Incremental Resync for Metro/Global Mirror is used in a Metro/Global Mirror Peer-to-Peer Remote Copy configuration in order to maintain a backup site if one of the three sites is lost. The purpose of the Incremental Resync function is to avoid always having to do a full volume resynchronization between the local and remote sites if an outage occurs at your intermediate site. When the intermediate site is lost, the local and remote sites can be connected, copying a subset of the data on the volumes to maintain a backup site.
The microcode records the bytes in flight at the local site. If the intermediate site is lost, then the local site volume may be established as the primary to the volume at the remote site.
Incremental Resync uses a change recording mechanism to track in flight changes. Because there can only be one function that uses change recording active on a device at a time, incremental resync prevents change recording for other purposes, such as incremental FlashCopy®. If an attempt is made to start Incremental Resync when another function using change recording is active, that start request fails. If an attempt is made to do an incremental FlashCopy from a PPRC primary that has Incremental Resync active, that FlashCopy request fails.
Like XRC, two change recording bitmaps are maintained. When data in flight is secured at the remote sight, the system updates the bitmaps. The local volume updates the change recording bitmaps checking the status of consistency group formation.