doc: Add initial design notes for the Health Checker

Signed-off-by: Niels de Vos <[email protected]>

docs/design/proposals/volume-condition.md

@@ -0,0 +1,25 @@
+# Suport for CSI `VolumeCondition` aka Volume Health Checker
+
+## health-checker API
+
+Under `internal/health-checker`  the Manager for health-checking is
+implemented. The Manager can start a checking process for a given path, return
+the (un)healthy state and stop the checking process when the volume is not
+needed anymore.
+
+The Manager is responsible for creating a suitble checker for the requested
+path. If the path a s block-device, the BlockChecker should be created. For a
+filesystem path (directory), the FileChecker is appropriate.
+
+## CephFS
+
+The health-checker writes to the file `csi-volume-condition.ts` in the root of
+the volume. This file containse a JSON formatted timestamp.
+
+A new `data` directory is introduced for newly created volumes. During the
+`NodeStageVolume` call the root of the volume is mounted, and the `data`
+directory is bind-mounted inside the container when `NodePublishVolume` is
+called.
+
+The `data` directory makes it possible to place Ceph-CSI internal files in the
+root of the volume, without that the user/application has access to it.