Polymorphic Volume Scheduling



Polly implements a centralized storage scheduling service that integrates with popular container schedulers of different application platforms for containerized workloads. It is an open source framework that supports use of external storage, with scheduled containerized workloads, at scale. It can be used to centralize the control of creating, mapping, snapshotting and deleting persistent data volumes on a multitude of storage platforms.

Key Features

What it does

Container runtime schedulers need to be integrated with every aspect of available hardware resources, including persistent storage. When requesting resources for an application the scheduler gets offers for CPU, RAM and disk.

To be able to offer persistent storage in a scalable way, the application and container scheduler needs awareness of the available resources from the underlying storage infrastructure.

Example workflow

  1. An application requires highly available storage with a specific set of policies applied
  2. The scheduler receives a request to start the application
  3. The scheduler checks with Polly or already has an off from Polly for storage resources
  4. Polly requests the volume(s) to be mapped to the container
  5. Scheduler issues request to start the container with persistent storage
  6. Container runtime orchestrates process of starting container and attaching persistent storage

Container runtime scheduler support

Cloud platform support

Storage platform support


Polly makes use of the open source storage plugin framework libStorage to enable storage orchestrator tools and container runtimes to make requests of storage. Any storage platform that has a driver implementation for the libStorage framework will work with Polly.

Hello Polly

In the grand tradition of technical documentation, the first true end-to-end example of Polly is called Hello Polly. It showcases a two-node deployment with the first node configured with REX-Ray talking to a Polly/libstorage server and the second node as merely a REX-Ray client talking to the Polly server on the first node. Both nodes have Docker (1.11+) installed and configured to leverage Polly for persistent storage.

The below example does have a few requirements:

Start Polly Vagrant Environment

Before bringing the Vagrant environment online, please ensure it is accomplished in a clean directory:

$ cd $(mktemp -d)

Inside the newly created, temporary directory, download the Polly Vagrantfile:

$ curl -fsSLO https://raw.githubusercontent.com/emccode/polly/master/Vagrantfile

Now it is time to bring the Polly environment online:

!!! note "note"

The next step could potentially open up the system on which the command
is executed to security vulnerabilities. The Vagrantfile brings the
VirtualBox web service online if it is not already running. However,
in the name of simplicity the Vagrantfile also disables the web server's
authentication module. Please do not disable authentication for the
VirtualBox web server if this example is being executed on an open network
or without some type of firewall in place.
$ vagrant up

Once the command has been completed successfully there will be two VMs online named node0 and node1. Both nodes are running Docker and REX-Ray; however, node0 has Polly configured to act as a libStorage server. Both REX-Ray instances will be talking to Polly for managing volumes for your container runtimes (i.e. Docker).

Now that the environment is online it is time to showcase Docker leveraging REX-Ray to create persistent storage as well as illustrating REX-Ray's distributed deployment capabilities.

Node 0

First, SSH into node0

$ vagrant ssh node0

From node0 use Docker with REX-Ray backed by a Polly server to create a new volume named hellopersistence:

vagrant@node0:~$ docker volume create --driver rexray --opt size=1 \
                 --name hellopersistence

You can verify that REX-Ray provisioned the volume by running the following command:

vagrant@node0:~$ rexray volume

Since the volume creation was actually created via a Polly server, you can verify that Polly is tracking the volumes created via the REX-Ray interface:

vagrant@node0:~$ polly volume ls

After the volume is created, mount it to the host and container using the --volume-driver and -v flag in the docker run command:

vagrant@node0:~$ docker run -tid --volume-driver=rexray \
                 -v hellopersistence:/mystore \
                 --name temp01 busybox

Create a new file named myfile on the file system backed by the persistent volume using docker exec:

vagrant@node0:~$ docker exec temp01 touch /mystore/myfile

Verify the file was successfully created by listing the contents of the persistent volume:

vagrant@node0:~$ docker exec temp01 ls /mystore

Remove the container that was used to write the data to the persistent volume:

vagrant@node0:~$ docker rm -f temp01

Finally, exit the SSH session to node0:

vagrant@node0:~$ exit

Node 1

It's time to connect to node1 and use the volume hellopersistence that was created in the previous section from node0.

!!! note "note"

While `node1` runs both the Docker and REX-Ray services like `node0`, the
REX-Ray service on `node1` in no way understands or is configured for the
VirtualBox storage driver. All interactions with the VirtualBox web service
occurs via `node0`'s Polly server with which `node1` communicates.

Use the vagrant command to SSH into node1:

$ vagrant ssh node1

Next, create a new container that mounts the existing volume, hellopersistence:

vagrant@node1:~$ docker run -tid --volume-driver=rexray \
                 -v hellopersistence:/mystore \
                 --name temp01 busybox

The next command validates the file myfile created from node0 in the previous section has persisted inside the volume across machines:

vagrant@node1:~$ docker exec temp01 ls /mystore

Finally, exit the SSH session to node1:

vagrant@node1:~$ exit

Cleaning Up

Be sure to kill the VirtualBox web server with a quick killall vboxwebsrv and to tear down the Vagrant environment with vagrant destroy -f. Omitting these commands will leave the web service and REX-Ray/Polly Vagrant nodes online and consume additional system resources.


REX-Ray with the use of Polly on the backend has been used to provide persistence for stateless containers!

Getting Help

Having issues? No worries, let's figure it out together.

GitHub and Slack

If a little extra help is needed, please don't hesitate to use GitHub issues or join the active conversation on the EMC {code} Community Slack Team in the #project-polly channel