Longhorn

Longhorn is a distributed block storage system for Kubernetes. Longhorn is lightweight, reliable, and easy-to-use. You can deploy Longhorn on an existing Kubernetes cluster with one simple command. Once Longhorn is deployed, it adds persistent volume support to the Kubernetes cluster.

Longhorn implements distributed block storage using containers and microservices. Longhorn creates a dedicated storage controller for each block device volume and sychronously replicates the volume across multiple replicas stored on multiple nodes. The storage controller and replicas are themselves orchestrated using Kubernetes. Longhorn supports snapshots, backups, and even allows you to schedule recurring snapshots and backups!

You can 了解更多 details of Longhorn and its design hereLonghorn - 图1.

Current status

Longhorn is a work in progress. It’s an alpha quality software at the moment. We appreciate your comments as we continue to work on it.

The latest release of Longhorn is v0.4.0, shipped with Longhorn Engine v0.4.0 as the default engine image.

Source code

Longhorn is 100% 完全开源 software. Project source code is spread across a number of repos:

  1. Longhorn engine — Core controller/replica logic https://github.com/rancher/longhorn-engine
  2. Longhorn manager — Longhorn orchestration, includes Flexvolume driver for Kubernetes https://github.com/rancher/longhorn-manager
  3. Longhorn UI — Dashboard https://github.com/rancher/longhorn-ui

Demo

Longhorn DemoLonghorn - 图3

Requirements

  • Minimal Requirements
  1. Docker v1.13+
  2. Kubernetes v1.8 - v1.12
    1. Kubernetes v1.13 support is a work-in-progress.
  3. Make sure open-iscsi has been installed in all nodes of the Kubernetes cluster. For GKE, recommended Ubuntu as guest OS image since it contains open-iscsi already.
    1. For Debian/Ubuntu, use apt-get install open-iscsi to install.
    2. For RHEL/CentOS, use yum install iscsi-initiator-utils to install.
  • Kubernetes driver Requirements

Longhorn can be used in Kubernetes to provide persistent storage through either Longhorn Container Storage Interface (CSI) driver or Longhorn Flexvolume driver. Longhorn will automatically deploy one of the drivers, depending on the Kubernetes cluster configuration. User can also specify the driver in the deployment yaml file. CSI is preferred.

  • Environment check script

We’ve wrote a script to help user to get enough information to configure the setup correctly.

Before installing, run:

  1. curl -sSfL https://raw.githubusercontent.com/rancher/longhorn/master/scripts/environment_check.sh | bash

Example result:

  1. daemonset.apps/longhorn-environment-check created
  2. waiting for pods to become ready (0/3)
  3. all pods ready (3/3)
  5.   MountPropagation is enabled!
  7. cleaning up...
  8. daemonset.apps "longhorn-environment-check" deleted
  9. clean up complete

Please make a note of MountPropagation feature gate status.

  • Requirement for the CSI driver
  1. Kubernetes v1.10+
    1. CSI is in beta release for this version of Kubernetes, and enabled by default.
  2. Mount propagation feature gate enabled.
    1. It’s enabled by default in Kubernetes v1.10. But some early versions of RKE may not enable it.
  3. If above conditions cannot be met, Longhorn will fall back to the Flexvolume driver.
  • Check if your setup satisfied CSI requirement
  1. Use the following command to check your Kubernetes server version
  1. kubectl version

Result:

  1. Client Version: version.Info{Major:"1", Minor:"10", GitVersion:"v1.10.3", GitCommit:"2bba0127d85d5a46ab4b778548be28623b32d0b0", GitTreeState:"clean", BuildDate:"2018-05-21T09:17:39Z", GoVersion:"go1.9.3", Compiler:"gc", Platform:"linux/amd64"}
  2. Server Version: version.Info{Major:"1", Minor:"10", GitVersion:"v1.10.1", GitCommit:"d4ab47518836c750f9949b9e0d387f20fb92260b", GitTreeState:"clean", BuildDate:"2018-04-12T14:14:26Z", GoVersion:"go1.9.3", Compiler:"gc", Platform:"linux/amd64"}

The Server Version should be v1.10 or above.

  1. The result of environment check script should contain MountPropagation is enabled!.
  • Requirement for the Flexvolume driver
  1. Kubernetes v1.8+
  2. Make sure curl, findmnt, grep, awk and blkid has been installed in the every node of the Kubernetes cluster.

Upgrading

For instructions on how to upgrade Longhorn App v0.1 or v0.2 to v0.3, see this document.

Deployment

Create the deployment of Longhorn in your Kubernetes cluster is straightforward.

  1. kubectl --kubeconfig=kube_configxxx.yml  apply  -f https://raw.githubusercontent.com/rancher/longhorn/master/deploy/longhorn.yml

For Google Kubernetes Engine (GKE) users, see here before proceeding.

Longhorn manager and Longhorn driver will be deployed as daemonsets in a separate namespace called longhorn-system, as you can see in the yaml file.

