It is possible to grow the replica volume, or the original volume, and still use FastResync. According to the DCO volume layout, growing the volume has the following different effects on the map that FastResync uses to track changes to the original volume:
For an instant snap DCO volume, the size of the map is increased and the size of the region that is tracked by each bit in the map stays the same.
For a version 0 DCO volume, the size of the map remains the same and the region size is increased.
In either case, the part of the map that corresponds to the grown area of the volume is marked as "dirty" so that this area is resynchronized. The snapback operation fails if it attempts to create an incomplete snapshot plex. In such cases, you must grow the replica volume, or the original volume, before invoking any of the commands vxsnap reattach, vxsnap restore, or vxassist snapback. Growing the two volumes separately can lead to a snapshot that shares physical disks with another mirror in the volume. To prevent this, grow the volume after the snapback command is complete.