Ceph (Rook)
5 minute read
Some of these date back to older versions but efforts are made to keep the most important - sometimes :)
Replication / configuration
Configuration data is stored in location: /var/lib/ceph Storage data is located on dedicated devices which are connected via OSD pods.
Replication: Like Ceph Clients, Ceph OSD Daemons use the CRUSH algorithm, but the Ceph OSD Daemon uses it to compute where replicas of objects should be stored (and for rebalancing). In a typical write scenario, a client uses the CRUSH algorithm to compute where to store an object, maps the object to a pool and placement group, then looks at the CRUSH map to identify the primary OSD for the placement group. The client writes the object to the identified placement group in the primary OSD. Then, the primary OSD with its own copy of the CRUSH map identifies the secondary and tertiary OSDs for replication purposes, and replicates the object to the appropriate placement groups in the secondary and tertiary OSDs (as many OSDs as additional replicas), and responds to the client once it has confirmed the object was stored successfully.
Prerequisite
Since version 1.4 lvm package present on the nodes is required. It applies for AWS machines (not tested on Ubuntu) Example installation command:
RHEL:
yum install lvm2 -y
Rook ceph design
https://rook.io/docs/rook/v1.4/ceph-storage.html
Cluster setup
Rook ceph cluster can be easily deployed using example default definitions from GH repo:
git clone --single-branch --branch release-1.4 https://github.com/rook/rook.git
open location:
rook/cluster/examples/kubernetes/ceph
and list examples:
-rw-r--r--. 1 root root 395 Jul 28 13:00 ceph-client.yaml
-rw-r--r--. 1 root root 1061 Jul 28 13:00 cluster-external-management.yaml
-rw-r--r--. 1 root root 886 Jul 28 13:00 cluster-external.yaml
-rw-r--r--. 1 root root 5300 Jul 28 13:00 cluster-on-pvc.yaml
-rw-r--r--. 1 root root 1144 Jul 28 13:00 cluster-test.yaml
-rw-r--r--. 1 root root 10222 Jul 28 14:47 cluster.yaml
-rw-r--r--. 1 root root 2143 Jul 28 13:00 common-external.yaml
-rw-r--r--. 1 root root 44855 Jul 28 13:00 common.yaml
-rw-r--r--. 1 root root 31424 Jul 28 13:00 create-external-cluster-resources.py
-rw-r--r--. 1 root root 2641 Jul 28 13:00 create-external-cluster-resources.sh
drwxr-xr-x. 5 root root 47 Jul 28 13:00 csi
-rw-r--r--. 1 root root 363 Jul 28 13:00 dashboard-external-https.yaml
-rw-r--r--. 1 root root 362 Jul 28 13:00 dashboard-external-http.yaml
-rw-r--r--. 1 root root 839 Jul 28 13:00 dashboard-ingress-https.yaml
-rw-r--r--. 1 root root 365 Jul 28 13:00 dashboard-loadbalancer.yaml
-rw-r--r--. 1 root root 1554 Jul 28 13:00 direct-mount.yaml
-rw-r--r--. 1 root root 3308 Jul 28 13:00 filesystem-ec.yaml
-rw-r--r--. 1 root root 780 Jul 28 13:00 filesystem-test.yaml
-rw-r--r--. 1 root root 4286 Jul 28 13:00 filesystem.yaml
drwxr-xr-x. 2 root root 115 Jul 28 13:00 flex
-rw-r--r--. 1 root root 4530 Jul 28 13:00 import-external-cluster.sh
drwxr-xr-x. 2 root root 183 Jul 28 13:00 monitoring
-rw-r--r--. 1 root root 1409 Jul 28 13:00 nfs.yaml
-rw-r--r--. 1 root root 495 Jul 28 13:00 object-bucket-claim-delete.yaml
-rw-r--r--. 1 root root 495 Jul 28 13:00 object-bucket-claim-retain.yaml
-rw-r--r--. 1 root root 2306 Jul 28 13:00 object-ec.yaml
-rw-r--r--. 1 root root 2313 Jul 28 13:00 object-openshift.yaml
-rw-r--r--. 1 root root 698 Jul 28 13:00 object-test.yaml
-rw-r--r--. 1 root root 488 Jul 28 13:00 object-user.yaml
-rw-r--r--. 1 root root 3573 Jul 28 13:00 object.yaml
-rw-r--r--. 1 root root 19075 Jul 28 13:00 operator-openshift.yaml
-rw-r--r--. 1 root root 18199 Jul 28 13:00 operator.yaml
-rw-r--r--. 1 root root 1080 Jul 28 13:00 pool-ec.yaml
-rw-r--r--. 1 root root 508 Jul 28 13:00 pool-test.yaml
-rw-r--r--. 1 root root 1966 Jul 28 13:00 pool.yaml
-rw-r--r--. 1 root root 410 Jul 28 13:00 rgw-external.yaml
-rw-r--r--. 1 root root 2273 Jul 28 13:00 scc.yaml
-rw-r--r--. 1 root root 682 Jul 28 13:00 storageclass-bucket-delete.yaml
-rw-r--r--. 1 root root 810 Jul 28 13:00 storageclass-bucket-retain-external.yaml
-rw-r--r--. 1 root root 681 Jul 28 13:00 storageclass-bucket-retain.yaml
-rw-r--r--. 1 root root 1251 Jul 28 13:00 toolbox.yaml
-rw-r--r--. 1 root root 6089 Jul 28 13:00 upgrade-from-v1.2-apply.yaml
-rw-r--r--. 1 root root 14957 Jul 28 13:00 upgrade-from-v1.2-crds.yaml
After creating basic setup (common.yaml, operator.yaml, cluster.yaml) install toolbox (toolbox.yaml) as well for checking the ceph cluster status.
