Add tutorial - migrate repo to LFS

- Link new doc to and from other docs
- Add tbs section

doc/topics/git/index.md

@@ -82,6 +82,8 @@ Git-related queries from GitLab.
 The following relate to Git Large File Storage:
 
 - [Getting Started with Git LFS](https://about.gitlab.com/2017/01/30/getting-started-with-git-lfs-tutorial/)
-- [GitLab Git LFS documentation](../../workflow/lfs/manage_large_binaries_with_git_lfs.md)
+- [Migrate an existing Git repo with Git LFS](migrate_to_git_lfs/index.md)
+- [GitLab Git LFS user documentation](../../workflow/lfs/manage_large_binaries_with_git_lfs.md)
+- [GitLab Git LFS admin documentation](../../workflow/lfs/lfs_administration.md)
 - [Git-Annex to Git-LFS migration guide](../../workflow/lfs/migrate_from_git_annex_to_git_lfs.md)
 - [Towards a production quality open source Git LFS server](https://about.gitlab.com/2015/08/13/towards-a-production-quality-open-source-git-lfs-server/)

doc/topics/git/migrate_to_git_lfs/index.md

+---
+type: tutorial, concepts
+description: "How to migrate an existing Git repository to Git LFS with BFG."
+last_updated: 2019-07-11
+---
+
+# Migrate a Git repo into Git LFS with BFG
+
+Using Git LFS can help you to reduce the size of your Git
+repository and improve its performance.
+
+However, simply adding the
+large files that are already in your repository to Git LFS,
+will not actually reduce the size of your repository because
+the files are still referenced by previous commits.
+
+Through the method described on this document, first migrate
+to Git LFS with [BFG](https://rtyley.github.io/bfg-repo-cleaner/)
+through a mirror repo, then clean up the repository's history,
+and lastly create LFS tracking rules to prevent new binary files
+from being added.
+
+This tutorial was inspired by the guide
+[Use BFG to migrate a repo to Git LFS](https://confluence.atlassian.com/bitbucket/use-bfg-to-migrate-a-repo-to-git-lfs-834233484.html).
+For more information on Git LFS, see the [references](#references)
+below.
+
+CAUTION: **Warning:**
+The method described on this guide rewrites Git history. Make
+sure to back up your repo before beginning and use it at your
+own risk.
+
+## Requirements
+
+Before beginning, make sure:
+
+- You have enough LFS storage for the files you want to convert.
+  Storage is required for the entire history of all files.
+- All the team members you share the repository with have pushed all changes.
+  Branches based on the repository before applying this method cannot be merged.
+  Branches based on the repo before applying this method cannot be merged.
+
+To follow this tutorial, you'll need:
+
+- Maintainer permissions to the existing Git repository
+  you'd like to migrate to LFS with access through the command line.
+- [Git](https://git-scm.com/book/en/v2/Getting-Started-Installing-Git)
+  and [Java Runtime Environment](https://www.java.com/en/download/manual.jsp)
+  (Java 7 or above) installed locally.
+- BFG installed locally:
+
+   ```bash
+   brew install bfg
+   ```
+
+- Git LFS installed locally:
+
+   ```bash
+   brew install git-lfs
+   ```
+
+NOTE: **Note:**
+This guide was tested on macOS Mojave.
+
+## Steps
+
+Consider an example upstream project, `git@gitlab.com:gitlab-tests/test-git-lfs-repo-migration.git`.
+
+1. Back up your repository:
+
+   Create a copy of your repository so that you can
+   recover it in case something goes wrong.
+
+1. Clone `--mirror` the repo:
+
+   Cloning with the mirror flag will create a bare repository.
+   This ensures you get all the branches within the repo.
+
+   It creates a directory called `<repo-name>.git`
+   (in our example, `test-git-lfs-repo-migration.git`),
+   mirroring the upstream project:
+
+   ```bash
+   git clone --mirror git@gitlab.com:gitlab-tests/test-git-lfs-repo-migration.git
+   ```
+
+1. Convert the Git history with BFG:
