Re: Handling large files with GIT

[Ben Clifford <benc@hawaga.org.uk>]

On Thu, 9 Feb 2006, Martin Langhoff wrote:

> I did suggest maildir,  where GIT is bound to do well as the content
> of the emails doesn't change but they just move around a lot. Though
> yes, trees are going to be nasty.

I've been keeping maildir in git for a few months, with mail being 
delivered into a git repo on one (permanently connected) host and me 
merging that branch into a repo on my laptop for reading (the intention 
being that I should be able to sync it back to the permanently connected 
host as I sometimes read mail there.

Alas, the merge part of this absolutely sucks -- as time goes by, its 
getting slower and slower (its taking an hour or so to do the merge, which 
has got to the point of being barely usable -- if it wasn't for the neat 
hack-value, I'd have given up on this by now).

I haven't really probed whats happening when I'm doing the merges in any 
depth, but I see a lot of index manipulation happening (git update-index, 
I think) to add and remove files where each invocation of that seems to be 
taking almost a whole second.

I wonder if the present merge algorithms perform especially badly in the 
case of a large number of files with lots of renames (and so lots of 
adds/removes) but no content changes? The merge should be able to happen 
entirely in the index, I think.

Perhaps one way to proceed would be for me to write a move-optimised merge 
strategy where I flip the index round and instead of saying "how has the 
content inside this filename changed?" I instead say "how has the filename 
associated with this content <hash> changed?"

A special-case on top of a move-optimised merge might be some 
maildir-aware filename handling that knows how to resolve conflicts when a 
particular content-hash has been renamed to two different names (eg. when 
one flag is added to a message in one repo and a different flag is added 
to a message in another repo).

Any advice/thoughts/suggestions-that-this-is-a-stupid-thing-to-do would be 
greatly appreciated.

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