I have a cluster with multiple nodes and i also have created a persistent volume in order to have persistence of my data in a Pod. What i am actually looking for is the following:
Lets say that we have a pod, named pod1 that is connected with the Persistent Volume and in a specific directory we have some files. If i have another different pod, named pod2 which is in the same node with the pod1, that wants to copy a file from the directory of the pod1 which is connected with the PV.
Is there any right way to configure this?
What you need is a shared persistent volume mounted in both pods.
You can achieve this in a couple of different ways:
Use a hostPath volume pointing to the same directory.
Use a volume type that supports RWX (read write many) access mode.
Related
In a Kubernetes cluster on Oracle cloud, I have a pod with an Apache server.
This pod needs a persistent volume so I used a persistentVolumeClaim and the cloud provider is able to automatically create an associated volume (Oracle Block Volume).
The access mode used by the PVC is readWriteOnce and therefore the volume created has the same access mode.
Everything work great.
Now I want to backup this volume using borg backup and borgmatic by starting a new pod regularly with a cronJob.
This backup pod needs to mount the volume in read only.
Question:
Can I use the previously defined PVC?
Do I need to create a new PVC with readOnly access mode?
As per documentation:
ReadWriteOnce:
the volume can be mounted as read-write by a single node. ReadWriteOnce access mode still can allow multiple pods to access the volume when the pods are running on the same node.
That means if you make a strict rule for deploying your pods to the same node, you can use the same PVC, here's the INSTRUCTION
Is there any way to access Google cloud Kubernetes persistent volume data without using pod. I cannot start pod due to data corruption in persistent volume. Have any command line tool or any other way.
If you have any concerns running pod with any specific application, in that case, you can run the Ubuntu POD and attach that pod to the PVC and access the data.
There also another option to clone the PV and PVC, perform the testing, and newly created PV and PVC while the old one will work as the backup option.
For cloning PV and PVC you can also use the tool : https://velero.io/
You can also attach the PVC to the POD in read-only mode and try accessing the data.
PersistentVolume resources are cluster resources that exist independently of Pods. This means that the disk and data represented by a PersistentVolume continue to exist as the cluster changes and as Pods are deleted and recreated.
It is possible to save data from your PersistentVolume with Status: Terminating and RetainPolicy set to default(delete). Your PersistentVolumes will not be terminated until there is a pod, deployment or to be more specific a PersistentVolumeClaim using it.
The steps we took to remedy our broken state are as follows:
The first thing you want to do is to create a snapshot of your PersistentVolumes.
In GKE console, go to Compute Engine -> Disks and find your volume there and create a snapshot of your volume. use
kubectl get pv | grep pvc-name
Use the snapshot to create a disk:
gcloud compute disks create name-of-disk --size=10 --source-snapshot=name-of-snapshot --type=pd-standard --zone=your-zone
At this point, stop the services using the volume and delete the volume and volume claim.
Re-create the volume manually with the data from the disk and update your volume claim to target a specific volume file.
For more information refer to the links below.
Accessing file shares from Google Kubernetes Engine clusters.
Configure a Pod to Use a PersistentVolume for Storage
I'm learning Kubernetes and there is something I don't get well.
There are 3 ways of setting up static storage:
Pods with volumes you attach diretctly the storage to
Pods with a PVC attached to its volume
StatefulSets with also PVC inside
I can understand the power of PVC when working together with StorageClass, but not when working with static storage and local storage like hostPath
To me, it sounds very similar:
In the first case I have a volume directly attached to a pod.
In the second case I have a volume statically attached to a PVC, which is also manually attached to a Pod. In the end, the volume will be statically attached to the Pod.
On both cases, the data will remain when the Pod is terminates and will be adopted by the next Pod which the corresponing definition, right?
The only profit I see from using PVCs over plain Pod is that you can define the acces mode. Apart of that. Is there a difference when working with hostpath?
On the other hand, the advantage of using a StatefulSet instead of a PVC is (if understood properly) that it get a headless service, and that the rollout and rollback mechanism works differently. Is that the point?