One of the two available drivers (CSI and Flexvolume) would be chosen automatically based on the environment of the user. User can also override the automatic choice if necessary. See here for the detail.

Noted that the volume created and used through one driver won’t be recongized by Kubernetes using the other driver. So please don’t switch driver (e.g. during upgrade) if you have existing volumes created using the old driver.

When you see those pods have started correctly as follows, you’ve deployed Longhorn successfully.

If Longhorn was deployed with CSI driver (csi-attacher/csi-provisioner/longhorn-csi-plugin exists):

  1. kubectl --kubeconfig=kube_configxxx.yml -n   longhorn-system get pod
  3. NAME                                        READY     STATUS    RESTARTS   AGE
  4. csi-attacher-0                              1/1       Running   0          6h
  5. csi-provisioner-0                           1/1       Running   0          6h
  6. engine-image-ei-57b85e25-8v65d              1/1       Running   0          7d
  7. engine-image-ei-57b85e25-gjjs6              1/1       Running   0          7d
  8. engine-image-ei-57b85e25-t2787              1/1       Running   0          7d
  9. longhorn-csi-plugin-4cpk2                   2/2       Running   0          6h
  10. longhorn-csi-plugin-ll6mq                   2/2       Running   0          6h
  11. longhorn-csi-plugin-smlsh                   2/2       Running   0          6h
  12. longhorn-driver-deployer-7b5bdcccc8-fbncl   1/1       Running   0          6h
  13. longhorn-manager-7x8x8                      1/1       Running   0          6h
  14. longhorn-manager-8kqf4                      1/1       Running   0          6h
  15. longhorn-manager-kln4h                      1/1       Running   0          6h
  16. longhorn-ui-f849dcd85-cgkgg                 1/1       Running   0          5d

Or with FlexVolume driver (longhorn-flexvolume-driver exists):

  1. kubectl --kubeconfig=kube_configxxx.yml -n   longhorn-system get pod
  3. NAME                                        READY     STATUS    RESTARTS   AGE
  4. engine-image-ei-57b85e25-8v65d              1/1       Running   0          7d
  5. engine-image-ei-57b85e25-gjjs6              1/1       Running   0          7d
  6. engine-image-ei-57b85e25-t2787              1/1       Running   0          7d
  7. longhorn-driver-deployer-5469b87b9c-b9gm7   1/1       Running   0          2h
  8. longhorn-flexvolume-driver-lth5g            1/1       Running   0          2h
  9. longhorn-flexvolume-driver-tpqf7            1/1       Running   0          2h
  10. longhorn-flexvolume-driver-v9mrj            1/1       Running   0          2h
  11. longhorn-manager-7x8x8                      1/1       Running   0          9h
  12. longhorn-manager-8kqf4                      1/1       Running   0          9h
  13. longhorn-manager-kln4h                      1/1       Running   0          9h
  14. longhorn-ui-f849dcd85-cgkgg                 1/1       Running   0          5d

Access the UI

Use kubectl --kubeconfig=kube_configxxx.yml -n longhorn-system get svc to get the external service IP for UI:

  1. NAME                TYPE           CLUSTER-IP      EXTERNAL-IP      PORT(S)        AGE
  2. longhorn-backend    ClusterIP      10.20.248.250   <none>           9500/TCP       58m
  3. longhorn-frontend   LoadBalancer   10.20.245.110   100.200.200.123   80:30697/TCP   58m

If the Kubernetes Cluster supports creating LoadBalancer, user can then use EXTERNAL-IP(100.200.200.123 in the case above) of longhorn-frontend to access the Longhorn UI. Otherwise the user can use <node_ip>:<port> (port is 30697in the case above) to access the UI.

Longhorn UI would connect to the Longhorn manager API, provides the overview of the system, the volume operations, and the snapshot/backup operations. It’s highly recommended for the user to check out Longhorn UI.

Noted that the current UI is unauthenticated at the moment.

Use Longhorn with Kubernetes

Longhorn provides the persistent volume directly to Kubernetes through one of the Longhorn drivers. No matter which driver you’re using, you can use Kubernetes StorageClass to provision your persistent volumes.

Use following command to create a default Longhorn StorageClass named longhorn.

