Add logging documentation (#206)

* Add logging documentation

---------

Signed-off-by: Monthon Klongklaew <monthonk@amazon.com>

docs/LOGGING.md

@@ -0,0 +1,43 @@
+# Logging
+
+There are two types of logging you can use for troubleshooting. The first set of logs are from the CSI Driver, and the other are from Mountpoint.
+
+## CSI Driver logs
+
+By default, CSI Driver logs are written to the driver pod’s `stderr` and those are captured by Kubernetes and may be retrieved with a corresponding API call:
+
+    POD_NAME=s3-app
+    NODE_NAME=$(kubectl get pod ${POD_NAME} -o=custom-columns=NODE:.spec.nodeName --no-headers | tail -1)
+    DRIVER_POD=$(kubectl get pods -n kube-system --field-selector spec.nodeName=${NODE_NAME} -o=custom-columns=NAME:.metadata.name | grep s3-csi-node)
+    kubectl logs ${DRIVER_POD} -n kube-system --container s3-plugin
+
+## Mountpoint logs
+
+Mountpoint logs are written in a node which your application pods are running. The location of the logs may vary by your operating systems, but they usually are written to host’s systemd journal. To fetch these logs, first you need to find the pod UID and node name for the pod.
+
+    POD_NAME=s3-app
+    kubectl get pods ${POD_NAME} -o=custom-columns=POD_UID:.metadata.uid,NODE:.spec.nodeName
+
+Note down the pod UID and node name.
+
+Next, connect to the Kubernetes node with the name discovered in the previous step. If you are using EKS with EC2 instances as worker nodes, you can find more details about how to connect to your nodes in the [AWS documentation](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/connect-to-linux-instance.html).
+
+Once you have a session on the node itself, use the UID noted earlier to find the corresponding Mountpoint logs.
+
+    POD_UID=f1015626-32ed-46e6-9634-c739c3a31312
+    PV_NAME=s3-pv
+    echo "MOUNT_PID: $(ps -ef | grep "$POD_UID.*$PV_NAME" | grep -v "grep" | awk '{print $2}')"
+
+If Mountpoint is no longer running since it was unable to mount or possibly died later, there may no longer be an active process to search for. If a value for `MOUNT_PID` was emitted meaning that the Mountpoint process is still running, note that down.
+
+If Mountpoint is still running, use the following to grab logs just from that Mountpoint process updating the value for `MOUNT_PID`:
+
+    MOUNT_PID=<MOUNT_PID>
+    UNIT=$(systemctl status $MOUNT_PID | grep --only-matching "mount-s3-.*\.service" | tail -1)
+    journalctl --unit $UNIT
+
+If Mountpoint is no longer running, you can use the following to get logs from all Mountpoint processes that have been executed since boot. This may include logs from multiple Mountpoint processes.
+
+    journalctl --boot -t mount-s3
+
+For more details about Mountpoint logging and the configuration options available, please refer to [Mountpoint's logging documentation](https://github.com/awslabs/mountpoint-s3/blob/main/doc/LOGGING.md).