Rsync-based replication

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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, Scribe 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 Scribe Rsync data mover then connects to the destination using ssh (exposed via a Service or load balancer) and sends any updates. At the conclusion of the transfer, the destination (optionally) creates a VolumeSnapshot to preserve the updated data.

Scribe 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; a minimal example is shown below:

---
apiVersion: scribe/v1alpha1
kind: ReplicationDestination
metadata:
  name: myDest
  namespace: myns
spec:
  rsync:
    copyMethod: Snapshot
    capacity: 10Gi
    accessModes: ["ReadWriteOnce"]

In the above example, a 10 GiB RWO volume will be provisioned using the default StorageClass 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 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 .status.latestImage.

Destination status

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

---
apiVersion: scribe/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: scribe-dest-test-20210114194305
  rsync:
    address: 10.99.236.225
    sshKeys: scribe-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.

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 None, this will be the PVC that is used as the destination by Scribe.

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

capacity

When Scribe 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:

  • None - 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 Scribe automatically provision the destination volume (using capacity, accessModes, etc.), the name of a pre-existing PVC may be specified here.

storageClassName

When Scribe 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.

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

Scribe 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

A minimal source configuration is shown here:

---
apiVersion: scribe.backube/v1alpha1
kind: ReplicationSource
metadata:
  name: mySource
  namespace: source
spec:
  sourcePVC: mysql-pv-claim
  trigger:
    schedule: "*/5 * * * *"
  rsync:
    sshKeys: scribe-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.

Source status

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

---
apiVersion: scribe.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.

  • None - Do no create a PiT copy. The Scribe 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.