  1. kubectl --kubeconfig=kube_configxxx.yml create  -f https://raw.githubusercontent.com/rancher/longhorn/master/examples/storageclass.yml

Now you can create a pod using Longhorn like this:

  1. kubectl --kubeconfig=kube_configxxx.yml create  -f https://raw.githubusercontent.com/rancher/longhorn/master/examples/pvc.yml

The yaml contains two parts:

  1. Create a PVC using Longhorn StorageClass.
  1. apiVersion: v1
  2. kind: PersistentVolumeClaim
  3. metadata:
  4. spec:
  5.   accessModes:
  6.     - ReadWriteOnce
  7.   storageClassName: longhorn
  8.   resources:
  9.     requests:
  10.       storage: 2Gi
  1. Use it in the a Pod as a persistent volume:
  1. apiVersion: v1
  2. kind: Pod
  3. metadata:
  4.   namespace: default
  5. spec:
  6.   containers:
  7.     image: nginx:stable-alpine
  8.     imagePullPolicy: IfNotPresent
  9.     volumeMounts:
  10.       mountPath: /data
  11.     ports:
  12.     - containerPort: 80
  13.   volumes:
  14.     persistentVolumeClaim:
  15.       claimName: longhorn-volv-pvc

More examples are available at ./examples/

Highlight features

Snapshot

A snapshot in Longhorn represents a volume state at a given time, stored in the same location of volume data on physical disk of the host. Snapshot creation is instant in Longhorn.

User can revert to any previous taken snapshot using the UI. Since Longhorn is a distributed block storage, please make sure the Longhorn volume is umounted from the host when revert to any previous snapshot, otherwise it will confuse the node filesystem and cause filesystem corruption.

Note about the block level snapshot

Longhorn is a crash-consistent block storage solution.

It’s normal for the OS to keep content in the cache before writing into the block layer. However, it also means if the all the replicas are down, then Longhorn may not contain the immediate change before the shutdown, since the content was kept in the OS level cache and hadn’t transfered to Longhorn system yet. It’s similar to if your desktop was down due to a power outage, after resuming the power, you may find some weird files in the hard drive.

To force the data being written to the block layer at any given moment, the user can run sync command on the node manually, or umount the disk. OS would write the content from the cache to the block layer in either situation.

Backup

A backup in Longhorn represents a volume state at a given time, stored in the secondary storage (backupstore in Longhorn word) which is outside of the Longhorn system. Backup creation will involving copying the data through the network, so it will take time.

A corresponding snapshot is needed for creating a backup. And user can choose to backup any snapshot previous created.

A backupstore is a NFS server or S3 compatible server.

A backup target represents a backupstore in Longhorn. The backup target can be set at Settings/General/BackupTarget

See here for details on how to setup backup target.

Recurring snapshot and backup

Longhorn supports recurring snapshot and backup for volumes. User only need to set when he/she wish to take the snapshot and/or backup, and how many snapshots/backups needs to be retains, then Longhorn will automatically create snapshot/backup for the user at that time, as long as the volume is attached to a node.

User can find the setting for the recurring snapshot and backup in the Volume Detail page.

Changing replica count of the volumes

The default replica count can be changed in the setting.

Also, when a volume is attached, the user can change the replica count for the volume in the UI.

Longhorn will always try to maintain at least given number of healthy replicas for each volume. If the current healthy replica count is less than specified replica count, Longhorn will start rebuilding new replicas. If the current healthy replica count is more than specified replica count, Longhorn will do nothing. In the later situation, if user delete one or more healthy replicas, or there are healthy replicas failed, as long as the total healthy replica count doesn’t dip below the specified replica count, Longhorn won’t start rebuilding new replicas.

Other features

Multiple disks

iSCSI

Base image

Usage guide

Restoring Stateful Set volumes

Google Kubernetes Engine

Upgrade

Deal with Kubernetes node failure

Troubleshooting

You can click Generate Support Bundle link at the bottom of the UI to download a zip file contains Longhorn related configuration and logs.

See here for the troubleshooting guide.

Uninstall Longhorn

  1. To prevent damage to the Kubernetes cluster, we recommend deleting all Kubernetes workloads using Longhorn volumes (PersistentVolume, PersistentVolumeClaim, StorageClass, Deployment, StatefulSet, DaemonSet, etc).

  2. Create the uninstallation job to cleanly purge CRDs from the system and wait for success:

  1. kubectl --kubeconfig=kube_configxxx.yml create  -f https://raw.githubusercontent.com/rancher/longhorn/master/uninstall/uninstall.yml
  2. kubectl --kubeconfig=kube_configxxx.yml -n   longhorn-system get job/longhorn-uninstall -w

Example output:

  1. $ kubectl --kubeconfig=kube_configxxx.yml create  -f https://raw.githubusercontent.com/rancher/longhorn/master/uninstall/uninstall.yml
  2. job.batch/longhorn-uninstall created
  3. $ kubectl --kubeconfig=kube_configxxx.yml -n   longhorn-system get job/longhorn-uninstall -w
  4. NAME                 DESIRED   SUCCESSFUL   AGE
  5. longhorn-uninstall   1         0            3s
  6. longhorn-uninstall   1         1            45s
  7. ^C
  1. Remove remaining components:
  1. kubectl --kubeconfig=kube_configxxx.yml delete  -f https://raw.githubusercontent.com/rancher/longhorn/master/deploy/longhorn.yml

License

Copyright (c) 2014-2019 Rancher Labs, Inc.Longhorn - 图4

Licensed under the Apache License, Version 2.0 (the “License”); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0Longhorn - 图5

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an “AS IS” BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.