Rsync-based replication

Rsync-based replication supports 1:1 asynchronous replication of volumes for use cases such as:

Disaster recovery
Mirroring to a test environment
Sending data to a remote site for processing

With this method, VolSync synchronizes data from a ReplicationSource to a ReplicationDestination using Rsync across an ssh connection. By using Rsync, the amount of data transferred during each synchronization is kept to a minimum, and the ssh connection ensures that the data transfer is both authenticated and secure.

The Rsync method is typically configured to use a “push” model for the data replication. A schedule or other trigger is used on the source side of the relationship to trigger each replication iteration.

During each iteration, (optionally) a point-in-time (PiT) copy of the source volume is created and used as the source data. The VolSync Rsync data mover then connects to the destination using ssh (exposed via a Service) and sends any updates. At the conclusion of the transfer, the destination (optionally) creates a VolumeSnapshot to preserve the updated data.

VolSync is configured via two CustomResources (CRs), one on the source side and one on the destination side of the replication relationship.

Destination configuration 

Start by configuring the destination; an example is shown below:

---
apiVersion: volsync.backube/v1alpha1
kind: ReplicationDestination
metadata:
  name: myDest
  namespace: myns
spec:
  rsync:
    copyMethod: Snapshot
    capacity: 10Gi
    accessModes: ["ReadWriteOnce"]
    storageClassName: my-sc
    volumeSnapshotClassName: my-vsc

In the above example, a 10 GiB RWO volume will be provisioned using the StorageClass my-sc to serve as the destination for replicated data. This volume is used by the rsync data mover to receive the incoming data transfers.

Since the copyMethod specified above is Snapshot, a VolumeSnapshot will be created, using the VolumeSnapshotClass named my-vsc, at the end of each synchronization interval. It is this snapshot that would be used to gain access to the replicated data. The name of the current VolumeSnapshot holding the latest synced data will be placed in the ReplicationDestination’s .status.latestImage.

Destination status 

VolSync provides status information on the state of the replication via the .status field in the ReplicationDestination object:

---
apiVersion: volsync.backube/v1alpha1
kind: ReplicationDestination
metadata:
  name: myDest
  namespace: myns
spec:
  rsync:
  # ... omitted ...
status:
  conditions:
    - lastTransitionTime: "2021-01-14T19:43:07Z"
      message: Reconcile complete
      reason: ReconcileComplete
      status: "True"
      type: Reconciled
  lastSyncDuration: 31.333710313s
  lastSyncTime: "2021-01-14T19:43:07Z"
  latestImage:
    apiGroup: snapshot.storage.k8s.io
    kind: VolumeSnapshot
    name: volsync-dest-test-20210114194305
  rsync:
    address: 10.99.236.225
    sshKeys: volsync-rsync-dest-src-test

In the above example,

No errors were detected (the Reconciled condition is True)
The destination ssh server is available at the IP specified in .status.rsync.address. This should be used when configuring the corresponding ReplicationSource.
The ssh keys for the source to use are available in the Secret .status.rsync.sshKeys. This Secret will need to be copied to the source so that it can authenticate.

After at least one synchronization has taken place, the following will also be available:

lastSyncTime contains the time of the last successful data synchronization.
latestImage references the object with the most recent copy of the data. If the copyMethod is Snapshot, this will be a VolumeSnapshot object. If the copyMethod is Direct, this will be the PVC that is used as the destination by VolSync.

Additional destination options 

There are a number of more advanced configuration parameters that are supported for configuring the destination. All of the following options would be placed within the .spec.rsync portion of the ReplicationDestination CustomResource.

accessModes

When VolSync creates the destination volume, this specifies the accessModes for the PVC. The value should be ReadWriteOnce or ReadWriteMany.

capacity

When VolSync creates the destination volume, this value is used to determine its size. This need not match the size of the source volume, but it must be large enough to hold the incoming data.

copyMethod

This specifies how the data should be preserved at the end of each synchronization iteration. Valid values are:

Direct - Do not create a point-in-time copy of the data.
Snapshot - Create a VolumeSnapshot at the end of each iteration

destinationPVC

Instead of having VolSync automatically provision the destination volume (using capacity, accessModes, etc.), the name of a pre-existing PVC may be specified here.

storageClassName

When VolSync creates the destination volume, this specifies the name of the StorageClass to use. If omitted, the system default StorageClass will be used.

volumeSnapshotClassName

When using a copyMethod of Snapshot, this value specifies the name of the VolumeSnapshotClass to use when creating a snapshot. If omitted, the system default VolumeSnapshotClass will be used.

sshKeys: This is the name of a Secret that contains the ssh keys for authenticating the connection with the source. If not provided, the destination keys will be automatically generated and corresponding source keys will be placed in a new Secret. The name of that new Secret will be placed in .status.rsync.sshKeys.
serviceType: VolSync creates a Service to allow the source to connect to the destination. This field determines the type of that Service. Allowed values are ClusterIP or LoadBalancer. The default is ClusterIP.
port: This determines the TCP port number that is used to connect via ssh. The default is 22.

