Data Replicas and System Capacity

You can create online data replicas in different ways. Each method consumes the capacity in the storage array differently.

Oracle FS System Manager (GUI) displays the relationship of the Clone LUNs and associated parent LUNs in a hierarchal relationship called a replica tree. A replica tree can also include clones of clones.

The Oracle FS System ensures that all of the logical volumes that are associated with a particular replica tree reside on the same Controller.

Note: This discussion of replica trees on a Controller does not apply to replicated objects created by the Oracle MaxRep for SAN.

If you change the home Controller of any of these logical volumes, the system changes the home Controller for all of them. The system updates the home Controller on the following objects:

Clone LUNs
Volume copies

Volume copies and logical volumes that are being migrated because of QoS changes continue to reside in the original replica tree until the data operations are complete. After the volume copy or the migration completes, the volume is removed from the original replica tree and becomes the root of a new replica tree.

After you start a volume copy operation or the system starts a data migration operation, if you rehome any object that is in the replica tree, the method for rehoming the Controller differs by replica type. If the system has not yet detached the copy from its source volume, the system rehomes the copy. If, however, the system has already detached the copy, the copy is no longer in the original replica tree; therefore the copy is not rehomed.

The following table summarizes the replica types and capacity usage.

Table 1 Capacity usage by online data replicas
Replica type	Description	Capacity usage
Clone LUN	Creates a readable and writable point-in-time snapshot of a LUN that only stores the delta between the source LUN and the clone. The Clone relies on the source LUN for unmodified LUN data.	Consumes system space allocated for clones. Only changes to the source or clone are stored.
volume copy	Creates a block-level, full-image, read-write copy of a logical volume. QoS attributes for a volume copy can differ from the QoS attributes of the original.	Consumes free space from system capacity that is equal to the current size of the volume.

The online data replicas have the following characteristics:

The replicas require no prior configuration (other than initial QoS allocation).
The system creates the replicas as an explicit one‑time operation.
The system creates the replicas on the same Oracle FS System as the source volume.
The system creates independent replicas from the source volumes. When data changes in the source volume, those changes are not reflected in the replica.

Note: Drive manufacturers often report drive capacities in decimal units (powers of 10). An Oracle FS System reports the capacity of physical storage and the size of logical volumes in binary units (powers of two):

1 MB = 1024² (1,048,576) bytes
1 GB = 1024³ (1,073,741,824) bytes
1 TB = 1024⁴ (1,099,511,627,776) bytes