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## IP Mirroring This section describes how data is replicated from the master node to the vice-master node, and how these nodes are resynchronized after failover or switchover. ### Data Partitions and Scoreboard Bitmaps When data is written to a replicated partition on the master node disk, the corresponding scoreboard bitmap is updated. The scoreboard bitmap maps one bit to a block of data on a replicated partition. When a block of data is changed, the corresponding bit in the scoreboard bitmap is set to 1. When the data has been replicated to the vice-master node, the corresponding bit in the scoreboard bitmap is set to zero. The scoreboard bitmap can reside on a partition on the master node disk or in memory. There are advantages and disadvantages to storing the scoreboard bitmap on the master node disk or in memory: Storing the scoreboard bitmap in memory means better performance because writing to memory is quicker than writing to disk. Scoreboard bitmaps in memory are copied to disk when the master node is shut down gracefully. Each replicated partition on a disk must have a corresponding partition for a scoreboard bitmap, even if the scoreboard bitmap is stored in memory. Storing the scoreboard bitmap in memory is a problem if the master node and vice-master node fail simultaneously. In this case, the scoreboard bitmap is lost and a full resynchronization is required when the nodes are rebooted. Storing the scoreboard bitmap on a disk partition is slower during normal operation because writing to disk is slower than writing to memory. However, if the master node and vice-master node fail simultaneously, the scoreboard bitmap can be used to resynchronize the nodes, without the need for a full resynchronization. For information about how to configure the scoreboard bitmap in memory or on disk, see "Changing the Location of the Scoreboard Bitmap" in the Netra High Availability Suite Foundation Services 2.1 6/03 Cluster Administration Guide. ### Replication During Normal Operation Replication is the act of copying data from the master node to the vice-master node. Through replication, the vice-master node has an up-to-date copy of the data on the master node. Replication enables the vice-master node to take over the master role at any time, transparently. After replication, the master node disk and vice-master node disk are synchronized, that is, the mirrored partitions contain exactly the same data. Replication occurs at the following times: When the master node and vice-master node are running, and data on the master node disk is changed After startup, to replicate the software installed on the replicated partitions, such as the diskless Solaris image After a failover or switchover The following figure illustrates a diskless node writing data to the master node, and that data being replicated to the vice-master node. Figure 7-2 Data Replication ### Replication During Failover and Switchover During failover or switchover, the master node goes out of service for a time before being re-established as the vice-master node. During this time, changes that are made to the new master node disk cannot be replicated to the vice-master node. Consequently, the cluster becomes unsynchronized. While the vice-master node is out of service, data continues to be updated on the master node disk, and the modified data blocks are identified in the scoreboard bitmap. Figure 7-3 illustrates Reliable NFS during failover or switchover. Figure 7-3 Reliable NFS During Failover or Switchover When the vice-master node is re-established, replication resumes. Any data written to the master node is replicated to the vice-master node. In addition, the scoreboard bitmap is examined to determine which data blocks have been changed while the vice-master node was out of service. Any changed data blocks are also replicated to the vice-master node. In this way, the cluster becomes synchronized again. The following figure illustrates the restoration of the synchronized state. Figure 7-4 Restoration of the Synchronized State While a cluster is unsynchronized, the data on the master node disk is not fully backed up. Do not schedule major tasks when a cluster is unsynchronized. You can verify whether a cluster is synchronized, as described in "To Verify That the Master Node and Vice-Master Node Are Synchronized" in the Netra High Availability Suite Foundation Services 2.1 6/03 Cluster Administration Guide. You can collect replication statistics by using the Node Management Agent as described in the Netra High Availability Suite Foundation Services 2.1 6/03 NMA Programming Guide. ## Master Node IP Address Failover For a failover to be transparent to a diskless node or dataless node, the following must be true: The diskless node or dataless node must be mounted onto the floating address of the master node. The floating address must always be configured and active on the node holding the master role, even after failover or switchover. For further information about the floating address of the master node, see Floating Address Triplet.
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