I am using a PVC with ReadWriteOnce access mode, which is used by a logstash Deployment which will run a stateful application and use this PVC.Each pod in the deployment will try to bind to the same persistent volume claim. In case of replicas > 1, it will fail (as it supports ReadWriteOnce, only the first one will be able to bind successfully). How do I specify that each pod is to be bound to a separate PV.
I don't want to define 3 separate yamls for each logstash replica / instance
apiVersion: apps/v1
kind: Deployment
metadata:
name: logstash
spec:
replicas: 3
template:
metadata:
labels:
app: logstash
spec:
containers:
image: "logstash-image"
imagePullPolicy: IfNotPresent
name: logstash
volumeMounts:
- mountPath: /data
name: logstash-data
restartPolicy: Always
volumes:
- name: logstash-data
persistentVolumeClaim:
claimName: logstash-vol
Need a way to do volume mount of different PVs to different pod replicas.
With Deployments you cannot do this properly. You should use StatefulSet with PVC template to achieve your target. The part of your StatefulSet YAML code snippet could look like this:
...
volumeClaimTemplates:
- metadata:
name: pv-data
spec:
accessModes:
- ReadWriteOnce
resources:
requests:
storage: 5G
assuming you have 3 replicas, you will see the pods are created one by one sequentially, and the PVC is requested during the pod creation.
The PVC is named as
volumeClaimTemplate name + pod-name + ordinal number and as result, you will have the list of newly created PVCs:
pv-data-<pod_name>-0
pv-data-<pod_name>-1
pv-data-<pod_name>-N
StatefulSet makes the names (not only names in fact) of your pods static and increments them depending on replica count, thats why every Pod will match its own PVC and PV respectively
Note: this is called dynamic provisioning. You should be familiar with
configuring kubernetes control plane components (like
controller-manager) to achieve this, because you will need
configured persistent storage (one of them) providers and understand
the retain policy of your data, but this is completely another
question...
Related
I have a PersistenceVolumeClaim defined by
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
name: my-pvc
spec:
storageClassName: "standard"
accessModes:
- ReadWriteOnce
resources:
requests:
storage: 1Gi
And the containers section of the deployment yaml looks like this
spec:
containers:
- name: my-container
image: abc/xyz:1.2.3
volumeMounts:
- mountPath: /var/store
name: mystore
volumes:
- name: mystore
persistentVolumeClaim:
claimName: my-pvc
I have a few questions about this set up.
Do each replica of my pod get 1GB storage space (Assuming the PersistentVolume has enough space)?
How would this behave if the pod replicas are on different kubernetes nodes?
Edit
I would like all replicas of my pod to have it's own storage (not a shared one). Is there a way to achieve this without creating a RWM volume?
Do each replica of my pod get 1GB storage space (Assuming the PersistentVolume has enough space)?
No. Since you use one PersistentVolumeClaim, you will get one PersistentVolume.
How would this behave if the pod replicas are on different kubernetes nodes?
It will not work, unless you use a volume type that can be used from multiple nodes at once, with access mode ReadWriteMany or ReadOnlyMany. But you have declared ReadWriteOnce in your PersistentVolumeClaim, so it will likely not work.
I would like all replicas of my pod to have it's own storage (not a shared one). Is there a way to achieve this without creating a RWM volume?
Yes, you can use StatefulSet instead of Deployment, and use the volumeClaimTemplates:-field.
I need to make use of PVC to specify the specs of the PV and I also need to make sure it uses a custom local storage path in the PV.
I am unable to figure out how to mention the hostpath in a PVC?
This is the PVC config:
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
name: mongo-pvc
spec:
accessModes:
- ReadWriteOnce
resources:
requests:
storage: 1Gi
And this is the mongodb deployment:
spec:
replicas: 1
selector:
matchLabels:
app: mongo
template:
metadata:
labels:
app: mongo
spec:
volumes:
- name: mongo-volume
persistentVolumeClaim:
claimName: mongo-pvc
containers:
- name: mongo
image: mongo
ports:
- containerPort: 27017
volumeMounts:
- name: mongo-volume
mountPath: /data/db
How and where do I mention the hostPath to be mounted in here?
