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> It looks like .git/logs contains the history. It looks like the file format is a space-separated list, with the format "$parentcommitsha1 $newcommitsha1 ... $

• mtdewcmu 13y ago
  >I've never looked at .git/logs, but it looks like that is used by the `git reflog` command. It's basically a history (or log) of every commit that a particular reference has pointed to[1]. For example, I cloned the git source code: I think it's more or less the DAG represented as an adjacency list. I'd have to think a bit about why there is a separate log file for each branch. It seems that there's some redundancy in doing that, and I'm wondering what the advantages and disadvantages are of splitting the history up in that way. >It's also of note that branches are referred to as 'references' too, hence storing them under `.git/refs/`. I've developed a loathing of excessive hierarchies/trees, so I'd rather see them flattened in a single directory. But that makes sense. >See: http://stackoverflow.com/questions/5290444/why-does-git-hash.. http://stackoverflow.com/questions/5290444/why-does-git-hash.... That's a good link. What's in an object? If an object corresponds to a commit, then it must aggregate data about changes to multiple files.
  • pyre 13y ago
    > I think it's more or less the DAG represented as an adjacency list. I'd have to think a bit about why there is a separate log file for each branch. It seems that there's some redundancy in doing that, and I'm wondering what the advantages and disadvantages are of splitting the history up in that way. Think of each branch as a pointer. Then realize that you can make that pointer point anywhere on the DAG, even to parts of the DAG that have no connection to each other. The `reflog` is a (local, non-comprehensive) history of where that pointer has pointed. That's why there is a separate log for each branch. I guess that technically they could have a single log file and add another field to specify the branch, but using the same directory tree structure as under .git/refs/ makes the mental model simpler (and probably a performance improvement not to have to parse the reflog for every branch just to see the reflog for one branch). > I've developed a loathing of excessive hierarchies/trees, so I'd rather see them flattened in a single directory. But that makes sense. I'm not sure what branches living under .git/refs has to do with excessive hierarchies/trees. There are enough things stored in the .git directory, that if you mashed them all together it wouldn't make any sense. > What's in an object? If you really care to dive deeper, you can check objects here: https://github.com/git/git/blob/master/object.h https://github.com/git/git/blob/master/object.h You can get a shorter version towards the bottom of the git manpage (e.g. `man git`): IDENTIFIER TERMINOLOGY <object> Indicates the object name for any type of object. <blob> Indicates a blob object name. <tree> Indicates a tree object name. <commit> Indicates a commit object name. <tree-ish> Indicates a tree, commit or tag object name. A command that takes a <tree-ish> argument ultimately wants to operate on a <tree> object but automatically dereferences <commit> and <tag> objects that point at a <tree>. <commit-ish> Indicates a commit or tag object name. A command that takes a <commit-ish> argument ultimately wants to operate on a <commit> object but automatically dereferences <tag> objects that point at a <commit>. <type> Indicates that an object type is required. Currently one of: blob, tree, commit, or tag. <file> Indicates a filename - almost always relative to the root of the tree structure GIT_INDEX_FILE describes.
• mtdewcmu 13y ago
  I'm guessing that the reason each branch has its own history is probably related to the goal of only appending new entries at the end of things. Since any branch can be under development, they need their own files. It sort of makes sense.
  • pyre 13y ago
    I still think that you're a little confused. The reflog is a "history of where this branch has pointed since the repository was created/cloned." If I clone a repository with a history of 100 commits on the 'master' branch, the reflog for the 'master' branch will only have one entry. You can completely delete the `.git/logs` and still run `git log` successfully. Here's an example: $ git clone blah DAG: A - B - C - D - E \ Z - X - Y Branches: master => E topic/new-feature => Y reflog: master E - clone from blah topic/new-feature Y - clone from blah Notice how cloning a repository with an existing DAG doesn't populate the reflog. It just give it a single entry saying that the branch was updated from 'nothing' to whatever commit it was pointing to remotely. Now let's change where 'master' is pointing: $ git reset master C DAG: A - B - C - D - E \ Z - X - Y Branches: master => C topic/new-feature => Y reflog: master E - clone from blah C - reset to C topic/new-feature Y - clone from blah Notice how the reflog is a history of the values that the branch was referencing, but is not the history as what you get when you run 'git log'. After the reset, 'git log master' would show you commits A, B and C, but A and B are nowhere in the reflog.
    • mtdewcmu 13y ago
      I see. I started reading the internals chapter at[1]. This free book seems better than the O'Reilly book, which I bought. So the DAG is actually stored inside objects. The contents of the objects directory could be described by a relational schema, and I think that would make it easier for a lot of people to understand (myself included): Blob - sha1hash (primary key) - contents (blob) Tree - sha1hash (primary key) TreeEntry - treeid (foreign key into Tree) - mode (mode of blob/subtree) - type ("blob" or "tree") - objectid (foreign key into Tree or Blob) - name Commit - sha1hash (primary key) - tree (foreign key into Tree) - parent (foreign key into Commit) - author - committer - comment The tree entries are actually denormalized and stored as a list inside the tree. You could represent this more accurately with XML. But who likes XML? [1] http://git-scm.com/book http://git-scm.com/book