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A Git fast forward is an extremely useful and efficient mechanism for harmonizing your project's main branch with changes introduced in a given feature branch. Git makes ample use of fast-forwarding merges behind the scenes, speeding up your development workflow in the process.
To fast-forward is to move forwards through a recording at a speed faster than that at which it would usually be played, for example two times or two point five times. The recordings are usually audio, video or computer data. It is colloquially known as 'f-forwarding'.
A non-fast-forward merge is a merge where the main branch had intervening changes between the branch point and the merge back to the main branch. In this case, a user can simulate a fast-forward by rebasing rather than merging. Rebasing works by abandoning some commits and creating new ones.
The Git merge --no-ff command merges the specified branch into the command in the current branch and ensures performing a merge commit even when it is a fast-forward merge. It helps in record-keeping of all performed merge commands in the concerning git repo.
When you try to merge one commit with a commit that can be reached by following the first commit’s history, Git simplifies things by moving the pointer forward because there is no divergent work to merge together – this is called a “fast-forward.”
For more : http://git-scm.com/book/en/v2/Git-Branching-Basic-Branching-and-Merging
In another way,
If Master has not diverged, instead of creating a new commit, git will just point master to the latest commit of the feature branch. This is a “fast forward.”
There won't be any "merge commit" in fast-forwarding merge.
In Git, to "fast forward" means to update the HEAD pointer in such a way that its new value is a direct descendant of the prior value. In other words, the prior value is a parent, or grandparent, or grandgrandparent, ...
Fast forwarding is not possible when the new HEAD is in a diverged state relative to the stream you want to integrate. For instance, you are on master and have local commits, and git fetch has brought new upstream commits into origin/master. The branch now diverges from its upstream and cannot be fast forwarded: your master HEAD commit is not an ancestor of origin/master HEAD. To simply reset master to the value of origin/master would discard your local commits. The situation requires a rebase or merge.
If your local master has no changes, then it can be fast-forwarded: simply updated to point to the same commit as the latestorigin/master. Usually, no special steps are needed to do fast-forwarding; it is done by merge or rebase in the situation when there are no local commits.
Is it ok to assume that fast-forward means all commits are replayed on the target branch and the HEAD is set to the last commit on that branch?
No, that is called rebasing, of which fast-forwarding is a special case when there are no commits to be replayed (and the target branch has new commits, and the history of the target branch has not been rewritten, so that all the commits on the target branch have the current one as their ancestor.)
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