Doc says that you set hostPath when creating a PV (the step before creating PVC).
apiVersion: v1
kind: PersistentVolume
metadata:
name: task-pv-volume
labels:
type: local
spec:
storageClassName: manual
capacity:
storage: 10Gi
accessModes:
- ReadWriteOnce
hostPath:
path: "/mnt/data"
After you create the PersistentVolumeClaim, the Kubernetes control plane looks for a PersistentVolume that satisfies the claim's requirements. If the control plane finds a suitable PersistentVolume with the same StorageClass, it binds the claim to the volume.
Please see https://kubernetes.io/docs/tasks/configure-pod-container/configure-persistent-volume-storage/
You don't (and can't) force a specific host path in a PersistentVolumeClaim.
Typically a Kubernetes cluster will be configured with a dynamic volume provisioner and that will create the matching PersistentVolume for you. Depending on how your cluster was installed that could be an Amazon EBS volume, a Google Cloud Platform persistent disk, an iSCSI volume, or some other type of storage; as an application author you don't really control that. (You tagged this question for GKE, and the GKE documentation has a section on dynamic volume provisioning.) You don't need to specify where on the host the volume might be mounted, and there's no way to provide this detail in the PersistentVolumeClaim.
With the YAML you show, and the context of this being on GKE, I'd expect Google to automatically provision a GCE persistent disk. If the pod gets rescheduled on a different node, the persistent disk will follow the pod to the new node. You don't need to worry about what specific host directory is being used; Kubernetes will manage this for you.
In most cases you'll want to avoid hostPath storage. You don't directly control which node your pods will run on, so you're not guaranteed that the pod will actually be scheduled on the node that has the data volume. It's appropriate for something like a log collector running in a DaemonSet, where you can guarantee that there is interesting content in that path on every node, but not for your general application database storage.
I have a three node GCE cluster and a single-pod GKE deployment with three replicas. I created the PV and PVC like so:
# Create a persistent volume for web content
apiVersion: v1
kind: PersistentVolume
metadata:
name: nginx-content
labels:
type: local
spec:
capacity:
storage: 5Gi
accessModes:
- ReadOnlyMany
hostPath:
path: "/usr/share/nginx/html"
--
# Request a persistent volume for web content
kind: PersistentVolumeClaim
apiVersion: v1
metadata:
name: nginx-content-claim
annotations:
volume.alpha.kubernetes.io/storage-class: default
spec:
accessModes: [ReadOnlyMany]
resources:
requests:
storage: 5Gi
They are referenced in the container spec like so:
spec:
containers:
- image: launcher.gcr.io/google/nginx1
name: nginx-container
volumeMounts:
- name: nginx-content
mountPath: /usr/share/nginx/html
ports:
- containerPort: 80
volumes:
- name: nginx-content
persistentVolumeClaim:
claimName: nginx-content-claim
Even though I created the volumes as ReadOnlyMany, only one pod can mount the volume at any given time. The rest give "Error 400: RESOURCE_IN_USE_BY_ANOTHER_RESOURCE". How can I make it so all three replicas read the same web content from the same volume?
First I'd like to point out one fundamental discrapency in your configuration. Note that when you use your PersistentVolumeClaim defined as in your example, you don't use your nginx-content PersistentVolume at all. You can easily verify it by running:
kubectl get pv
on your GKE cluster. You'll notice that apart from your manually created nginx-content PV, there is another one, which was automatically provisioned based on the PVC that you applied.
Note that in your PersistentVolumeClaim definition you're explicitely referring the default storage class which has nothing to do with your manually created PV. Actually even if you completely omit the annotation:
annotations:
volume.alpha.kubernetes.io/storage-class: default
it will work exactly the same way, namely the default storage class will be used anyway. Using the default storage class on GKE means that GCE Persistent Disk will be used as your volume provisioner. You can read more about it here:
Volume implementations such as gcePersistentDisk are configured
through StorageClass resources. GKE creates a default StorageClass for
you which uses the standard persistent disk type (ext4). The default
StorageClass is used when a PersistentVolumeClaim doesn't specify a
StorageClassName. You can replace the provided default StorageClass
with your own.
But let's move on to the solution of the problem you're facing.
Solution:
First, I'd like to emphasize you don't have to use any NFS-like filesystems to achive your goal.
If you need your PersistentVolume to be available in ReadOnlyMany mode, GCE Persistent Disk is a perfect solution that entirely meets your requirements.
It can be mounted in ro mode by many Pods at the same time and what is even more important by many Pods, scheduled on different GKE nodes. Furthermore it's really simple to configure and it works on GKE out of the box.
