Merge the Gitaly docs with the ones hosted in GitLab

- Bring https://gitlab.com/gitlab-org/gitaly/blob/master/doc/configuration/README.md
  in doc/administration/gitaly/reference.md
- Bring https://gitlab.com/gitlab-org/gitaly/blob/master/cmd/gitaly-debug/README.md
  in doc/administration/gitaly/index.md#gitaly-debug

doc/administration/gitaly/index.md

@@ -10,11 +10,15 @@ In the rest of this page, Gitaly server is referred to the standalone node that
 only runs Gitaly, and Gitaly client to the GitLab Rails node that runs all other
 processes except Gitaly.
 
+## Architecture
+
+Here's a high-level architecture overview of how Gitaly is used.
+
+![Gitaly architecture diagram](img/architecture_v12_4.png)
+
 ## Configuring Gitaly
 
-The Gitaly service itself is configured via a TOML configuration file.
-This file is documented [in the Gitaly
-repository](https://gitlab.com/gitlab-org/gitaly/blob/master/doc/configuration/README.md).
+The Gitaly service itself is configured via a [TOML configuration file](reference.md).
 
 In case you want to change some of its settings:
 
@@ -562,6 +566,29 @@ and this mechanism (the concurrency limiter) will do nothing.
 
 ## Troubleshooting Gitaly
 
+### `gitaly-debug`
+
+The `gitaly-debug` command provides "production debugging" tools for Gitaly and Git
+performance. It is intended to help production engineers and support
+engineers investigate Gitaly performance problems.
+
+If you're using GitLab 11.6 or newer, this tool should be installed on
+your GitLab / Gitaly server already at `/opt/gitlab/embedded/bin/gitaly-debug`.
+If you're investigating an older GitLab version you can compile this
+tool offline and copy the executable to your server:
+
+```sh
+git clone https://gitlab.com/gitlab-org/gitaly.git
+cd cmd/gitaly-debug
+GOOS=linux GOARCH=amd64 go build -o gitaly-debug
+```
+
+To see the help page of `gitaly-debug` for a list of supported sub-commands, run:
+
+```sh
+gitaly-debug -h
+```
+
 ### Commits, pushes, and clones return a 401
 
 ```
@@ -571,17 +598,6 @@ remote: GitLab: 401 Unauthorized
 You will need to sync your `gitlab-secrets.json` file with your GitLab
 app nodes.
 
-### `gitaly-debug`
-
-Since GitLab 11.6, Gitaly comes with a command-line tool called
-`gitaly-debug` that can be run on a Gitaly server to aid in
-troubleshooting. In GitLab 11.6 its only sub-command is
-`simulate-http-clone` which allows you to measure the maximum possible
-Git clone speed for a specific repository on the server.
-
-For an up to date list of sub-commands see [the gitaly-debug
-README](https://gitlab.com/gitlab-org/gitaly/blob/master/cmd/gitaly-debug/README.md).
-
 ### Client side GRPC logs
 
 Gitaly uses the [gRPC](https://grpc.io/) RPC framework. The Ruby gRPC

doc/administration/gitaly/reference.md

+---
+type: reference
+---
+
+# Gitaly reference
+
+Gitaly is configured via a [TOML](https://github.com/toml-lang/toml)
+configuration file. Unlike installations from source, in Omnibus GitLab, you
+would not edit this file directly.
+
+The configuration file is passed as an argument to the `gitaly`
+executable. This is usually done by either Omnibus GitLab or your
+[init](https://en.wikipedia.org/wiki/Init) script.
+
+An [example config file](https://gitlab.com/gitlab-org/gitaly/blob/master/config.toml.example)
+can be found in the Gitaly project.
+
+## Format
+
+At the top level, `config.toml` defines the items described on the table below.
+
+| Name | Type | Required | Description |
+| ---- | ---- | -------- | ----------- |
+| `socket_path` | string | yes (if  `listen_addr` is not set) | A path which Gitaly should open a Unix socket. |
+| `listen_addr` | string | yes (if `socket_path` is not set) | TCP address for Gitaly to listen on. |
+| `tls_listen_addr` | string | no | TCP over TLS address for Gitaly to listen on. |
+| `bin_dir`    | string | yes    | Directory containing Gitaly's executables. |
+| `prometheus_listen_addr` | string | no | TCP listen address for Prometheus metrics. If not set, no Prometheus listener is started. |
+
+For example:
+
+```toml
+socket_path = "/home/git/gitlab/tmp/sockets/private/gitaly.socket"
+listen_addr = "localhost:9999"
+tls_listen_addr = "localhost:8888"
+bin_dir = "/home/git/gitaly"
+prometheus_listen_addr = "localhost:9236"
