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List:       git
Subject:    Re: GSoC - Some questions on the idea of "Better big-file support".
From:       Jeff King <peff () peff ! net>
Date:       2012-03-30 19:54:04
Message-ID: 20120330195404.GA20189 () sigill ! intra ! peff ! net
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On Fri, Mar 30, 2012 at 03:11:40PM -0400, Bo Chen wrote:

> Just make clear one of my confusions. Delta operation is to find out
> the differences between different versions of the same file, right?
> As I know, delta encoding is to re-encode a file based on the
> differences between neighboring blocks, thus can help compress a file
> since after delta encoding, we will have more similar data within the
> file. Can anyone elaborate a little bit what is the relation between
> delta operation in git and delta encoding listed above? Thanks.

Sort of. Git is snapshot based. So each version of a file is its own
"object", and from a high-level view, we store all objects. But we store
the logical objects themselves in packfiles, in which the actual
representation of the object may be stored as a difference to another
object (which is likely to be a different version of the same file, but
does not have to be).

Here's some background reading:

  http://progit.org/book/ch1-3.html

  http://progit.org/book/ch9-4.html

> I am wondering why we cannot divide the 2  2GB files into chunks and
> delta chunks by chunks. Is that any difference, except a little more
> IOs?

It's more complicated than that. What if the file is re-ordered? You
would want to compare early chunks in one version against later chunks
in the other. So yes, you can reduce memory pressure by doing more I/O,
but doing too much I/O will be very slow. Coming up with a solution is
part of what this project is about. And chunking is part of that
solution.

> > Read about rsync algorithm [2]. Bup [1] implements the same (I think)
> > algorithm, but on top of git. For preliminary patches, have a look at
> > jc/split-blob series at commit 4a1242d in git.git.
> 
> Make clear my another confusion. The file which has been updated
> (added, deleted, and modified) is first delta-compressed, and then
> synchronize to the remote repo by some mechanism (rsync?). I am
> wondering what is the the relationship between delta operation and
> rsync.

No, the updated file is delta compressed into a packfile, and the
packfile is transmitted. Rsync comes into play because it uses a novel
chunking algorithm, which was copied by bup (and is referred to as the
"bupsplit" algorithm). Read up on how bup works and why it was invented.

-Peff
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