In case you want to use your storage in ReadWriteMany mode, I agree that something like NFS may be the only solution as GCE Persistent Disk doesn't provide such capability.
Let's take a closer look how we can configure it.
We need to start from defining our PVC. This step was actually already done by yourself but you got lost a bit in further steps. Let me explain how it works.
The following configuration is correct (as I mentioned annotations section can be omitted):
# Request a persistent volume for web content
kind: PersistentVolumeClaim
apiVersion: v1
metadata:
name: nginx-content-claim
spec:
accessModes: [ReadOnlyMany]
resources:
requests:
storage: 5Gi
However I'd like to add one important comment to this. You said:
Even though I created the volumes as ReadOnlyMany, only one pod can
mount the volume at any given time.
Well, actually you didn't. I know it may seem a bit tricky and somewhat surprising but this is not the way how defining accessModes really works. In fact it's a widely misunderstood concept. First of all you cannot define access modes in PVC in a sense of putting there the constraints you want. Supported access modes are inherent feature of a particular storage type. They are already defined by the storage provider.
What you actually do in PVC definition is requesting a PV that supports the particular access mode or access modes. Note that it's in a form of a list which means you may provide many different access modes that you want your PV to support.
Basically it's like saying: "Hey! Storage provider! Give me a volume that supports ReadOnlyMany mode." You're asking this way for a storage that will satisfy your requirements. Keep in mind however that you can be given more than you ask. And this is also our scenario when asking for a PV that supports ReadOnlyMany mode in GCP. It creates for us a PersistentVolume which meets our requirements we listed in accessModes section but it also supports ReadWriteOnce mode. Although we didn't ask for something that also supports ReadWriteOnce you will probably agree with me that storage which has a built-in support for those two modes fully satisfies our request for something that supports ReadOnlyMany. So basically this is the way it works.
Your PV that was automatically provisioned by GCP in response for your PVC supports those two accessModes and if you don't specify explicitely in Pod or Deployment definition that you want to mount it in read-only mode, by default it is mounted in read-write mode.
You can easily verify it by attaching to the Pod that was able to successfully mount the PersistentVolume:
kubectl exec -ti pod-name -- /bin/bash
and trying to write something on the mounted filesystem.
The error message you get:
"Error 400: RESOURCE_IN_USE_BY_ANOTHER_RESOURCE"
concerns specifically GCE Persistent Disk that is already mounted by one GKE node in ReadWriteOnce mode and it cannot be mounted by another node on which the rest of your Pods were scheduled.
If you want it to be mounted in ReadOnlyMany mode, you need to specify it explicitely in your Deployment definition by adding readOnly: true statement in the volumes section under Pod's template specification like below:
apiVersion: apps/v1
kind: Deployment
metadata:
name: nginx-deployment
labels:
app: nginx
spec:
replicas: 3
selector:
matchLabels:
app: nginx
template:
metadata:
labels:
app: nginx
spec:
containers:
- name: nginx
image: nginx:1.14.2
ports:
- containerPort: 80
volumeMounts:
- mountPath: "/usr/share/nginx/html"
name: nginx-content
volumes:
- name: nginx-content
persistentVolumeClaim:
claimName: nginx-content-claim
readOnly: true
Keep in mind however that to be able to mount it in readOnly mode, first we need to pre-populate such volume with data. Otherwise you'll see another error message, saying that unformatted volume cannot be mounted in read only mode.
The easiest way to do it is by creating a single Pod which will serve only for copying data which was already uploaded to one of our GKE nodes to our destination PV.
Note that pre-populating PersistentVolume with data can be done in many different ways. You can mount in such Pod only your PersistentVolume that you will be using in your Deployment and get your data using curl or wget from some external location saving it directly on your destination PV. It's up to you.
In my example I'm showing how to do it using additional local volume that allows us to mount into our Pod a directory, partition or disk (in my example I use a directory /var/tmp/test located on one of my GKE nodes) available on one of our kubernetes nodes. It's much more flexible solution than hostPath as we don't have to care about scheduling such Pod to particular node, that contains the data. Specific node affinity rule is already defined in PersistentVolume and Pod is automatically scheduled on specific node.
