If enabled, Rook uses [FlexVolume](https://github.com/kubernetes/community/blob/master/contributors/devel/sig-storage/flexvolume.md) to integrate with Kubernetes for performing storage operations. In some operating systems where Kubernetes is deployed, the [default Flexvolume plugin directory](https://github.com/kubernetes/community/blob/master/contributors/devel/sig-storage/flexvolume.md#prerequisites) (the directory where FlexVolume drivers are installed) is **read-only**.
Some Kubernetes deployments require you to configure kubelet with a FlexVolume plugin directory that is accessible and read/write (`rw`). These steps need to be carried out on **all nodes** in your cluster. Rook needs to be told where this directory is in order for the volume plugin to work.
If you are not using a platform that is listed above and the path `/usr/libexec/kubernetes/kubelet-plugins/volume/exec/` is read/write, you don't need to configure anything.
That is because `/usr/libexec/kubernetes/kubelet-plugins/volume/exec/` is the kubelet default FlexVolume path and Rook assumes the default FlexVolume path if not set differently.
If running `mkdir -p /usr/libexec/kubernetes/kubelet-plugins/volume/exec/` should give you an error about read-only filesystem, you need to use [another read/write FlexVolume path](#other-common-readwrite-flexvolume-paths) and configure it on the Rook operator and kubelet.
### Other common read/write FlexVolume paths
Because `/var/lib/kubelet` is read write on most systems, these are the other commonly used paths:
Kubespray uses the [kubelet_flexvolumes_plugins_dir](https://github.com/kubernetes-sigs/kubespray/blob/v2.11.0/inventory/sample/group_vars/k8s-cluster/k8s-cluster.yml#L206) variable to define where it sets the plugin directory.
Kubespray prior to the v2.11.0 release [used a non-standard FlexVolume plugin directory](https://github.com/kubernetes-sigs/kubespray/blob/f47a66622743aa31970cebeca7968a0939cb700d/roles/kubernetes/node/defaults/main.yml#L53): `/var/lib/kubelet/volume-plugins`.
The Kubespray configured kubelet is already configured to use that directory.
If you are using kubespray v2.10.x or older, continue with [configuring the FlexVolume path](#configuring-the-flexvolume-path) to configure Rook to use the FlexVolume path.
#### As of v2.11.0 and newer
Kubespray v2.11.0 included https://github.com/kubernetes-sigs/kubespray/pull/4752 which sets the same plugin directory assumed by rook by default: `/usr/libexec/kubernetes/kubelet-plugins/volume/exec`.
No special configuration of the directory is needed in Rook unless:
* Kubespray is deployed onto a platform where the default path is not writable, or
* you have explicitly defined a custom path in the [kubelet_flexvolumes_plugins_dir](https://github.com/kubernetes-sigs/kubespray/blob/v2.11.0/inventory/sample/group_vars/k8s-cluster/k8s-cluster.yml#L206) variable
If you have not defined one, and the default path is not writable, the [alternate configuration](https://github.com/kubernetes-sigs/kubespray/blob/v2.11.0/roles/kubernetes/preinstall/tasks/0040-set_facts.yml#L189) is `/var/lib/kubelet/volumeplugins`
If needed, continue with [configuring the FlexVolume path](#configuring-the-flexvolume-path) to configure Rook to use the FlexVolume path.
To find out which FlexVolume directory path you need to set on the Rook operator, please look at the OpenShift docs of the version you are using, [latest OpenShift Flexvolume docs](https://docs.openshift.org/latest/install_config/persistent_storage/persistent_storage_flex_volume.html#flexvolume-installation) (they also contain the FlexVolume path for Atomic).
Continue with [configuring the FlexVolume path](#configuring-the-flexvolume-path) to configure Rook to use the FlexVolume path.
Rancher provides an easy way to configure kubelet. The FlexVolume flag will be shown later on in the [configuring the FlexVolume path](#configuring-the-flexvolume-path).
It can be provided to the kubelet configuration template at deployment time or by using the `up to date` feature if Kubernetes is already deployed.
If you're using [rke](https://github.com/rancher/rke), run `rke up`, this will update and restart your kubernetes cluster system components, in this case the kubelet docker instance(s)
will get restarted with the new volume bind and volume plugin dir flag.
The default FlexVolume path for Rancher is `/usr/libexec/kubernetes/kubelet-plugins/volume/exec` which is also the default
FlexVolume path for the Rook operator.
If the default path as above is used no further configuration is required, otherwise if a different path is used
the Rook operator will need to be reconfigured, to do this continue with [configuring the FlexVolume path](#configuring-the-flexvolume-path) to configure Rook to use the FlexVolume path.
Follow [these instructions](tectonic.md) to configure the Flexvolume plugin for Rook on Tectonic during ContainerLinux node ignition file provisioning.
If you want to use Rook with an already provisioned Tectonic cluster, please refer to the [ContainerLinux](#containerlinux) section.
Continue with [configuring the FlexVolume path](#configuring-the-flexvolume-path) to configure Rook to use the FlexVolume path.
Use the [most common read/write FlexVolume path](#most-common-readwrite-flexvolume-path) for the next steps.
If your kubelet is running as a (Docker, rkt, etc) container you need to make sure that this directory from the host is reachable by the kubelet inside the container.
Continue with [configuring the FlexVolume path](#configuring-the-flexvolume-path) to configure Rook to use the FlexVolume path.
If the environment specific section doesn't mention a FlexVolume path in this doc or external docs, please refer to the [most common read/write FlexVolume path](#most-common-readwrite-flexvolume-path) section, before continuing to [configuring the FlexVolume path](#configuring-the-flexvolume-path).
You must provide the above found FlexVolume path when deploying the [rook-operator](https://github.com/rook/rook/blob/master/cluster/examples/kubernetes/ceph/operator.yaml) by setting the environment variable `FLEXVOLUME_DIR_PATH`.
(In the `operator.yaml` manifest replace `<PathToFlexVolumes>` with the path or if you use helm set the `agent.flexVolumeDirPath` to the FlexVolume path)
The location where you can set the kubelet FlexVolume path (flag) depends on your platform.
Please refer to your platform documentation for that and/or the [platform specific FlexVolume path](#platform-specific-flexvolume-path) for information about that.
After adding the flag to kubelet, kubelet must be restarted for it to pick up the new flag.