+```
+
+### Authentication
+
+Gitaly can be configured to reject requests that do not contain a
+specific bearer token in their headers. This is a security measure to
+be used when serving requests over TCP:
+
+```toml
+[auth]
+# A non-empty token enables authentication.
+token = "the secret token"
+```
+
+Authentication is disabled when the token setting in `config.toml` is absent or
+an empty string.
+
+It is possible to temporarily disable authentication with the `transitioning`
+setting. This allows you to monitor if all clients are
+authenticating correctly without causing a service outage for clients
+that are not configured correctly yet:
+
+```toml
+[auth]
+token = "the secret token"
+transitioning = true
+```
+
+CAUTION: **Warning:**
+Remember to disable `transitioning` when you are done
+changing your token settings.
+
+All authentication attempts are counted in Prometheus under
+the `gitaly_authentications_total` metric.
+
+### TLS
+
+Gitaly supports TLS encryption. You will need to bring your own certificates as
+this isn't provided automatically.
+
+| Name | Type | Required | Description |
+| ---- | ---- | -------- | ----------- |
+| `certificate_path` | string | no | Path to the certificate. |
+| `key_path` | string | no | Path to the key. |
+
+```toml
+tls_listen_addr = "localhost:8888"
+
+[tls]
+certificate_path = '/home/git/cert.cert'
+key_path = '/home/git/key.pem'
+```
+
+[Read more](index.md#tls-support) about TLS in Gitaly.
+
+### Storage
+
+GitLab repositories are grouped into directories known as "storages"
+(e.g., `/home/git/repositories`) containing bare repositories managed
+by GitLab with names (e.g., `default`).
+
+These names and paths are also defined in the `gitlab.yml` configuration file of
+GitLab. When you run Gitaly on the same machine as GitLab, which is the default
+and recommended configuration, storage paths defined in Gitaly's `config.toml`
+must match those in `gitlab.yml`.
+
+| Name | Type | Required | Description |
+| ---- | ---- | -------- | ----------- |
+| `storage` | array | yes | An array of storage shards. |
+| `path` | string | yes | The path to the storage shard. |
+| `name` | string | yes | The name of the storage shard. |
+
+For example:
+
+```toml
+[[storage]]
+path = "/path/to/storage/repositories"
+name = "my_shard"
+
+[[storage]]
+path = "/path/to/other/repositories"
+name = "other_storage"
+```
+
+### Git
+
+The following values can be set in the `[git]` section of the configuration file.
+
+| Name | Type | Required | Description |
+| ---- | ---- | -------- | ----------- |
+| `bin_path` | string | no | Path to Git binary. If not set, will be resolved using `PATH`. |
+| `catfile_cache_size` | integer | no | Maximum number of cached [cat-file processes](#cat-file-cache). Default is `100`. |
+
+#### `cat-file` cache
+
+A lot of Gitaly RPCs need to look up Git objects from repositories.
+Most of the time we use `git cat-file --batch` processes for that. For
+better performance, Gitaly can re-use these `git cat-file` processes
+across RPC calls. Previously used processes are kept around in a
+["git cat-file cache"](https://about.gitlab.com/2019/07/08/git-performance-on-nfs/#enter-cat-file-cache).
+In order to control how much system resources this uses, we have a maximum number
+of cat-file processes that can go into the cache.
+
+The default limit is 100 `cat-file`s, which constitute a pair of
+`git cat-file --batch` and `git cat-file --batch-check` processes. If
+you are seeing errors complaining about "too many open files", or an
+inability to create new processes, you may want to lower this limit.
+
+Ideally, the number should be large enough to handle normal
+traffic. If you raise the limit, you should measure the cache hit ratio
+before and after. If the hit ratio does not improve, the higher limit is
+probably not making a meaningful difference. Here is an example
+Prometheus query to see the hit rate:
+
+```text
+sum(rate(gitaly_catfile_cache_total{type="hit"}[5m])) / sum(rate(gitaly_catfile_cache_total{type=~"(hit)|(miss)"}[5m]))
+```
+
+### `gitaly-ruby`
+
+A Gitaly process uses one or more `gitaly-ruby` helper processes to
+execute RPC's implemented in Ruby instead of Go. The `[gitaly-ruby]`
+section of the config file contains settings for these helper processes.