Thank you in advance!
Extracted from this blog:
The biggest difference is that the Kubernetes scheduler understands
which node a Local Persistent Volume belongs to. With HostPath
volumes, a pod referencing a HostPath volume may be moved by the
scheduler to a different node resulting in data loss. But with Local
Persistent Volumes, the Kubernetes scheduler ensures that a pod using
a Local Persistent Volume is always scheduled to the same node.
Using hostPath does not garantee that a pod will restart on the same node. So you pod can attach /tmp/storage on k8s-node-1, then if you delete and re-create the pod, it may attach tmp/storage on k8s-node-[2-n]
On the contrary, if you use PVC/PV with local persistent storage class, then if you delete and re-create a pod, it will stick on the node which handle the local persistent storage.
StatefulSet creates pods and has volumeClaimTemplate field, which creates a dedicated PVC for each pod. So each pod created by the statefulSet will have its own dedicated storage, linked with Pod->PVC->PV->Storage. So StatefulSet use also the PVC/PV mechanism.
More details are available here.
I've read through all the docs and a few SO posts and can't find an answer to this question:
Where does minikube persist its persistent volumes in my local mac filing system?
Thanks
First of all keep in mind that Kubernetes is running on Minikube cluster. Minikube itself run on Virtual Machine, so all data would be stored in this VM not on your MacOS.
When you want to point exact place where you would like to save this data in Kubernetes you could choose between:
hostpath
A hostPath volume mounts a file or directory from the host node's filesystem into your Pod. This is not something that most Pods will need, but it offers a powerful escape hatch for some applications.
local
A local volume represents a mounted local storage device such as a disk, partition or directory.
Local volumes can only be used as a statically created PersistentVolume. Dynamic provisioning is not supported yet.
Compared to hostPath volumes, local volumes can be used in a durable and portable manner without manually scheduling Pods to nodes, as the system is aware of the volume's node constraints by looking at the node affinity on the PersistentVolume.
However, Minikube supports only hostpath.
In this case you should check Minikube documentation about Persistent Volumes
minikube supports PersistentVolumes of type hostPath out of the box. These PersistentVolumes are mapped to a directory inside the running minikube instance (usually a VM, unless you use --driver=none, --driver=docker, or --driver=podman). For more information on how this works, read the Dynamic Provisioning section below.
minikube is configured to persist files stored under the following
directories, which are made in the Minikube VM (or on your localhost
if running on bare metal). You may lose data from other directories on
reboots.
/data
/var/lib/minikube
/var/lib/docker
/tmp/hostpath_pv
/tmp/hostpath-provisioner
If you would like to mount directory from host you would need to use minikube mount.
$ minikube mount <source directory>:<target directory>
For more details, please check Minikube Mounting filesystems documentation.
If you are using the volume type hostPath the files are saved on your node.
To access your node filesystem you can use the command: minikube ssh and under your mounted path you'll find your documents.
As per this official document, Kubernetes Persistent Volumes support three types of access modes.
ReadOnlyMany
ReadWriteOnce
ReadWriteMany
The given definitions of them in the document is very high-level. It would be great if someone can explain them in little more detail along with some examples of different use cases where we should use one vs other.
You should use ReadWriteX when you plan to have Pods that will need to write to the volume, and not only read data from the volume.
You should use XMany when you want the ability for Pods to access the given volume while those workloads are running on different nodes in the Kubernetes cluster. These Pods may be multiple replicas belonging to a Deployment, or may be completely different Pods. There are many cases where it's desirable to have Pods running on different nodes, for instance if you have multiple Pod replicas for a single Deployment, then having them run on different nodes can help ensure some amount of continued availability even if one of the nodes fails or is being updated.
If you don't use XMany, but you do have multiple Pods that need access to the given volume, that will force Kubernetes to schedule all those Pods to run on whatever node the volume gets mounted to first, which could overload that node if there are too many such pods, and can impact the availability of Deployments whose Pods need access to that volume as explained in the previous paragraph.
