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The witness adds resiliency to Metro Volume by handling an expanded scope of failure scenarios while mitigating risk of a split-brain situation. Some of the additional outcomes available with the witness are not possible solely with the polarization mechanism. The table below explores infrastructure failure scenarios for preferred and non-preferred and the expected outcomes when the witness is deployed. These resources should be used in the planning process when deploying Metro Volume and determining the location of the preferred and non-preferred roles as well as the workloads residing on them.
Note: If there are multiple infrastructure failures, the expected outcomes are dependent on the failures happening concurrently. If there is an ordered delay between failures, the actual outcome may not match the expected outcome.
Figure | Preferred | Non-preferred | Link: Replication | Link: Preferred to witness | Link: Non-preferred to witness | Witness | Expected Outcome |
OK | OK | OK | OK | OK | OK | Volume is online on both arrays | |
OK | OK | OK | X | OK | OK | ||
OK | OK | OK | OK | X | OK | ||
OK | OK | OK | X | X | OK | ||
OK | OK | OK | OK | OK | X | ||
X | OK | OK | OK | OK | OK | Volume is online on the Non-preferred * | |
| X | OK | OK | X | OK | OK | |
X | OK | OK | OK | X | OK | Volume is offline on both arrays | |
X | OK | OK | OK | OK | X | ||
OK | X | OK | OK | OK | OK | Volume is online on the Preferred | |
OK | X | OK | X | OK | OK | Volume is offline on both arrays | |
OK | X | OK | OK | X | OK | Volume is online on the Preferred | |
OK | X | OK | OK | OK | X | Volume is offline on both arrays | |
OK | OK | X | OK | OK | OK | Volume is online on the Preferred | |
OK | OK | X | X | OK | OK | Two scenarios depending on the order of the failures ** | |
OK | OK | X | OK | X | OK | Volume is online on the Preferred | |
OK | OK | X | X | X | OK | Volume is offline on both arrays | |
OK | OK | X | OK | OK | X |
* Improvement in Metro Volume availability due to witness
** Two scenarios depending on the order of the failures
Scenario #1 Failure Order:
Scenario #2 Failure Order:
Note: Host connectivity status is not considered in any failure scenario response.
The multi-failure situations as shown in Table 9 expect the individual failures at almost the same time. In failure situations where both Preferred and Non-Preferred are not able to access the witness over some time, the metro sessions revert to Preferred (bias) mode and the expected outcome mirrors a configuration without a witness involved. The illustration below shows situations where the Preferred will remain online after a series of failure events.