+
+These processes are known to occasionally suffer from memory leaks.
+Gitaly restarts its `gitaly-ruby` helpers when their memory exceeds the
+`max_rss` limit.
+
+| Name | Type | Required | Description |
+| ---- | ---- | -------- | ----------- |
+| `dir` | string | yes | Path to where gitaly-ruby is installed (needed to boot the process).|
+| `max_rss` | integer | no | Resident set size limit that triggers a gitaly-ruby restart, in bytes. Default is `200000000` (200MB). |
+| `graceful_restart_timeout` | string | no | Grace period before a gitaly-ruby process is forcibly terminated after exceeding `max_rss`. Default is `10m` (10 minutes).|
+| `restart_delay` | string | no |Time that gitaly-ruby memory must remain high before a restart. Default is `5m` (5 minutes).|
+| `num_workers` | integer | no |Number of gitaly-ruby worker processes. Try increasing this number in case of `ResourceExhausted` errors. Default is `2`, minimum is `2`.|
+| `linguist_languages_path` | string | no | Override for dynamic `languages.json` discovery. Defaults to an empty string (use of dynamic discovery).|
+
+Example:
+
+```toml
+[gitaly-ruby]
+dir = "/home/git/gitaly/ruby"
+max_rss = 200000000
+graceful_restart_timeout = "10m"
+restart_delay = "5m"
+num_workers = 2
+```
+
+### GitLab Shell
+
+For historical reasons
+[GitLab Shell](https://gitlab.com/gitlab-org/gitlab-shell) contains
+the Git hooks that allow GitLab to validate and react to Git pushes.
+Because Gitaly "owns" Git pushes, GitLab Shell must therefore be
+installed alongside Gitaly. This will be [simplified in the
+future](https://gitlab.com/gitlab-org/gitaly/issues/1226).
+
+| Name | Type | Required | Description |
+| ---- | ---- | -------- | ----------- |
+| `dir` | string | yes | The directory where GitLab Shell is installed.|
+
+Example:
+
+```toml
+[gitlab-shell]
+dir = "/home/git/gitlab-shell"
+```
+
+### Prometheus
+
+You can optionally configure Gitaly to record histogram latencies on GRPC method
+calls in Prometheus.
+
+| Name | Type | Required | Description |
+| ---- | ---- | -------- | ----------- |
+| `grpc_latency_buckets` | array | no | Prometheus stores each observation in a bucket, which means you'd get an approximation of latency. Optimizing the buckets gives more control over the accuracy of the approximation. |
+
+Example:
+
+```toml
+prometheus_listen_addr = "localhost:9236"
+
+[prometheus]
+grpc_latency_buckets = [0.001, 0.005, 0.025, 0.1, 0.5, 1.0, 10.0, 30.0, 60.0, 300.0, 1500.0]
+```
+
+### Logging
+
+The following values configure logging in Gitaly under the `[logging]` section.
+
+| Name | Type | Required | Description |
+| ---- | ---- | -------- | ----------- |
+| `format` | string | no | Log format: `text` or `json`. Default: `text`. |
+| `level`  | string | no | Log level: `debug`, `info`, `warn`, `error`, `fatal`, or `panic`. Default: `info`. |
+| `sentry_dsn` | string | no | Sentry DSN for exception monitoring. |
+| `sentry_environment` | string | no | [Sentry Environment](https://docs.sentry.io/enriching-error-data/environments/) for exception monitoring. |
+| `ruby_sentry_dsn` | string | no | Sentry DSN for gitaly-ruby exception monitoring. |
+
+While the main Gitaly application logs go to stdout, there are some extra log
+files that go to a configured directory, like the GitLab Shell logs.
+Gitlab Shell does not support `panic` or `trace` level logs. `panic` will fall
+back to `error`, while `trace` will fall back to `debug`. Any other invalid log
+levels will default to `info`.
+
+Example:
+
+```toml
+[logging]
+level = "warn"
+dir = "/home/gitaly/logs"
+format = "json"
+sentry_dsn = "https://<key>:<secret>@sentry.io/<project>"
+ruby_sentry_dsn = "https://<key>:<secret>@sentry.io/<project>"
+```
+
+## Concurrency
+
+You can adjust the `concurrency` of each RPC endpoint.
+
+| Name | Type | Required | Description |
+| ---- | ---- | -------- | ----------- |
+| `concurrency` | array | yes | An array of RPC endpoints. |
+| `rpc` | string | no | The name of the RPC endpoint (`/gitaly.RepositoryService/GarbageCollect`). |
+| `max_per_repo` | integer | no | Concurrency per RPC per repo. |
+
+Example:
+
+```toml
+[[concurrency]]
+rpc = "/gitaly.RepositoryService/GarbageCollect"
+max_per_repo = 1
+```