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Add more detail to cluster volumes docs
Signed-off-by: Drew Erny <derny@mirantis.com>
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Drew Erny
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1 changed files with 144 additions and 30 deletions
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@ -2,50 +2,97 @@ Cluster Volumes
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===============
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Docker Cluster Volumes is a new feature which allows using CSI plugins to
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create cluster-aware volumes
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create cluster-aware volumes.
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The Container Storage Interface is a platform-agnostic API for storage
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providers to write storage plugins which are compatible with many container
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orchestrators. By leveraging the CSI, Docker Swarm can provide intelligent,
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cluster-aware access to volumes across many supported storage providers.
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## Installing a CSI plugin
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CSI, the Container Storage Interface, defines an API for storage providers to
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write storage plugins which are cross-compatible between various container
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orchestrators. However, most CSI plugins are shipped with configuration
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specific to Kubernetes. Docker CSI Plugins use the same binaries as those for
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Kubernetes, but in a different environment and sometimes with different
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configuration.
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Docker accesses CSI plugins through the Docker managed plugin system, using the
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`docker plugin` command.
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If a plugin is already adapted for and available for Docker, it can be
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installed through the `docker plugin install` command. Though such plugins may
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require configuration specific to the user's environment, they will ultimately
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be detected by and work automatically with Docker once enabled.
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If a plugin is available for Docker, it can be installed through the `docker
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plugin install` command. Plugins may require configuration specific to the
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user's environment, they will ultimately be detected by and work automatically
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with Docker once enabled.
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Currently, there is no way to automatically deploy a Docker Plugin across all
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nodes in a cluster. Therefore, users must ensure the Docker Plugin is installed
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on all nodes in the cluster on which it is desired.
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Docker Swarm worker nodes report their active plugins to the Docker Swarm
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managers, and so it is not necessary to install a plugin on every worker node
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if this is not desired. However, the plugin must be installed on every manager
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node, or a leadership change could result in Docker Swarm no longer having the
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ability to call the plugin.
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The CSI plugin must be installed on all manager nodes. If a manager node does
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not have the CSI plugin installed, a leadership change to that manager nodes
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will make Swarm unable to use that driver.
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### Creating a Docker CSI Plugin
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Docker Swarm worker nodes report their active plugins to the Docker Swarm
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managers, so it is not necessary to install a plugin on every worker node. The
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plugin only needs to be installed on those nodes need access to the volumes
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provided by that plugin.
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### Multiple Instances of the Same Plugin
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In some cases, it may be desirable to run multiple instances of the same
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plugin. For example, there may be two different instances of some storage
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provider which each need a differently configured plugin.
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To run more than one instance of the same plugin, set the `--alias` option when
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installing the plugin. This will cause the plugin to take a local name
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different from its original name.
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Ensure that when using plugin name aliases, the plugin name alias is the same
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on every node.
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## Creating a Docker CSI Plugin
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Most CSI plugins are shipped with configuration specific to Kubernetes. They
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are often provided in the form of Helm charts, and installation information may
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include Kubernetes-specific steps. Docker CSI Plugins use the same binaries as
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those for Kubernetes, but in a different environment and sometimes with
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different configuration.
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Before following this section, readers should ensure they are acquainted with
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the
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[Docker Engine managed plugin system](https://docs.docker.com/engine/extend/).
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Docker CSI plugins use this system to run.
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Docker Plugins consist of a root filesystem and a `config.json`. The root
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filesystem can generally be exported from whatever image is built for the
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plugin. The `config.json` specifies how the plugin is used. Several
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CSI-specific concerns, as well as some general but poorly-documented features,
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are outlined here.
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### Basic Requirements
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Docker CSI plugins are identified with a special interface type. There are two
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related interfaces that CSI plugins can expose. In the `config.json`, this
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should be set as such.
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related interfaces that CSI plugins can expose.
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* `docker.csicontroller/1.0` is used for CSI Controller plugins.
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* `docker.csinode/1.0` is used for CSI Node plugins.
