Multi-Target Incremental Resynchronization (MTIR)

To implement a Multi-Target PPRC environment, it is important to understand how the environment is configured.

It is assumed that you have an understanding of Copy Services terminology and functions. For example, Peer-to-Peer Remote Copy or PPRC is a general term that is used to refer to Metro Mirror and Global Copy.

Figure 1 presents a high-level overview of a configuration featuring volumes h1, h2, and h3 on systems at both local and remote sites, connected by PPRC paths. It's not mandatory to establish paths between h2 and h3 before setting up the environment. However, it is recommended. The later-provided scenarios assume that these paths have been created in both directions prior to the creation of the pairs..
Figure 1. Multi-Target PPRC configuration
This figure represents a Multi-Target PPRC configuration

By convention, the scenarios begin at the location (indicated by the arrow CP, representing the control point) where host commands are issued or where production I/O is directed. It is essential to maintain a stable control point throughout the process, ensuring that the control point is at the same location as the production systems or where they are recovered. Each site, such as h1, h2, and so on, may represent multiple volumes within the same storage system.

The state of the relationship for a volume is indicated by two characters: the first character signifies whether the volume is a primary (P) or secondary (S), and the second character indicates whether it is duplex (d), suspended (s), or pending (p). Two secondary volumes that share the same primary volume are known as partner volumes. For example, volumes h2 and h3 are partner volumes because they share the primary volume h1. Each primary volume is equipped with a recording mechanism to track out-of-sync changes for each relationship, enabling faster subsequent resynchronization for local site volumes. The secondary volume associated with a recording mechanism is indicated in parentheses.

When a Multi-Target PPRC configuration is established, the storage system internally creates suspended PPRC relationships between the two partner volumes. These are known as Multi-Target Incremental Resynchronization pairs. The PPRC relationship monitors out-of-sync data between the partner volumes, facilitating incremental resynchronization later. Incremental resync is crucial to minimize the data volume required for copying and the time needed to restore a mirroring environment.