IMPORTANT:
ensure the osd container is created and running. It requires a storage device to be available on the nodes.
During cluster startup it searches for the devices available and creates osd containers for them.
Kubelet nodes have to use a default flag enable-controller-attach-detach set to true. Otherwise PVC will not attach to the pod.
Location of the file where we can find the flag:
/var/lib/kubelet/kubeadm-flags.env
on every worker nodes with kubelet. After that we need to restart kubelet:
systemctl restart kubelet
If cluster is working we can create a storage which can be one of a type:
Block: Create block storage to be consumed by a pod
Object: Create an object store that is accessible inside or outside the
Kubernetes cluster
Shared Filesystem: Create a filesystem to be shared across multiple pods
Eg.
-> filesystem.yaml and then
-> storageclass.yaml
CRD:
There are 2 ways cluster can be set up:
- Host-based Cluster
- PVC-based Cluster
PVC example:
---
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
name: rbd-pvc
spec:
accessModes:
- ReadWriteOnce
resources:
requests:
storage: 1Gi
storageClassName: rook-ceph-block
Application using PVC example:
---
apiVersion: apps/v1
kind: Deployment
metadata:
name: postgresql
namespace: default
labels:
k8s-app: postgresql
kubernetes.io/cluster-service: "true"
spec:
replicas: 1
selector:
matchLabels:
k8s-app: postgresql
template:
metadata:
labels:
k8s-app: postgresql
kubernetes.io/cluster-service: "true"
spec:
containers:
- name: postgres
image: postgres:10.1
ports:
- containerPort: 5432
env:
- name: POSTGRES_DB
value: dbdb
- name: POSTGRES_USER
value: test
- name: POSTGRES_PASSWORD
value: test
- name: PGDATA
value: /var/lib/postgresql/data/pgdata
volumeMounts:
- mountPath: "/var/lib/postgresql/data"
name: "image-store"
volumes:
- name: image-store
persistentVolumeClaim:
claimName: rbd-pvc
readOnly: false
Choosing Block Storage which allows a single pod to mount storage, be aware that if one node where Your application is hosted will crash, all the pods located on the crashed node will go into terminating state and application will be unavailable since terminating pods blocking access to ReadWriteOnce volume and new pod can't create. You have to manually delete volume attachment or use CephFS instead of RBD.
Related discussion: https://stackoverflow.com/questions/61186199/why-does-kubernetes-not-terminating-pods-after-a-node-crash
Internal k8s automated setup and tests
Step by step procedure for setting environment up and testing it (together with backup/restore) is available in the following repo: https://github.com/mkyc/k8s-rook-ceph
Useful links:
Good starting point:
https://rook.io/docs/rook/v1.4/ceph-quickstart.html
Toolbox for debugging:
https://rook.io/docs/rook/v1.4/ceph-toolbox.html
Filesystem storage:
https://rook.io/docs/rook/v1.4/ceph-filesystem.html
Custom Resource Definitions:
https://rook.io/docs/rook/v1.4/ceph-cluster-crd.html
Add/remove osd nodes: https://access.redhat.com/documentation/en-us/red_hat_ceph_storage/2/html/administration_guide/adding_and_removing_osd_nodes
Useful rook ceph guide: https://www.cloudops.com/2019/05/the-ultimate-rook-and-ceph-survival-guide/
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