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.
This refresh process is controlled by the RTRACKS (the number of updated tracks) and RFREQUENCY (time) options of the XSET command. You should not refresh the bitmaps too often because it slows SDM performance for large numbers of volumes. When a subsequent XADDPAIR command is issued for a suspended volume, the SDM will read the hardware bitmaps and only copy the changed tracks.