• Considerations for synchronous replication

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    Considerations for synchronous replication

    Synchronization states

    Replication sessions configured with synchronous replication (RPO set to zero) can have the following synchronization states. Each of these describe the state of the user data on the destination LUN. For an accurate information on the synchronization state of a replication, refer to the synchronization state on the source system:

    • Consistent – The data on the destination system is consistent from the host perspective. The destination LUN is the latest or point in time copy of the source LUN.
    • In Sync – The data on the destination system is consistent from the host perspective. The destination LUN is an exact copy of the source LUN.
    • Syncing – The data on the destination system is getting updated from the source system. The updates represent the pending changes on the source system. These changes are not yet replicated to the destination system because of the following reasons:
      • Replication session is new or performing a failback operation, which requires a full sync.
      • Replication session was previously in a Consistent or In Sync state. A connection issue has created a backlog of changes on the source system. The backlog will be updated to the destination system once the connection issue is resolved.
      • Replication session was paused resulting in a backlog of changes on the source system. The backlog will be updated to the destination system once the replication session is resumed.
    • Out of Sync – The state of the data on the destination LUN cannot be determined by the replication session. The update to the destination system may be incomplete. This can occur when a connection issue disrupts a replication that is in a Syncing state, the replication session is paused on the source system, or the replication session was deleted locally on the destination system.
    • Inconsistent – The state is reported only when replication is failed over. It appears when the synchronization state of the session was not In Sync or Consistent, prior to failover. In this case, it is recommended that you check for the consistency of the destination storage resource.
    Synchronization state transitions

    The following table lists the synchronization state transitions for some of the replication actions:

    Action
    Synchronization state transition
    Initial synchronization
    Unknown > Out of Sync > Syncing > Consistent > In Sync
    Pause
    In Sync > Consistent or Syncing > Out of Sync
    Resume
    Consistent or Out of Sync > Syncing > Consistent > In Sync
    Fail over
    Syncing > Inconsistent or Out of Sync > Inconsistent
    If the state was In Sync or Consistent before a failover occurred, that state is retained.
    Replication connection down
    Syncing > Out of Sync or In Sync > Consistent (if there is I/O)

    Fibre Channel configuration for synchronous replication

    Every replication session is tied to a Storage Processor (SP) via a replication interface. For synchronous replication, a Fibre Channel (FC) connection is used for data communication, and an IP interface used for management commands. Before creating a replication interface or connection, it is recommended that you configure the FC zoning.

    Only one FC port can be configured and used for synchronous replication. Once an FC port is assigned, it will continue to act as the synchronous replication port and can never be changed. To determine the FC port used for synchronous replication, in the CLI console, run the command /remote/sys show -detail. Port information, similar to the following example, will appear in the output:

                        Synchronous FC ports = spb_fc4, spa_fc4
                      

    For more information, see the Unisphere CLI User Guide.

    RecoverPoint and the native Replication feature cannot simultaneously operate on the same volume or Fibre Channel port of the storage system. Even if you do not configure or use synchronous replication, do not configure the synchronous replication port for RecoverPoint. There is no such restriction on the Ethernet ports for iSCSI. You can configure RecoverPoint to use any available Ethernet port.
    Zoning configuration

    Ensure that you complete the following:

    • Configure the port as an F-port to avoid any potential issues, and remove other types of ports from the list of allowed port types.
    • For a synchronous replication connection, connect SPA port of the source system to the SPA port of the destination system. Similarly, SPB port of the source system to the SPB port of the destination system.

      For example,

      • OB_H1450 SPA Port 4 <--> OB_H1443 SPA Port 4
      • OB_H1450 SPB Port 4 <--> OB_H1443 SPB Port 4
    • The SPA and SPB ports on the same side (source or destination) must have the identical port configuration. However, the configuration of ports on the source system can be different from the port configuration on the destination system. For example, SPA Port 4 <--> SPA I/O Module 0 Port 0 and SPB Port 4 <--> SPB I/O Module 0 Port 0 are supported.
    • It is recommended to have a separate FC port for FC host access. But, the synchronous replication FC port can be shared with host access.
    • Although setting the port speeds to Auto works, it is recommended that you set the port speeds for the FC ports on both systems and the FC switch port.
    High availability consideration

    To ensure high availability, consider the following:

    • If the FC connection for an SP is disrupted, set the SP ownership for the source LUN to the other SP. SP ownership on the destination LUN automatically switches to the corresponding SP on the destination system.
    • If an SP on the destination system shuts down or is faulted (not in a normal state), the system will automatically change the SP ownership on the destination LUN. Ensure that you manually set the SP ownership for the source LUN to the other SP.