To create it we need 3 things:
StorageClass:
apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
name: local-storage
provisioner: kubernetes.io/no-provisioner
volumeBindingMode: WaitForFirstConsumer
PersistentVolume definition:
apiVersion: v1
kind: PersistentVolume
metadata:
name: example-pv
spec:
capacity:
storage: 10Gi
volumeMode: Filesystem
accessModes:
- ReadWriteOnce
persistentVolumeReclaimPolicy: Delete
storageClassName: local-storage
local:
path: /var/tmp/test
nodeAffinity:
required:
nodeSelectorTerms:
- matchExpressions:
- key: kubernetes.io/hostname
operator: In
values:
- <gke-node-name>
and finally PersistentVolumeClaim:
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
name: myclaim
spec:
accessModes:
- ReadWriteOnce
volumeMode: Filesystem
resources:
requests:
storage: 10Gi
storageClassName: local-storage
Then we can create our temporary Pod which will serve only for copying data from our GKE node to our GCE Persistent Disk.
apiVersion: v1
kind: Pod
metadata:
name: mypod
spec:
containers:
- name: myfrontend
image: nginx
volumeMounts:
- mountPath: "/mnt/source"
name: mypd
- mountPath: "/mnt/destination"
name: nginx-content
volumes:
- name: mypd
persistentVolumeClaim:
claimName: myclaim
- name: nginx-content
persistentVolumeClaim:
claimName: nginx-content-claim
Paths you can see above are not really important. The task of this Pod is only to allow us to copy our data to the destination PV. Eventually our PV will be mounted in completely different path.
Once the Pod is created and both volumes are successfully mounted, we can attach to it by running:
kubectl exec -ti my-pod -- /bin/bash
Withing the Pod simply run:
cp /mnt/source/* /mnt/destination/
That's all. Now we can exit and delete our temporary Pod:
kubectl delete pod mypod
Once it is gone, we can apply our Deployment and our PersistentVolume finally can be mounted in readOnly mode by all the Pods located on various GKE nodes:
apiVersion: apps/v1
kind: Deployment
metadata:
name: nginx-deployment
labels:
app: nginx
spec:
replicas: 3
selector:
matchLabels:
app: nginx
template:
metadata:
labels:
app: nginx
spec:
containers:
- name: nginx
image: nginx:1.14.2
ports:
- containerPort: 80
volumeMounts:
- mountPath: "/usr/share/nginx/html"
name: nginx-content
volumes:
- name: nginx-content
persistentVolumeClaim:
claimName: nginx-content-claim
readOnly: true
Btw. if you are ok with the fact that your Pods will be scheduled only on one particular node, you can give up on using GCE Persistent Disk at all and switch to the above mentioned local volume. This way all your Pods will be able not only to read from it but also to write to it at the same time. The only caveat is that all those Pods will be running on a single node.
You can achieve this with a NFS like file system. On Google Cloud, Filestore is the right product for this (NFS managed). You have a tutorial here for achieving your configuration
You will need to use a shared volume claim with ReadWriteMany (RWX) type if you want to share the volume across different nodes and provide highly scalable solution. Like using NFS server.
You can find out how to deploy an NFS server here:
https://www.shebanglabs.io/run-nfs-server-on-ubuntu-20-04/
And then you can mount volumes (directories from NFS server) as follows:
https://www.shebanglabs.io/how-to-set-up-read-write-many-rwx-persistent-volumes-with-nfs-on-kubernetes/
I've used such a way to deliver shared static content between +8 k8s deployments (+200 pods) serving 1 Billion requests a month over Nginx. and it did work perfectly with that NFS setup :)
Google provides NFS like filesystem called as Google Cloud Filestore. You can mount that on multiple pods.
We have success creating the pods, services and replication controllers according to our project requirements. Now we are planning to setup persistence storage in AWS using Kubernetes. I have created the YAML file to create an EBS volume in AWS, it's working fine as expected. I am able to claim volume and successfully mount to my pod (this is for single replica only).
But when I am trying to create more the one replica, my pods are not creating successfully. When I am trying to create volumes, it's creating in only one availability zone. If my pod is created in a different zone node, since my volume is already created in different zone, due to that my pod is not creating successfully. How to create volumes in different zones for same application? How to make it successful, along with replica? How to create my persistent volumes claims?
