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Dell EMC Metro node 7.0.1 Product Guide

Metro HA with cross-connect failure management

This section describes how metro node Metro HA with cross-connect rides through failures of hosts, storage arrays, clusters, metro node Witness, and the inter-cluster link.

Host failure

If hosts at one site fail, then VMware HA restarts the virtual machines on the surviving hosts. Since surviving hosts are connected to the same datastore, VMware can restart the virtual machines on any of the surviving hosts.

Cluster failure

If a metro node cluster fails:

  • Metro node Witness guides the surviving cluster to continue.
  • VMware re-routes I/O to the surviving cluster.
  • No disruption to I/O.

Storage array failure

If one or more storage arrays at one site fail:

  • All distributed volumes continue I/O to the surviving leg.
  • No disruption to the metro node clusters or the virtual machines.
  • I/O is disrupted only to local virtual volumes on the metro node cluster attached to the failed array.

Metro node Witness failure

If metro node Witness fails or becomes unreachable (link outage):

  • Both metro node clusters call home to report that metro node Witness is not reachable.
  • No disruption to I/O, metro node clusters, or the virtual machines.

Inter-cluster link failure

If the inter-cluster link fails:

  • Metro node Witness guides the preferred cluster to continue.
  • I/O suspends at the non-preferred cluster.
  • VMware re-routes I/O to the continuing cluster.
  • No disruption to I/O.

The following table summarizes how metro node HA with cross-connect manages failures.

Table 1. How metro node Metro HA recovers from failure
Failure description Failure handling
Host failure (Site 1) VMware HA software automatically restarts the affected applications at Site 2.
Metro node cluster failure 
(Site 1) Metro node Witness detects the failure and enables all volumes on the surviving cluster.
Inter-cluster link failure
  • If the cross-connects use different physical links from those used to connect the metro node clusters, applications are unaffected. Every volume continues to be available in one data center or the other.
  • If the cross-connect links use the same physical links as those used to connect the metro node clusters, an application restart is required.
Storage array failure Applications are unaffected. Metro node dynamically redirects I/O to the mirrored copy on the surviving array.
NOTE This example assumes that all distributed volumes are also mirrored on the local cluster. If not, then the application remains available because the data can be fetched or sent from or to the remote cluster. However, each read/write operation now incurs a performance cost.
Failure of 
metro node Witness Both clusters call home. As long as both clusters continue to operate and there is no inter-cluster link partition, applications are unaffected.
CAUTION If either cluster fails or if there is an inter-cluster link partition, the system is at a risk of data unavailability. If the metro node Witness outage is expected to be long, disable the metro node Witness functionality to prevent the possible data unavailability.

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