title	description	moduleStatus
The sds-replicated-volume module	The sds-replicated-volume module: General Concepts and Principles.	preview

{{< alert level="warning" >}} The module is only guaranteed to work if the system requirements are met. As for any other configurations, the module may work, but its smooth operation is not guaranteed. {{< /alert >}}

This module manages replicated block storage based on DRBD. Currently, LINSTOR is used as a control-plane/backend (without the possibility of direct user configuration). The module allows you to create a Storage Pool as well as a StorageClass by creating Kubernetes custom resources. To create a Storage Pool, you will need the LVMVolumeGroup configured on the cluster nodes. The LVM configuration is done by the sds-node-configurator module.

Caution! Before enabling the sds-replicated-volume module, you must enable the sds-node-configurator module.

Caution. Data synchronization during volume replication is carried out in synchronous mode only, asynchronous mode is not supported.

After you enable the sds-replicated-volume module in the Deckhouse configuration, you will only have to create storage pools and StorageClasses.

Caution! The user is not allowed to create a StorageClass for the replicated.csi.storage.deckhouse.io CSI driver.

To ensure the proper functioning of the sds-replicated-volume module, follow these steps:

• Configure LVMVolumeGroup. Before creating a StorageClass, create the LVMVolumeGroup resource for the sds-node-configurator module on the cluster nodes.

• Enable the sds-node-configurator module.
  Ensure that the sds-node-configurator module is enabled before enabling the sds-replicated-volume module.

{{< alert level="warning" >}} Direct configuration of the LINSTOR backend by the user is prohibited. {{< /alert >}}

{{< alert level="info" >}} Data synchronization during volume replication occurs only in synchronous mode. Asynchronous mode is not supported. {{< /alert >}}

• Create Storage Pools and Corresponding StorageClasses.

  Users are prohibited from creating StorageClasses for the replicated.csi.storage.deckhouse.io CSI driver.

  After the sds-replicated-volume module is activated in the Deckhouse configuration, the cluster will automatically be set up to work with the LINSTOR backend. You only need to create the storage pools and StorageClasses.

The module supports two operating modes: LVM and LVMThin.
Each mode has its own characteristics, advantages, and limitations. Learn more about the differences in the FAQ.

Quickstart guide

Note that all commands must be run on a machine that has administrator access to the Kubernetes API.

Enabling modules

Enabling the sds-node-configurator module:

1. Create a ModuleConfig resource to enable the module:

   kubectl apply -f - <<EOF
   apiVersion: deckhouse.io/v1alpha1
   kind: ModuleConfig
   metadata:
     name: sds-node-configurator
   spec:
     enabled: true
     version: 1
   EOF

2. Wait for the sds-node-configurator module to reach the Ready state. At this stage, there is no need to check pods in the d8-sds-node-configurator namespace.

   kubectl get module sds-node-configurator -w

Enabling the sds-replicated-volume module:

3. Activate the sds-replicated-volume module. Before enabling, it is recommended to review the available settings.
   The example below launches the module with default settings, which will result in creating service pods for the sds-replicated-volume component on all cluster nodes, installing the DRBD kernel module, and registering the CSI driver:

   kubectl apply -f - <<EOF
   apiVersion: deckhouse.io/v1alpha1
   kind: ModuleConfig
   metadata:
     name: sds-replicated-volume
   spec:
     enabled: true
     version: 1
   EOF

4. Wait for the sds-replicated-volume module to reach the Ready state.

   kubectl get module sds-replicated-volume -w

5. Make sure that all pods in d8-sds-replicated-volume and d8-sds-node-configurator namespaces are Running or Completed and are running on all nodes where DRBD resources are intended to be used:

   kubectl -n d8-sds-replicated-volume get pod -o wide -w
   kubectl -n d8-sds-node-configurator get pod -o wide -w

Configuring storage on nodes

You need to create LVM volume groups on the nodes using LVMVolumeGroup custom resources. As part of this quickstart guide, we will create a regular Thick storage. See usage examples to learn more about custom resources.

