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Document the mode and max-buffer-size log options (#5613)

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* Document how and why to configure the `mode` and `max-buffer-size` log options
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Stephen Kuenzli
2017-12-27 15:08:47 -07:00
committed by Misty Stanley-Jones
parent ec82a87307
commit 8ad2680824
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engine/admin/logging/overview.md
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@@ -89,6 +89,28 @@ json-file
{% endraw %}
```
## Configure the delivery mode of log messages from container to log driver
Docker provides two modes for delivering messages from the container to the log driver:
* (default) direct, blocking delivery from container to driver
* non-blocking delivery that stores log messages in an intermediate per-container ring buffer for consumption by driver
The `non-blocking` message delivery mode prevents applications from blocking due to logging back pressure. Applications will likely fail in unexpected ways when STDERR or STDOUT streams block.
> **WARNING**: When the buffer is full and a new message is enqueued, the oldest message in memory is dropped. Dropping messages is often preferred to blocking the log-writing process of an application.
{: .warning}
The `mode` log option controls whether to use the `blocking` (default) or `non-blocking` message delivery.
The `max-buffer-size` log option controls the size of the ring buffer used for intermediate message storage when `mode` is set to `non-blocking`. `max-buffer-size` defaults to 1 megabyte.
The following example starts an Alpine container with log output in non-blocking mode and a 4 megabyte buffer:
```bash
$ docker run -it --log-opt mode=non-blocking --log-opt max-buffer-size=4m alpine ping 127.0.0.1
```
### Use environment variables or labels with logging drivers
Some logging drivers add the value of a container's `--env|-e` or `--label`