---
kind: PersistentVolumeClaim
apiVersion: v1
metadata:
name: mongo-pvc
labels:
type: amazonEBS
spec:
accessModes:
- ReadWriteOnce
resources:
requests:
storage: 10Gi
---
apiVersion: v1
kind: ReplicationController
metadata:
labels:
name: mongo-pp
name: mongo-controller-pp
spec:
replicas: 2
template:
metadata:
labels:
name: mongo-pp
spec:
containers:
- image: mongo
name: mongo-pp
ports:
- name: mongo-pp
containerPort: 27017
hostPort: 27017
volumeMounts:
- mountPath: "/opt/couchbase/var"
name: mypd1
volumes:
- name: mypd1
persistentVolumeClaim:
claimName: mongo-pvc
When you are using ReadWriteOnce volumes (ones that can not be mounted to multiple pods at the same time), simple PV/PVC creation will not cut it.
Both PV and PVC are pretty "singular" in a way that if you refer in Deployment to a particular claim name, your pods will all try to get the same one claim and the same one pv bound to that claim, resulting in a race condition where only one of the pods will be the first and only allowed to mount that RWO storage.
To mitigate this, you should use not PVC directly but via volumeClaimTemplates that will create PVC dynamicaly for every new pod scaled, like below :
volumeClaimTemplates:
- metadata:
name: claimname
spec:
accessModes: [ "ReadWriteOnce" ]
resources:
requests:
storage: 1Gi
I think the problem your a facing is caused by the underlying storage mechanism, in this case EBS.
When scaling Pods behind a replication controller, each replica will attempt to mount the same persistent volume. If you look at the K8 docs in regards to EBS, you will see the following:
There are some restrictions when using an awsElasticBlockStore volume:
the nodes on which pods are running must be AWS EC2 instances those
instances need to be in the same region and availability-zone as the
EBS volume EBS only supports a single EC2 instance mounting a volume
So by default, when you scale up behind a replication controller, Kubernetes will try to spread across different nodes, this means that a second node is trying to mount this volume which is not allowed for EBS.
Basically, I see that you have two options.
Use a different volume type. nfs, Glusterfs etc
Use a StatefulSet instead of a replication controller and have each replica mount an independent volume. Would require database replication but provide high availability.
I have a small problem and need to know what is the best way to approach this/solve my issue.
I have deployed few pods on Kubernetes and so far I have enjoyed learning about and working with Kubernetes. Did all the persistent volume, volume claim...etc. and can see my data on the host, as I need those files for further processing.
Now the issue is 2 pods (2 replicas) sharing the same volume claim are writing to the same location on the host, expected, but unfortunately causing the data to be duplicated in the output file.
What I need is:
To have a unique output of each pod on the host. Is the only way to achieve this is by
having two deployment files, in my case, and each to use a different volume claim/persistent
volume ? At the same time not sure if this is an optimal approach for future updates, upgrades, availability of certain number of pods ... etc.
Or can I still have one deployment file with 2 or more replicas and still avoid the output duplication when sharing the same pvc ?
Please note that I have one node deployment and that's why I'm using hostpath at the moment.
creating pv:
kind: PersistentVolume
apiVersion: v1
metadata:
name: ls-pv
labels:
type: local
spec:
storageClassName: manual
capacity:
storage: 100Gi
accessModes:
- ReadWriteOnce
hostPath:
path: "/ls-data/my-data2"
claim-pv:
kind: PersistentVolumeClaim
apiVersion: v1
metadata:
name: ls-pv-claim
spec:
storageClassName: manual
accessModes:
- ReadWriteOnce
resources:
requests:
storage: 100Gi
How I use my pv inside my deployment:
apiVersion: extensions/v1beta1
kind: Deployment
metadata:
name: logstash
namespace: default
labels:
component: logstash
spec:
replicas: 2
selector:
matchLabels:
component: logstash
#omitted
ports:
- containerPort: 5044
name: logstash-input
protocol: TCP
- containerPort: 9600
name: transport
protocol: TCP
volumeMounts:
- name: ls-pv-store
mountPath: "/logstash-data"
volumes:
- name: ls-pv-store
persistentVolumeClaim:
claimName: ls-pv-claim
Depending on what exactly you are trying to achieve you could use Statefulsets instead of Deployments. Each Pod spawn from the Statefulset's Pod template can have it's own separate PersistentVolumeClaim that is created from the volumeClaimTemplate (see the link for an example). You will need a StorageClass set up for this.
If you are looking for something simpler you write to /mnt/volume/$HOSTNAME from each Pod. This will also ensure that they are using separate files as the hostnames for the Pods are unique.