Cloud Extensions high availability model

Cloud Extensions provide high availability using an active-passive model. Workloads use a primary Cloud Edge node, but use iSCSI multipath to also connect to the secondary node. If the primary node fails, the workloads failover to use the secondary node. When the primary node recovers, Migrate for Compute Engine fails back.

Impaired Cloud Extensions

A Cloud Extension with an Impaired status functions differently depending on whether only one or both of its Cloud Edge nodes have failed:

  • If only one of the Cloud Edge nodes fails, most operations remain available, but write throughput decreases.
  • If both Cloud Edge nodes fail, VMs relying on that Cloud Extension will no longer have access to storage and will fail. Additionally, the Cloud Extension can no longer run new VMs in the cloud. In this case, you can either:
    1. Force running VMs to move back to their source environment.
    2. If the VM is fully cached, run the Prepare to Detach operation.
    3. Stop VMs.

A Cloud Extension can also be Impaired if the Migrate for Compute Engine Backend or the Migrate for Compute Engine Manager fail. In this case, a limited set of actions are available.

Cloud Extensions can be impaired due to:

  1. Incomplete deployment or initial failed health checks. This is most likely due to improperly configured networks or permissions. After fixing the underlying causes, repair the Cloud Extension. Repairing the cloud extension attempts to recreate the missing components, run relevant health checks, or both. If the repair succeeds, the Cloud Extension is Active.
  2. A VM health issue. Stopping and starting the Cloud Extension might fix this issue after affected VMs restart on a healthy host.

Avoiding data loss

When one Cloud Edge node fails, the Cloud Extension enters a fail-safe mode. To avoid data loss, all data are written to the Cloud Extension's object store on Cloud Storage, which reduces the Cloud Extension performance.

What's next

For more information, see Monitoring Cloud Extensions.