So putting all that together:
If you need to write to the volume, and you may have multiple Pods needing to write to the volume where you'd prefer the flexibility of those Pods being scheduled to different nodes, and ReadWriteMany is an option given the volume plugin for your K8s cluster, use ReadWriteMany.
If you need to write to the volume but either you don't have the requirement that multiple pods should be able to write to it, or ReadWriteMany simply isn't an available option for you, use ReadWriteOnce.
If you only need to read from the volume, and you may have multiple Pods needing to read from the volume where you'd prefer the flexibility of those Pods being scheduled to different nodes, and ReadOnlyMany is an option given the volume plugin for your K8s cluster, use ReadOnlyMany.
If you only need to read from the volume but either you don't have the requirement that multiple pods should be able to read from it, or ReadOnlyMany simply isn't an available option for you, use ReadWriteOnce. In this case, you want the volume to be read-only but the limitations of your volume plugin have forced you to choose ReadWriteOnce (there's no ReadOnlyOnce option). As a good practice, consider the containers.volumeMounts.readOnly setting to true in your Pod specs for volume mounts corresponding to volumes that are intended to be read-only.
ReadOnlyMany – the volume can be mounted read-only by many nodes
By this method, multiple pods running on multiple nodes can use a single volume and read data.
If a pod mounts a volume with ReadOnlyMany access mode, other pod can mount it and perform only read operation. Right now GCP is not supporting this method.
This means a volume can be mounted on one or many nodes of your Kubernetes cluster and you can only perform read operation.
You have one pod is running on node and you are reading stored file from volume. While on same volume you can not perform writes.
As it's ReadOnlyMany, if your pod is scheduled to another node, then also volume and data will be available to perform read operation.
ReadWriteMany – the volume can be mounted as read-write by many nodes
By this method, multiple pods running on multiple nodes can use a single volume and read/write data.
If a pod mounts a volume with ReadWriteMany access mode, other pod can also mount it.
This means the volume can be mounted on one or many node of your Kubernetes cluster and you can perform both, read and write operation.
You have one pod running on a node and you are reading & writing the stored file from the volume.
As it's ReadWriteMany if your pod schedule to another node then also the volume and data will be available there to perform read/write operation.
for this, you can use NFS (MinIO, GlusterFS) or EFS filesystem also.
ReadWriteOnce – the volume can be mounted as read-write by a single node
If a pod mounts a volume with ReadWriteOnce access mode, no other pod can mount it. In GCE (Google Compute Engine) the only allowed modes are ReadWriteOnce and ReadOnlyMany. So either one pod mounts the volume ReadWrite, or one or more pods mount the volume ReadOnlyMany.
This means the volume can be mounted on only one node of your kubernetes cluster and you can only perform read operation.
You have one pod running on node and you are reading stored file from volume. While on same volume you cannot perform writes.
As it's ReadWriteOnce if your pod is scheduled to another node then may mossible volume will be attached to the node and you can not get access of data there.
In Kubernetes you provision storage either statically(using a storage class) or dynamically (Persistent Volume). Once the storage is available to bound and claimed, you need to configure it in what way your Pods or Nodes are connecting to the storage (a persistent volume). That could be configured in below four modes.
ReadOnlyMany (ROX)
In this mode multiple pods running on different Nodes could connect to the storage and carry out read operation.
ReadWriteMany (RWX)
In this mode multiple pods running on different Nodes could connect to the storage and carry out read and write operation.
ReadWriteOnce (RWO)
In this mode multiple pods running in only one Node could connect to the storage and carry out read and write operation.
ReadWriteOncePod (RWOP)
In this mode the volume can be mounted as read-write by a single Pod. Use ReadWriteOncePod access mode if you want to ensure that only one pod across whole cluster can read that PVC or write to it. This is only supported for CSI volumes and Kubernetes version 1.22+.
Folow the documentation to get more insight.