To configure the storage:

• List all the BlockDevice resources available in your cluster:

kubectl get bd

NAME                                           NODE       CONSUMABLE   SIZE      PATH
dev-0a29d20f9640f3098934bca7325f3080d9b6ef74   worker-0   true         30Gi      /dev/vdd
dev-457ab28d75c6e9c0dfd50febaac785c838f9bf97   worker-0   false        20Gi      /dev/vde
dev-49ff548dfacba65d951d2886c6ffc25d345bb548   worker-1   true         35Gi      /dev/vde
dev-75d455a9c59858cf2b571d196ffd9883f1349d2e   worker-2   true         35Gi      /dev/vdd
dev-ecf886f85638ee6af563e5f848d2878abae1dcfd   worker-0   true         5Gi       /dev/vdb

• Create an LVMVolumeGroup resource for worker-0:

kubectl apply -f - <<EOF
apiVersion: storage.deckhouse.io/v1alpha1
kind: LVMVolumeGroup
metadata:
  name: "vg-1-on-worker-0" # The name can be any fully qualified resource name in Kubernetes. This LVMVolumeGroup resource name will be used to create ReplicatedStoragePool in the future
spec:
  type: Local
  local:
    nodeName: "worker-0"
  blockDeviceSelector:
    matchExpressions:
      - key: kubernetes.io/metadata.name
        operator: In
        values:
          - dev-0a29d20f9640f3098934bca7325f3080d9b6ef74
          - dev-ecf886f85638ee6af563e5f848d2878abae1dcfd
  actualVGNameOnTheNode: "vg-1" # the name of the LVM VG to be created from the above block devices on the node 
EOF

• Wait for the created LVMVolumeGroup resource to become Ready:

kubectl get lvg vg-1-on-worker-0 -w

• The resource becoming Ready means that an LVM VG named vg-1 made up of the /dev/vdd and /dev/vdb block devices has been created on the worker-0 node.

• Next, create an LVMVolumeGroup resource for worker-1:

kubectl apply -f - <<EOF
apiVersion: storage.deckhouse.io/v1alpha1
kind: LVMVolumeGroup
metadata:
  name: "vg-1-on-worker-1"
spec:
  type: Local
  local:
    nodeName: "worker-1"
  blockDeviceSelector:
    matchExpressions:
      - key: kubernetes.io/metadata.name
        operator: In
        values:
          - dev-49ff548dfacba65d951d2886c6ffc25d345bb548
  actualVGNameOnTheNode: "vg-1"
EOF

• Wait for the created LVMVolumeGroup resource to become Ready:

kubectl get lvg vg-1-on-worker-1 -w

• The resource becoming Ready means that an LVM VG named vg-1 made up of the /dev/vde block device has been created on the worker-1 node.

• Create an LVMVolumeGroup resource for worker-2:

kubectl apply -f - <<EOF
apiVersion: storage.deckhouse.io/v1alpha1
kind: LVMVolumeGroup
metadata:
  name: "vg-1-on-worker-2"
spec:
  type: Local
  local:
    nodeName: "worker-2"
  blockDeviceSelector:
    matchExpressions:
      - key: kubernetes.io/metadata.name
        operator: In
        values:
          - dev-75d455a9c59858cf2b571d196ffd9883f1349d2e
  actualVGNameOnTheNode: "vg-1"
EOF

• Wait for the created LVMVolumeGroup resource to become Ready:

kubectl get lvg vg-1-on-worker-2 -w

• The resource becoming Ready means that an LVM VG named vg-1 made up of the /dev/vdd block device has been created on the worker-2 node.