+
+   ```bash
+   bfg --convert-to-git-lfs "*.{png,mp4,jpg,gif}" --no-blob-protection test-git-lfs-repo-migration.git
+   ```
+
+   It is scanning all the history, and looking for any files with
+   that extension, and then converting them to an LFS pointer.
+
+1. Clean up the repository:
+
+   ```bash
+   # cd path/to/mirror/repo:
+   cd test-git-lfs-repo-migration.git
+   # clean up the repo:
+   git reflog expire --expire=now --all && git gc --prune=now --aggressive
+   ```
+
+   You can also take a look on how to further [clean the repo](../../../user/project/repository/reducing_the_repo_size_using_git.md),
+   but it's not necessary for the purposes of this guide.
+
+1. Install Git LFS in the mirror repository:
+
+   ```bash
+   git lfs install
+   ```
+
+1. [Unprotect the default branch](../../../user/project/protected_branches.md),
+   so that we can force-push the rewritten repository:
+
+   1. Navigate to your project's **Settings > Repository** and
+   expand **Protected Branches**.
+   1. Scroll down to locate the protected branches and click
+   **Unprotect** the default branch.
+
+1. Force-push to GitLab:
+
+   ```bash
+   git push --force
+   ```
+
+1. Track the files you want with LFS:
+
+   ```bash
+   # cd path/to/upstream/repo:
+   cd test-git-lfs-repo-migration
+   # You may need to reset your local copy with upstream's `master` after force-pushing from the mirror:
+   git reset --hard origin/master
+   # Track the files with LFS:
+   git lfs track "*.gif" "*.png" "*.jpg" "*.psd" "*.mp4" ".gitattributes" "img/"
+   ```
+
+   Now all existing the files you converted, as well as the new
+   ones you add, will be properly tracked with LFS.
+
+1. [Re-protect the default branch](../../../user/project/protected_branches.md):
+
+   1. Navigate to your project's **Settings > Repository** and
+   expand **Protected Branches**.
+   1. Select the default branch from the **Branch** dropdown menu,
+   and set up the
+   **Allowed to push** and **Allowed to merge** rules.
+   1. Click **Protect**.
+
+<!-- ## Troubleshooting
+
+Include any troubleshooting steps that you can foresee. If you know beforehand what issues
+one might have when setting this up, or when something is changed, or on upgrading, it's
+important to describe those, too. Think of things that may go wrong and include them here.
+This is important to minimize requests for support, and to avoid doc comments with
+questions that you know someone might ask.
+
+Each scenario can be a third-level heading, e.g. `### Getting error message X`.
+If you have none to add when creating a doc, leave this section in place
+but commented out to help encourage others to add to it in the future. -->
+
+## References
+
+- [Getting Started with Git LFS](https://about.gitlab.com/2017/01/30/getting-started-with-git-lfs-tutorial/)
+- [Migrate from Git Annex to Git LFS](../../../workflow/lfs/migrate_from_git_annex_to_git_lfs.md)
+- [GitLab's Git LFS user documentation](../../../workflow/lfs/manage_large_binaries_with_git_lfs.md)
+- [GitLab's Git LFS administrator documentation](../../../workflow/lfs/lfs_administration.md)
+- Alternative method to [migrate an existing repo to Git LFS](https://github.com/git-lfs/git-lfs/wiki/Tutorial#migrating-existing-repository-data-to-lfs)
+
+<!--
+Test project:
+https://gitlab.com/gitlab-tests/test-git-lfs-repo-migration
+-->

doc/workflow/lfs/manage_large_binaries_with_git_lfs.md

@@ -84,6 +84,10 @@ that are on the remote repository, eg. for a branch from origin:
 git lfs fetch origin master
 ```
 
+### Migrate an existing repo to Git LFS
+
+Read the documentation on how to [migrate an existing Git repo with Git LFS](../../topics/git/migrate_to_git_lfs/index.md).
+
 ## File Locking
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-ce/issues/35856) in GitLab 10.5.