Source configuration 

An example source configuration is shown here:

---
apiVersion: volsync.backube/v1alpha1
kind: ReplicationSource
metadata:
  name: mySource
  namespace: source
spec:
  sourcePVC: mysql-pv-claim
  trigger:
    schedule: "*/5 * * * *"
  rsync:
    sshKeys: volsync-rsync-dest-src-database-destination
    address: my.host.com
    copyMethod: Clone

In the above example, the PVC named mysql-pv-claim will be replicated every 5 minutes using the Rsync replication method. At the start of each iteration, a clone of the source PVC will be created to generate a point-in-time copy for the iteration. The source will then use the ssh keys in the named Secret (.spec.rsync.sshKeys) to authenticate to the destination. The connection will be made to the address specified in .spec.rsync.address.

The synchronization schedule, .spec.trigger.schedule, is defined by a cronspec, making the schedule very flexible. Both intervals (shown above) as well as specific times and/or days can be specified.

When configuring the source, the user must manually create the Secret referenced in .spec.rsync.sshKeys by copying the contents from the Secret generated previously on the destination (and made available in the destination’s .status.rsync.sshKeys).

Additionally, this ReplicationSource specifies a copyMethod of Clone which will directly generate a point-in-time copy of the source volume. However, not all CSI drivers support volume cloning (most notably the ebs-csi driver). In such cases, the copyMethod: Snapshot can be used to indirectly create a copy of the volume by first taking a snapshot, then restoring it. In this case, the user should also provide the volumeSnapshotClassName: <vsc-name> option to indicate which VolumeSnapshotClass VolSync should use when creating the temporary snapshot.

Source status 

The state of the replication from the source’s point of view is available in the .status field of the ReplicationSource:

---
apiVersion: volsync.backube/v1alpha1
kind: ReplicationSource
metadata:
  name: mySource
  namespace: source
spec:
  sourcePVC: mysql-pv-claim
  trigger:
    schedule: "*/5 * * * *"
  rsync:
    # ... omitted ...
status:
  conditions:
    - lastTransitionTime: "2021-01-14T19:42:38Z"
      message: Reconcile complete
      reason: ReconcileComplete
      status: "True"
      type: Reconciled
  lastSyncDuration: 7.774288635s
  lastSyncTime: "2021-01-14T20:10:07Z"
  nextSyncTime: "2021-01-14T20:15:00Z"
  rsync: {}

In the above example,

No errors were detected (the Reconciled condition is True).
The last synchronization was completed at .status.lastSyncTime and took .status.lastSyncDuration seconds.
The next scheduled synchronization is at .status.nextSyncTime.

Note

The length of time required to synchronize the data is determined by the rate of change for data in the volume and the bandwidth between the source and destination. In order to avoid missed intervals, ensure there is sufficient bandwidth between the source and destination such that lastSyncTime remains safely below the synchronization interval (.spec.trigger.schedule).

Additional source options 

There are a number of more advanced configuration parameters that are supported for configuring the source. All of the following options would be placed within the .spec.rsync portion of the ReplicationSource CustomResource.

accessModes

When using a copyMethod of Clone or Snapshot, this field allows overriding the access modes for the point-in-time (PiT) volume. The default is to use the access modes from the source PVC.

capacity

When using a copyMethod of Clone or Snapshot, this allows overriding the capacity of the PiT volume. The default is to use the capacity of the source volume.

copyMethod

This specifies the method used to create a PiT copy of the source volume. Valid values are:

Clone - Create a new volume by cloning the source PVC (i.e., use the source PVC as the volumeSource for the new volume.
Direct - Do no create a PiT copy. The VolSync data mover will directly use the source PVC.
Snapshot - Create a VolumeSnapshot of the source PVC, then use that snapshot to create the new volume. This option should be used for CSI drivers that support snapshots but not cloning.

storageClassName

This specifies the name of the StorageClass to use when creating the PiT volume. The default is to use the same StorageClass as the source volume.

volumeSnapshotClassName

When using a copyMethod of Snapshot, this specifies the name of the VolumeSnapshotClass to use. If not specified, the cluster default will be used.

address: This specifies the address of the replication destination’s ssh server. It can be taken directly from the ReplicationDestination’s .status.rsync.address field.
sshKeys: This is the name of a Secret that contains the ssh keys for authenticating the connection with the destination. If not provided, the source keys will be automatically generated and corresponding destination keys will be placed in a new Secret. The name of that new Secret will be placed in .status.rsync.sshKeys.
path: This determines the path within the destination volume where the data should be written. In order to create a replica of the source volume, this should be left as the default of /.
port: This determines the TCP port number that is used to connect via ssh. The default is 22.
sshUser: This is the username to use when connecting to the destination. The default value is “root”.

For a concrete example, see the database synchronization example.

Rsync-specific considerations 

This section explains some additional considerations when setting up rsync-based replication.

Copying the SSH key secret 

When setting up the replication, it is necessary for the ReplicationSource to have a copy of the SSH keys so that it can connect to the network endpoint created by the ReplicationDestination. While these keys can be generated manually, the recommended method is to allow VolSync to generate the keys when setting up the ReplicationDestination. The resulting Secret should then be copied to the source cluster.

