doc: Add initial design notes for the Health Checker

Signed-off-by: Niels de Vos <ndevos@ibm.com>
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Niels de Vos 2023-09-14 17:45:01 +02:00 committed by mergify[bot]
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# Support 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 suitable checker for the requested
path. If the path is a 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 contains 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.