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* Combined plugins should include both interfaces.
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Additionally, the interface field of the config.json includes a `socket` field.
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This can be set to any value, but the CSI plugin should have its `CSI_ENDPOINT`
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environment variable set appropriately.
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In the `config.json`, this should be set as such:
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```json
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"interface": {
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"types": ["docker.csicontroller/1.0","docker.csinode/1.0"]
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"types": ["docker.csicontroller/1.0","docker.csinode/1.0"],
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"socket": "my-csi-plugin.sock"
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},
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"env": [
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{
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"name": "CSI_ENDPOINT",
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"value": "/run/docker/plugins/my-csi-plugin.sock"
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}
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]
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```
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Additionally, the CSI specification states that CSI plugins should have
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The CSI specification states that CSI plugins should have
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`CAP_SYS_ADMIN` privileges, so this should be set in the `config.json` as
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well:
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@ -55,9 +102,81 @@ well:
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}
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```
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Other configuration is largely specific to the CSI plugin.
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### Propagated Mount
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#### Split-Component Plugins
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In order for the plugin to expose volumes to Swarm, it must publish those
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volumes to a Propagated Mount location. This allows a mount to be itself
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mounted to a different location in the filesystem. The Docker Plugin system
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only allows one Propagated Mount, which is configured as a string representing
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the path in the plugin filesystem.
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When calling the CSI plugin, Docker Swarm specifies the publish target path,
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which is the path in the plugin filesystem that a volume should ultimately be
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used from. This is also the path that needs to be specified as the Propagated
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Mount for the plugin. This path is hard-coded to be `/data/published` in the
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plugin filesystem, and as such, the plugin configuration should list this as
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the Propagated Mount:
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```json
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"propagatedMount": "/data/published"
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```
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### Configurable Options
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Plugin configurations can specify configurable options for many fields. To
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expose a field as configurable, the object including that field should include
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a field `Settable`, which is an array of strings specifying the name of
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settable fields.
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For example, consider a plugin that supports a config file.
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```json
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"mounts": [
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{
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"name": "configfile",
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"description": "Config file mounted in from the host filesystem",
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"type": "bind",
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"destination": "/opt/my-csi-plugin/config.yaml",
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"source": "/etc/my-csi-plugin/config.yaml"
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}
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]
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```
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This configuration would result in a file located on the host filesystem at
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`/etc/my-csi-plugin/config.yaml` being mounted into the plugin filesystem at
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`/opt/my-csi-plugin/config.yaml`. However, hard-specifying the source path of
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the configuration is undesirable. Instead, the plugin author can put the
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`Source` field in the Settable array:
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```json
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"mounts": [
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{
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"name": "configfile",
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"description": "Config file mounted in from the host filesystem",
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"type": "bind",
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"destination": "/opt/my-csi-plugin/config.yaml",
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"source": "",
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"settable": ["source"]
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}
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]
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```
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When a field is exposed as settable, the user can configure that field when
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installing the plugin.
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```
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$ docker plugin install my-csi-plugin configfile.source="/srv/my-csi-plugin/config.yaml"
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```
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Or, alternatively, it can be set while the plugin is disabled:
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```
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$ docker plugin disable my-csi-plugin
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$ docker plugin set my-csi-plugin configfile.source="/var/lib/my-csi-plugin/config.yaml"
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$ docker plugin enable
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```
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### Split-Component Plugins
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For split-component plugins, users can specify either the
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`docker.csicontroller/1.0` or `docker.csinode/1.0` plugin interfaces. Manager
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@ -198,11 +317,6 @@ Cluster Volume availability can be one of three states:
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A Volume can only be removed from the cluster entirely if its availability is
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set to `drain`, and it has been fully unpublished from all nodes.
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#### Force-Removing Volumes
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There are cases where a Volume can get caught in a state where Swarm cannot
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verify their removal. In these cases,
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## Unsupported Features
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The CSI Spec allows for a large number of features which Cluster Volumes in
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