• Now that we have all the LVM VGs created on the nodes, create a ReplicatedStoragePool out of those VGs:

kubectl apply -f -<<EOF
apiVersion: storage.deckhouse.io/v1alpha1
kind: ReplicatedStoragePool
metadata:
  name: data
spec:
  type: LVM
  lvmVolumeGroups: # Here, specify the names of the LVMVolumeGroup resources you created earlier
    - name: vg-1-on-worker-0
    - name: vg-1-on-worker-1
    - name: vg-1-on-worker-2
EOF

• Wait for the created ReplicatedStoragePool resource to become Completed:

kubectl get rsp data -w

• Confirm that the data Storage Pool has been created on nodes worker-0, worker-1 and worker-2:

alias linstor='kubectl -n d8-sds-replicated-volume exec -ti deploy/linstor-controller -- linstor'
linstor sp l

╭─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╮
┊ StoragePool          ┊ Node     ┊ Driver   ┊ PoolName ┊ FreeCapacity ┊ TotalCapacity ┊ CanSnapshots ┊ State ┊ SharedName                    ┊
╞═════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════╡
┊ DfltDisklessStorPool ┊ worker-0 ┊ DISKLESS ┊          ┊              ┊               ┊ False        ┊ Ok    ┊ worker-0;DfltDisklessStorPool ┊
┊ DfltDisklessStorPool ┊ worker-1 ┊ DISKLESS ┊          ┊              ┊               ┊ False        ┊ Ok    ┊ worker-1;DfltDisklessStorPool ┊
┊ DfltDisklessStorPool ┊ worker-2 ┊ DISKLESS ┊          ┊              ┊               ┊ False        ┊ Ok    ┊ worker-2;DfltDisklessStorPool ┊
┊ data                 ┊ worker-0 ┊ LVM      ┊ vg-1     ┊    35.00 GiB ┊     35.00 GiB ┊ False        ┊ Ok    ┊ worker-0;data                 ┊
┊ data                 ┊ worker-1 ┊ LVM      ┊ vg-1     ┊    35.00 GiB ┊     35.00 GiB ┊ False        ┊ Ok    ┊ worker-1;data                 ┊
┊ data                 ┊ worker-2 ┊ LVM      ┊ vg-1     ┊    35.00 GiB ┊     35.00 GiB ┊ False        ┊ Ok    ┊ worker-2;data                 ┊
╰─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╯

• Create a ReplicatedStorageClass resource for a zone-free cluster (see use cases for details on how zonal ReplicatedStorageClasses work):

kubectl apply -f -<<EOF
apiVersion: storage.deckhouse.io/v1alpha1
kind: ReplicatedStorageClass
metadata:
  name: replicated-storage-class
spec:
  storagePool: data # Here, specify the name of the ReplicatedStoragePool you created earlier
  reclaimPolicy: Delete
  topology: Ignored # - note that setting "ignored" means there should be no zones (nodes labeled topology.kubernetes.io/zone) in the cluster
EOF

• Wait for the created ReplicatedStorageClass resource to become Created:

kubectl get rsc replicated-storage-class -w

• Confirm that the corresponding StorageClass has been created:

kubectl get sc replicated-storage-class

• If StorageClass with the name replicated-storage-class is shown, then the configuration of the sds-replicated-volume module is complete. Now users can create PVs by specifying StorageClass with the name replicated-storage-class. Given the above settings, a volume will be created with 3 replicas on different nodes.

System requirements and recommendations

Requirements

{{< alert level="info" >}} Applicable to both single-zone clusters and clusters using multiple availability zones. {{< /alert >}}

• Use stock kernels provided with supported distributions.
• A network infrastructure with a bandwidth of 10 Gbps or higher is required for network connectivity.
• To achieve maximum performance, the network latency between nodes should be between 0.5–1 ms.
• Do not use another SDS (Software Defined Storage) to provide disks for SDS Deckhouse.

Recommendations

• Avoid using RAID. The reasons are detailed in the FAQ.
• Use local physical disks. The reasons are detailed in the FAQ.
• In order for cluster to be operational, but with performance degradation, network latency should not be higher than 20ms between nodes