rn-17: add 'snapshot' article

Author: chriscool

rev_news/drafts/edition-17.md

@@ -25,9 +25,50 @@ This edition covers what happened during the month of June 2016.
 ### Reviews
 -->
 
-<!---
 ### Support
--->
+
+*[Fwd: what is a snapshot](http://thread.gmane.org/gmane.comp.version-control.git/297701/)
+
+Ovatta Bianca asked:
+
+> I read in every comparison of git vs other version control systems,
+> that git does not record differences but takes "snapshots"
+> I was wondering what a "snapshot" is? Does git store at every commit
+> the entire files which have been modified even if only a few bytes
+> were changed out of a large file?
+
+Philip Oakley answered:
+
+> A snaphot is like a tar or zip of all your tracked files. This means it is
+> easier to determine (compared to lots of diffs) the current content.
+>
+> Keeping all the snapshots as separate loose items, when the majority of
+> their content is unchanged would be very inefficient, so git then uses, at
+> the right time, an efficient (and obviously lossless) compression technique
+> to 'zip' all the snapshots together so that the final repository is
+> 'packed'. The overall effect is a very efficient storage scheme.
+
+and pointed to [some documentation on Git's web site](https://git-scm.com/book/en/v2/Git-Internals-Plumbing-and-Porcelain)
+for "some good explanations".
+
+Jeff King alias Peff explained how the relationship between Git
+objects already makes things efficient:
+
+> Each commit is a snapshot in that it points to the sha1 of the root
+> tree, which points to the sha1 of other trees and blobs. And following
+> that chain gives you the whole state of the tree, without having to care
+> about other commits.
+>
+> And if the next commit changes only a few files, the sha1 for all the
+> other files will remain unchanged, and git does not need to store them
+> again. So already, before any explicit compression has happened, we get
+> de-duplication of identical content from commit to commit, at the file
+> and tree level.
+>
+> And then when a file does change, we store the whole new version, then
+> delta compress it during "git gc", etc, as you describe.
+
+And Jakub Narębski detailed the "loose" and the "packed" format.
 
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