Synchronization and resynchronization functions are designed to occur at any time. This function has the intent of restoring recovery capability in as short a time as possible.
The synchronization process stops if the residual count builds up on the primary subsystem. It is possible for the total number of residuals for a single storage control session to exceed a specified value. When this occurs, the SDM suspends all synchronization activity for volumes in that SSID until the residual count drops below the threshold. This prevents any significant delays caused by the synchronization process. It also ensures that you do not have to wait for a period of low activity to perform this process. The HaltThreshold parmlib parameter controls this residual count level.
Both planned and unplanned outage support use volume level bitmaps to provide a partial resynchronization capability where only changed tracks are copied. While a volume pair is in duplex status, the bitmaps are continually updated to maintain a record of tracks that have changed data. These bitmaps must be refreshed periodically to minimize the amount of data that must be copied during the resynchronization process.