SnapMirror replicas are initiated upon normal Snapshot copy creation or when using special rolling Snapshot copies.
Each time a Snapshot copy of a LUN is created—manually or because of a Snapshot copy schedule—SnapDrive determines whether the LUN from which a Snapshot copy was made resides on a SnapMirror source volume. If so, then after the Snapshot copy has been made, SnapDrive may send a SnapMirror update request to all the destination volumes associated with the source volume for that LUN.
When you initiate a Snapshot copy of a LUN on a SnapMirror source through SnapDrive, a window with a check box labeled “Initiate SnapMirror Update” is displayed. The check box is selected by default.
You can also create a special type of Snapshot copy called a “rolling” Snapshot copy, using the Update SnapMirror operation in SnapDrive. These Snapshot copies are used exclusively to facilitate frequent SnapMirror volume replication. Like regular Snapshot copies, rolling Snapshot copies are replicated to the SnapMirror destination volume as soon as they are created.
SnapDrive creates a new rolling Snapshot copy every time you initiate a mirror update operation (using the Update Mirror option in the Action menu or the sdcli snap update_mirror command) for a specific LUN drive residing on a SnapMirror source volume.
To guarantee that at least one rolling Snapshot copy for each LUN is always available on the destination volume, SnapDrive maintains a maximum of two rolling Snapshot copies on the source volume.