Below is an example of a ReplicationDestination object. The VolSync operator has generated the SSH keys that should be used in the source, and it has provided the name of the Secret containing them in the .status.rsync.sshKeys field:

ReplicationDestination with SSH key Secret highlighted

apiVersion: volsync.backube/v1alpha1
kind: ReplicationDestination
metadata:
  creationTimestamp: "2022-02-17T13:56:16Z"
  generation: 1
  name: database-destination
  namespace: dest
  resourceVersion: "2307"
  uid: 71f0512b-8a6b-438c-9b9a-0dd2c0f4e7b8
spec:
  rsync:
    accessModes:
    - ReadWriteOnce
    capacity: 2Gi
    copyMethod: Snapshot
    serviceType: ClusterIP
status:
  conditions:
  - lastTransitionTime: "2022-02-17T13:56:30Z"
    message: Reconcile complete
    reason: ReconcileComplete
    status: "True"
    type: Reconciled
  lastSyncStartTime: "2022-02-17T13:56:16Z"
  rsync:
    address: 10.96.150.107
    sshKeys: volsync-rsync-dst-src-database-destination

This Secret exists in the same Namespace as the associated Replicationdestination. It has the following contents:

Secret as created by VolSync

apiVersion: v1
data:
  destination.pub: c3NoL...
  source: LS0tL...
  source.pub: c3NoLX...
kind: Secret
metadata:
  creationTimestamp: "2022-02-17T13:56:30Z"
  name: volsync-rsync-dst-src-database-destination
  namespace: dest
  ownerReferences:
  - apiVersion: volsync.backube/v1alpha1
    blockOwnerDeletion: true
    controller: true
    kind: ReplicationDestination
    name: database-destination
    uid: 71f0512b-8a6b-438c-9b9a-0dd2c0f4e7b8
  resourceVersion: "2296"
  uid: 61ab5402-318f-46df-b36f-cd209f3d1455
type: Opaque

The above Secret contains 3 fields: the source’s public, the source’s private, and the destination’s public keys.

This Secret must be copied to the source cluster, into the same Namespace where the source PVC and ReplicationSource will reside. That can be accomplished as follows:

$ kubectl -n dest get secret volsync-rsync-dst-src-database-destination -oyaml > secret.yaml

Once saved to the local file, prepare it for the new cluster/namespace by removing the following fields from the metadata area:

creationTimestamp
namespace
ownerReferences
resourceVersion
uid

After removing the above fields, the Secret is as follows:

Prepared secret.yaml

apiVersion: v1
data:
  destination.pub: c3NoL...
  source: LS0tL...
  source.pub: c3NoLX...
kind: Secret
metadata:
  name: volsync-rsync-dst-src-database-destination
type: Opaque

Assuming the source objects will be in Namespace source, this Secret can be added to the source cluster via:

$ kubectl -n source create -f secret.yaml
secret/volsync-rsync-dst-src-database-destination created

This Secret should then be referenced when creating the corresponding ReplicationSource. For example:

ReplicationSource showing reference to SSH key Secret

apiVersion: volsync.backube/v1alpha1
kind: ReplicationSource
metadata:
  name: database-source
  namespace: source
spec:
  sourcePVC: mysql-pv-claim
  trigger:
    schedule: "*/10 * * * *"
  rsync:
    sshKeys: volsync-rsync-dest-src-database-destination
    address: my.host.com
    copyMethod: Clone

Choosing between Service types (ClusterIP vs LoadBalancer)

When using Rsync-based replication, the ReplicationSource needs to be able to make a network connection to the ReplicationDestination. This requires network connectivity from the source to the destination cluster.

When a ReplicationDestination object is created, VolSync creates a corresponding Service object to serve as the network endpoint. The type of Service (LoadBalancer or ClusterIP) should be specified in the ReplicationDestination’s .spec.rsync.serviceType field.

ReplicationDestination with service type highlighted

apiVersion: volsync.backube/v1alpha1
kind: ReplicationDestination
metadata:
  name: database-destination
  namespace: dest
spec:
  rsync:
    accessModes:
    - ReadWriteOnce
    capacity: 2Gi
    copyMethod: Snapshot
    serviceType: ClusterIP

The clusters’ networking configuration between the two clusters affects the proper choice of Service type.

If ClusterIP is specified, the Service will receive an IP address allocated from the “cluster network” address pool. By default, this collection of addresses are not accessible from outside the cluster, making it a poor choice for cross-cluster replication. However, various networking addons such as Submariner bridge the cluster networks, making this a good option.

If LoadBalancer is specified, an externally accessible IP address will be allocated. This requires cluster support for load balancers such as those provided by the various cloud providers or MetalLB in the case of physical clusters. While this is the easiest method for allocating an accessible address in cloud environments, load balancers tend to incur additional costs and be limited in number.

To summarize the above trade-offs, when running on one of the public clouds, using a LoadBalancer is a quick way to get started and will work for replicating small numbers of volumes. If replicating a large number of volumes, an overlay network solution such as Submariner in combination with ClusterIP addresses will likely be more scalable.