docs: Add docs of backend logging behavior (#6608)

docs/pages/observability/logging.md

@@ -20,6 +20,9 @@ enabled via the `DYN_LOGGING_SPAN_EVENTS` environment variable.
 | `DYN_LOG` | Log levels per target `<default_level>,<module_path>=<level>,<module_path>=<level>` | `info` | `DYN_LOG=info,dynamo_runtime::system_status_server:trace` |
 | `DYN_LOG_USE_LOCAL_TZ` | Use local timezone for timestamps (default is UTC) | `false` | `true` |
 | `DYN_LOGGING_CONFIG_PATH` | Path to custom TOML logging configuration | none | `/path/to/config.toml` |
+| `VLLM_LOGGING_LEVEL` | vLLM backend log level (independent of `DYN_LOG`) | `INFO` | `DEBUG` |
+| `TLLM_LOG_LEVEL` | TensorRT-LLM backend log level (independent of `DYN_LOG`) | `INFO` | `DEBUG` |
+| `DYN_SKIP_SGLANG_LOG_FORMATTING` | Disable Dynamo's SGLang log configuration | `false` | `true` |
 | `OTEL_SERVICE_NAME` | Service name for trace and span information | `dynamo` | `dynamo-frontend` |
 | `OTEL_EXPORT_ENABLED` | Enable OTLP trace exporting | `false` | `true` |
 | `OTEL_EXPORTER_OTLP_TRACES_ENDPOINT` | OTLP exporter endpoint | `http://localhost:4317` | `http://tempo:4317` |
@@ -259,6 +262,55 @@ Notice how the `x_request_id` field appears in all log entries, alongside the `t
 
 
 
+## Backend Engine Log Levels
+
+Dynamo's `DYN_LOG` environment variable controls Dynamo's own logging. Each
+inference backend has its own log level control that is **independent** of
+`DYN_LOG`.
+
+### vLLM
+
+vLLM log level is controlled by the `VLLM_LOGGING_LEVEL` environment variable.
+It defaults to `INFO` and is completely independent of `DYN_LOG`.
+
+```bash
+# Set vLLM to debug while keeping Dynamo at info
+export DYN_LOG=info
+export VLLM_LOGGING_LEVEL=DEBUG
+```
+
+Valid values: `DEBUG`, `INFO`, `WARNING`, `ERROR`, `CRITICAL`.
+
+### TensorRT-LLM
+
+TensorRT-LLM log level is controlled by the `TLLM_LOG_LEVEL` environment
+variable. It defaults to `INFO` and is completely independent of `DYN_LOG`.
+
+```bash
+# Set TRT-LLM to info while keeping Dynamo at warn
+export DYN_LOG=warn
+export TLLM_LOG_LEVEL=INFO
+```
+
+Valid values: `TRACE`, `DEBUG`, `INFO`, `WARNING`, `ERROR`, `INTERNAL_ERROR`.
+
+**Note:** `TLLM_LOG_LEVEL` is read once at TensorRT-LLM import time. It must
+be set before the process starts.
+
+### SGLang
+
+SGLang logging is currently configured through Dynamo and follows the
+`DYN_LOG` level by default. To disable Dynamo's SGLang log configuration
+and manage it independently, set:
+
+```bash
+export DYN_SKIP_SGLANG_LOG_FORMATTING=true
+```
+
+Alternatively, pass the `--log-level` argument to the SGLang worker
+command to set the SGLang engine's log level directly (e.g.
+`--log-level DEBUG`). This is independent of `DYN_LOG`.
+
 ## Related Documentation
 
 - [Distributed Runtime Architecture](../design-docs/distributed-runtime.md)