Home > Storage > PowerMax and VMAX > Data Protection > Dell PowerMax and VMAX All Flash: SRDF/Metro Overview and Best Practices > PowerMaxOS/Solutions Enabler 9.0 (and later) witness enhancements
Currently, whenever a witness-protected SRDF/Metro session is activated, which occurs as devices are made RW on the SRDF link, the arrays on the two sides of the session negotiate to determine the witness they will use. Under PowerMaxOS and later, the two sides of a witness-protected SRDF/Metro session also negotiate to determine the side better suited to continue servicing host IOs in the event of a failure. That side will then become the ‘winner’ side, reported as the R1. In general, the added negotiation between the two sides of the session will select the side that currently has a more available DR configuration or that has fewer array-level HW/SW issues as the ‘winner’ side. The behavior is the same for array and virtual witness options.
The witness selection criteria with this release and greater is the following (in priority order):
The first of these criteria that one array has and the other does not stops the selection process. The side with the matched criteria is the preferred winner. The two sides repeat this selection process regularly for each SRDF/Metro session to ensure that the winner remains the one that is most preferable. This means that the winning side may change during a session.
As a result, the traditional bias and set bias options will no longer be supported nor necessary for configurations supported by a PowerMaxOS or later witness. As such, when choosing to use a witness of based on the PowerMaxOS release or later to protect the SRDF/Metro configuration, the user is choosing to let the witness select the winner side in the event of a failure. Set bias operations will therefore only be supported for configurations with an ActiveBias (non-witness) SRDF pair state.
For suspend operations, the set bias option has been renamed to ‘Keep’ to specify which side will retain host accessibility (designated by the R1 SRDF personality). For example, suspending an SRDF/Metro session with set bias now becomes keep:
symrdf -sg <SgName> -sid <SymmID> -rdfg <GrpNum>
[-bypass] [-noprompt] [-i <Interval>] [-c <Count>]
[-v | -noecho] [-force] [-symforce]
. . .
suspend [-keep <R1 | R2 >]
Similar -keep syntax available with –g, –cg, -sg, -file options.