta graph (e.g., A deltas against
B, which deltas against C, which deltas against A; at one point you had
a full copy of one object which let you create the cycle, but you later
deleted it as redundant with the delta, and now you cannot reconstruct
any of the objects).

As an extension to that is that a thin-pack will make the "validity"
of a single pack dependent on its surroundings, making "verify-pack"
useless.  It may say "everything is fine", but corruption of another
pack that holds a base object a delta in it depends on will render
the pack unusable.

With the current arrangement, if you grab a single pack and re-idx
it, you know you can reconstruct all the objects in it.

The original reason was not cycles; it was primarily because we did
not want to have to map more than one packfile while reconstructing
the delta chain for a single object (this was way before the "open
small mmap windows into a large packfile" was invented).