message: Doc: add p2p proxy field in nydusd config
  Show how nydus integrate with Dragonfly P2P service.
diff: @@ -84,21 +84,12 @@ We are working on enabling cloud-hypervisor support for nydus.
  // localfs | oss | registry
  "type": "localfs",
  "config": {
  - // Access remote storage backend via P2P proxy, e.g. Dragonfly client
  - "proxy": "http://p2p-proxy:65001",
  - // Fallback to remote storage backend if P2P proxy ping failed
  - "proxy_fallback": true,
  - // Endpoint of P2P proxy health check
  - "proxy_ping_url": "http://p2p-proxy:40901/server/ping",
  - // Interval of P2P proxy checking, in seconds
  - "proxy_check_interval": 5,
  // Drop the read request once http request timeout, in seconds
  "timeout": 5,
  // Drop the read request once http connection timeout, in seconds
  "connect_timeout": 5,
  // Retry count when read request failed
  "retry_limit": 0,
  - ...
  }
  },
  "cache": {
  @@ -206,6 +197,41 @@ We are working on enabling cloud-hypervisor support for nydus.
  }
  ```
  +##### Enable P2P Proxy for Storage Backend
  +
  +Add `device.backend.config.proxy` field to enable HTTP proxy for storage backend. For example, use P2P distribution service to reduce network workload and latency in large scale container cluster using [Dragonfly](https://d7y.io/) (enable centralized dfdaemon mode).
  +
  +```
  +{
  + "device": {
  + "backend": {
  + "type": "registry",
  + "config": {
  + "proxy": {
  + // Access remote storage backend via P2P proxy, e.g. Dragonfly dfdaemon server URL
  + "url": "http://p2p-proxy:65001",
  + // Fallback to remote storage backend if P2P proxy ping failed
  + "fallback": true,
  + // Endpoint of P2P proxy health checking
  + "ping_url": "http://p2p-proxy:40901/server/ping",
  + // Interval of P2P proxy health checking, in seconds
  + "check_interval": 5
  + },
  + ...
  + }
  + },
  + ...
  + },
  + ...
  +}
  +```
  +
  +Once the configuration is loaded successfully on nydusd starting, we will see the log as shown below:
  +
  +```
  +INFO [storage/src/backend/connection.rs:136] backend config: CommonConfig { proxy: ProxyConfig { url: "http://p2p-proxy:65001", ping_url: "http://p2p-proxy:40901/server/ping", fallback: true, check_interval: 5 }, timeout: 5, connect_timeout: 5, retry_limit: 0 }
  +```
  +
  ### Mount Bootstrap Via API
  To mount a bootstrap via api, first launch nydusd without a bootstrap:
  