add command reference doc (#119)

* add command reference

* add subcommand: format, mount, benchmark

* format style

* format style

* add command reference link in README

* typo

README.md

@@ -19,7 +19,7 @@ The highlighted features are:
 
 ---
 
-[Architecture](#architecture) | [Getting Started](#getting-started) | [POSIX Compatibility](#posix-compatibility) | [Performance Benchmark](#performance-benchmark) | [Supported Object Storage](#supported-object-storage) | [Status](#status) | [Roadmap](#roadmap) | [Reporting Issues](#reporting-issues) | [Contributing](#contributing) | [Community](#community) | [Usage Tracking](#usage-tracking) | [License](#license) | [Credits](#credits) | [FAQ](#faq)
+[Architecture](#architecture) | [Getting Started](#getting-started) | [POSIX Compatibility](#posix-compatibility) | [Performance Benchmark](#performance-benchmark) | [Supported Object Storage](#supported-object-storage) | [Command Reference](docs/command_reference.md) | [Status](#status) | [Roadmap](#roadmap) | [Reporting Issues](#reporting-issues) | [Contributing](#contributing) | [Community](#community) | [Usage Tracking](#usage-tracking) | [License](#license) | [Credits](#credits) | [FAQ](#faq)
 
 ---
 
@@ -91,6 +91,10 @@ After that you can access the volume just like a local directory.
 
 To get all options, just run `./juicefs mount -h`.
 
+### Command Reference
+
+There is a [command reference](docs/command_reference.md) to see all options of the subcommand.
+
 ### Kubernetes
 
 There is a [Kubernetes CSI driver](https://github.com/juicedata/juicefs-csi-driver) to use JuiceFS in Kubernetes easily.

docs/command_reference.md

+# Command Reference
+
+There are many commands to help you manage your filesystem. This page provides a detailed reference for these commands.
+
+## Overview
+
+If you run `juicefs` by itself, it will print all available subcommands. In addition, you can add `-h/--help` flag after each subcommand to get more information of that subcommand.
+
+```
+NAME:
+   juicefs - A POSIX filesystem built on redis and object storage.
+
+USAGE:
+   juicefs [global options] command [command options] [arguments...]
+
+VERSION:
+   0.9.3-27 (2021-01-26 c516f3c)
+
+COMMANDS:
+   format     format a volume
+   mount      mount a volume
+   benchmark  run benchmark, including read/write/stat big/small files
+   help, h    Shows a list of commands or help for one command
+
+GLOBAL OPTIONS:
+   --debug, -v    enable debug log (default: false)
+   --quiet, -q    only warning and errors (default: false)
+   --trace        enable trace log (default: false)
+   --help, -h     show help (default: false)
+   --version, -V  print only the version (default: false)
+
+COPYRIGHT:
+   AGPLv3
+```
+
+Usage: `juicefs [command] [command options] [arguments ...]`
+
+Add `-h` or `--help` after all commands, getting arguments list and help information.
+
+***Note:*** If `juicefs` is not placed in your `$PATH`, you should run the script with the path to the script. For example, if `juicefs` is placed in current directory, you should use `./juicefs`. It is recommended to place `juicefs` in your `$PATH` for the convenience.
+
+The documentation below gives you detailed information about each subcommand.
+
+## juicefs format
+
+### Description
+
+Format a volume. It's the first step for initializing a new filesystem volume.
+
+### Synopsis
+
+```
+juicefs format [command options] REDIS-URL NAME
+```
+
+### Options
+
+`--block-size value`  
+size of block in KiB (default: 4096)
+
+`--compress value`   
+compression algorithm (lz4, zstd, none) (default: "lz4")
+
+`--storage value`    
+Object storage type (e.g. s3, gcs, oss, cos) (default: "file")
+
+`--bucket value`     
+A bucket URL to store data (default: `"/Users/surui/.juicefs/local"`)
+
+`--access-key value`  
+Access key for object storage (env ACCESS_KEY)
+
+`--secret-key value`  
+Secret key for object storage (env SECRET_KEY)
+
+`--force`             
+overwrite existing format (default: false)
+
+## juicefs mount
+
+### Description
+
+Mount a volume. the volume shoud be formatted first.
+
+### Synopsis
+
+```
+juicefs mount [command options] REDIS-URL MOUNTPOINT
+```
+
+### Options
+
+`-d, --background`  
+run in background (default: false)
+
+`--no-syslog`   
+disable syslog (default: false)
+
+`-o value`  
+other fuse options
+
+`--attr-cache value`    
+attributes cache timeout in seconds (default: 1)
+
+`--entry-cache value`   
+file entry cache timeout in seconds (default: 1)
+
+`--dir-entry-cache value`   
+dir entry cache timeout in seconds (default: 1)
+
+`--enable-xattr`    
+enable extended attributes (xattr) (default: false)
+
+`--get-timeout value`   
+the max number of seconds to download an object (default: 60)
+
+`--put-timeout value`   
+the max number of seconds to upload an object (default: 60)
+
+`--io-retries value`    
+number of retries after network failure (default: 30)
+
+`--max-uploads value`   
+number of connections to upload (default: 20)
+
+`--buffer-size value`   
+total read/write buffering in MB (default: 300)
+
+`--prefetch value`       
+prefetch N blocks in parallel (default: 3)
+
+`--writeback`   
+Upload objects in background (default: false)
+
+`--cache-dir value`       
+directory to cache object (default: `"/Users/surui/.juicefs/cache"`)
+
+`--cache-size value`    
+size of cached objects in MiB (default: 1024)
+
+`--free-space-ratio value`      
+min free space (ratio) (default: 0.1)
+
+`--cache-partial-only` 
+cache only random/small read (default: false)
+
+`--no-usage-report`  
+do not send usage report (default: false)
+
+## juicefs benchmark
+
+### Description
+
+Run benchmark, include read/write/stat big and small files.
+
+### Synopsis
+
+```
+juicefs benchmark [command options] [arguments...]
+```
+
+### Options
+
+`--dest value`  
+path to run benchmark (default: `"/jfs/benchmark"`)
+
+`--block-size value`    
+block size in MiB (default: 1)
+
+`--bigfile-file-size value` 
+size of big file in MiB (default: 1024)
+
+`--smallfile-file-size value`   
+size of small file in MiB (default: 0.1)
+
+`--smallfile-count value`   
+number of small files (default: 100)
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