A “split-brain” happens when two or more nodes in a cluster can not communicate, and yet, believe that they are the only survivor and should keep writing to storage, essentially corrupting the data. SRDF/Metro takes a deterministic approach to not allow a “split-brain” to occur when the two storage systems can not communicate. Two complimentary methods are used to protect against a “split-brain”: SRDF/Metro Bias rules and SRDF/Metro Witness rules.
Under Bias rules, one side of the SRDF/Metro paired devices (where the R1 devices are) has the ‘Bias’, and therefore it is pre-determined to “win”, or resume I/Os, if replication stopped unexpectedly. The other side (where the R2 devices are), is pre-determined to immediately stop servicing I/Os.
SRDF/Metro synchronized device-pairs protected by Bias rules alone show a state of ‘ActiveBias’.
While using Bias rules is an incontestable method for preventing a “split-brain” (only one side can resume I/Os), it is not flexible, as a “disaster” may then occur where the Bias side is, requiring manual intervention to make the non-Biased side available. To protect against this situation, SRDF/Metro uses Witness rules in addition to Bias rules.
Note: Starting with PowerMaxOS 5978 Q3 2019 release, SRDF/Metro regularly takes additional factors into account to determine the Bias side, such as which of the storage systems has an active SRDF/A protection, available storage directors, etc. That means that the Bias may change dynamically during SRDF/Metro replication to reflect these factors.
Under Witness rules, an added component called a Witness serves as a real-time arbitrator when an active/active SRDF group unexpectedly stops replicating. SRDF/Metro, with the help of the Witness, quickly determines, in real-time, which storage system is deemed best to continue servicing I/Os for that SRDF group, based on the situation and failure conditions.
These factors are important for working with Witness:
SRDF/Metro synchronized device-pairs protected by Witness rules show a state of ActiveActive.