Merge branch 'add_lfs_doc' into 'master'

Documentation for Git LFS

Doc explaining how to use Git LFS, how to use Git LFS with GitLab, known issues and troubleshooting tips.

See merge request !1813

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+# Git LFS
+
+Managing large files such as audio, video and graphics files has always been one of the shortcomings of Git.
+The general recommendation is to not have Git repositories larger than 1GB to preserve performance.
+
+GitLab already supports [managing large files with git annex](http://doc.gitlab.com/ee/workflow/git_annex.html) (EE only), however in certain
+environments it is not always convenient to use different commands to differentiate between the large files and regular ones.
+
+Git LFS makes this simpler for the end user by removing the requirement to learn new commands.
+<!-- more -->
+
+## How it works
+
+Git LFS client talks with the GitLab server over HTTPS. It uses HTTP Basic Authentication to authorize client requests.
+Once the request is authorized, Git LFS client receives instructions from where to fetch or where to push the large file.
+
+## Requirements
+
+* Git LFS is supported in GitLab starting with version 8.2
+* Git LFS client version 0.6.0 and up
+
+## GitLab and Git LFS
+
+### Configuration
+
+Git LFS objects can be large in size. By default, they are stored on the server GitLab is installed on.
+
+There are two configuration options to help GitLab server administrators:
+
+* Enabling/disabling Git LFS support
+* Changing the location of LFS object storage
+
+#### Omnibus packages
+
+In `/etc/gitlab/gitlab.rb`:
+
+```ruby
+gitlab_rails['lfs_enabled'] = false
+gitlab_rails['lfs_storage_path'] = "/mnt/storage/lfs-objects"
+```
+
+#### Installations from source
+
+In `config/gitlab.yml`:
+
+```yaml
+  lfs:
+    enabled: false
+    storage_path: /mnt/storage/lfs-objects
+```
+
+## Known limitations
+
+* Git LFS v1 original API is not supported since it was deprecated early in LFS development, starting with Git LFS version 0.6.0
+* When SSH is set as a remote, Git LFS objects still go through HTTPS
+* Any Git LFS request will ask for HTTPS credentials to be provided so good Git credentials store is recommended
+* Currently, storing GitLab Git LFS objects on a non-local storage (like S3 buckets) is not supported
+* Git LFS always assumes HTTPS so if you have GitLab server on HTTP you will have to add the url to Git config manually (see #troubleshooting-tips)
+
+## Using Git LFS
+
+Lets take a look at the workflow when you need to check large files into your Git repository with Git LFS:
+For example, if you want to upload a very large file and check it into your Git repository:
+
+```bash
+git clone git@gitlab.example.com:group/project.git
+git lfs init                          # initialize the Git LFS project project
+git lfs track "*.iso"                 # select the file extensions that you want to treat as large files
+```
+
+Once a certain file extension is marked for tracking as a LFS object you can use Git as usual without having to redo the command to track a file with the same extension:
+
+```bash
+cp ~/tmp/debian.iso ./                # copy a large file into the current directory
+git add .                             # add the large file to the project
+git commit -am "Added Debian iso"     # commit the file meta data
+git push origin master                # sync the git repo and large file to the GitLab server
+```
+
+Downloading a single large file is also very simple:
+
+```bash
+git clone git@gitlab.example.com:group/project.git
+git lfs fetch debian.iso              # download the large file
+```
+
+
+## Troubleshooting
+
+### error: Repository or object not found
+
+There are a couple of reasons why this error can occur:
+
+* Wrong version of LFS client used:
+
+Check the version of Git LFS on the client machine with `git lfs version`. Only version 0.6.0 and newer are supported.
+
+* Project is using deprecated LFS API
+
+Check the Git config of the project for traces of deprecated API with `git lfs -l`. If `batch = false` is set in the config, remove the line and try using Git LFS client newer than 0.6.0.
+
+### Invalid status for <url> : 501
+
+When attempting to push a LFS object to a GitLab server that doesn't have Git LFS support enabled, server will return status `error 501`. Check with your GitLab administrator why Git LFS is not enabled on the server. See [Configuration section](#configuration) for instructions on how to enable LFS support.
+
+### getsockopt: connection refused
+
+If you push a LFS object to a project and you receive an error similar to: `Post <URL>/info/lfs/objects/batch: dial tcp IP: getsockopt: connection refused`,
+the LFS client is trying to reach GitLab through HTTPS. However, your GitLab instance is being served on HTTP.
+
+This behaviour is caused by Git LFS using HTTPS connections by default when a `lfsurl` is not set in the Git config.
+
+To prevent this from happening, set the lfs url in project Git config:
+
+```bash
+
+git config --add lfs.url "http://gitlab.example.com/group/project.git/info/lfs/objects/batch"
+```
+
+### Credentials are always required when pushing an object
+
+Given that Git LFS uses HTTP Basic Authentication to authenticate the user pushing the LFS object on every push for every object, user HTTPS credentials are required.
+
+By default, Git has support for remembering the credentials for each repository you use. This is described in [Git credentials man pages](https://git-scm.com/docs/gitcredentials).
+
+For example, you can tell Git to remember the password for a period of time in which you expect to push the objects:
+
+```bash
+git config --global credential.helper 'cache --timeout=3600'
+```
+
+This will remember the credentials for an hour after which Git operations will require re-authentication.
+
+If you are using OS X you can use `osxkeychain` to store and encrypt your credentials. For Windows, `wincred` is available.
+
+More details about various methods of storing the user credentials can be found on [Git Credential Storage documentation](https://git-scm.com/book/en/v2/Git-Tools-Credential-Storage)
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