SRDF/Metro: Array failure - disable witness?

Hi,

Is this from a customer - just curious on who the information is flowing to.

The Witness Failure Situation has a few scenarios :

1. Total witness failure (which means both sides of SRDF/Metro loose connectivity to witness)  - Then we move automatically into Bias mode.
2. Partial Witness Failure : In this situation one of the VMAX3 is still communicating with witness but the other side might not. We refer to this as Witness Degraded mode. In this situation the side which is still in communication with witness will be surviving site. Lets say S1(VMAX3) has bias and both S1(VMAX3) and S2(VMAX3) were connected to witness "W" . Now if S1 looses its connection to "W" then we are in Witness degraded mode and any failure between S1 and S2 will result in S2 being the winner which will be continuously available without any disruption. 

Hope this addresses your questions.

Thanks

Deepak

Hi there,

I work for a EMC Partner and the questions came up because we do DR planning for one of our customers.

Sorry for saying so, but this was not the question. We are well aware on the failure scenarios mentioned in the Technical Note “EMC VMAX3 SRDF/Metro Overview and Best Practices” and also in the SRDF CLI User Guide.

The things we are not sure about is, what happen if say S1 is down for a longer period of time (eg. DC destroyed during natural disaster and needs to be rebuild from ground up) . S2 and the witness array are up and running. Then the witness or the communication to the witness fails – what will happen on the S2.

Reason for asking: we need to define the steps which need to be performed to make sure the S2 survives a witness failure without disrupting production.

Q1: do we need to move Bias (swap identity) for all volumes to S2 or has this been done by the witness already?

Q2: how can the witness be disabled without disrupting production?

Q3: is there a procedure available how to handle such a case?

Thanks

Tom

Von: deepak_vokaliga

Gesendet: Donnerstag, 3. März 2016 20:05

An: Tom Schmuki

Betreff: Re: - SRDF/Metro: Array failure - disable witness?

ECN

SRDF/Metro: Array failure - disable witness?

reply from deepak_vokaliga in VMAX - View the full discussion

Thanks for the clarification. Lets see if i get it right this time

In the situation you explained -  S1 goes down(looses connections to W), W and S2 are still up and then W also goes for a toss so S2 is the lone survivor.

Q1:        do we need to move Bias (swap identity) for all volumes to S2 or has this been done by the witness already?

> No , user doesn't have to do anything - S2 will continue to be available to the host and there will be no disruption until S2 is intact.

Q2:        how can the witness be disabled without disrupting production?

>  You can take out the witness anytime - VMAX3 will automatically move into Bias mode. In this specific situation there is no impact on S2 if you remove the witness.

Q3:        is there a procedure available how to handle such a case?

> With my ansnwers to Q1 and Q2 - this question should be taken care of.

Some more details : (S1 and W are down)

> if user brings up S1 first and has not corrected the problem with "W" communication on both sides ,then if you re-start SRDF/Metro - we will automatically start in Bias mode.  To start SRDF/Metro in witness protected mode - both sides should be able to communicate to W.

Hope this helps

Thanks

Deepak