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How an out-of-order Snapshot copy transfer works

The transfer of a Snapshot copy that does not conform to the usual sequence scheduled by a SnapVault policy is an out-of-order Snapshot copy transfer.

In SnapVault relationships, Snapshot copies are selected and transferred from the primary volume to the secondary volume, according to the configured SnapVault policy. Only Snapshot copies that are newer than the common Snapshot copy between the primary and secondary volume are transferred. However, you can use the snapmirror update command to initiate the transfer of a Snapshot copy that was not originally selected and transferred.

When you initiate an out-of-order transfer, an older Snapshot copy is used to establish the base. To avoid subsequent transfers of Snapshot copies that already exist on the SnapVault secondary volume, the list of Snapshot copies that are selected for transfer in this update cycle are reconciled against the Snapshot copies that are already present on the secondary volume. Snapshot copies that are already present on the secondary volume are discarded from the transfer list.

Example of a new base that is established from an out-of-order Snapshot copy transfer

In this example, the SnapVault policy has a schedule in which only the even-numbered Snapshot copies on the primary volume are transferred to the secondary volume. Before the out-of-order transfer begins, the primary volume contains Snapshot copies 2 through 6; the secondary volume contains only the even-numbered Snapshot copies (noted as "SC" in the figures). Snapshot copy 4 is the common Snapshot copy that is used to establish the base, as shown in the following figure:

After Snapshot copy 3 is transferred to the secondary volume, out of order, it becomes the new common Snapshot copy that is used to establish the base, as shown in the following figure:

Note: Although Snapshot copy 3 is now the base, the exported Snapshot copy is still Snapshot copy 4.

When Snapshot copies are selected for subsequent updates according to the SnapVault policy, the policy selects Snapshot copy 4 and Snapshot copy 6 for transfer to the secondary volume. When the transfer list is reconciled, Snapshot copy 4 is removed from the transfer list because it already exists on the secondary volume. Only Snapshot copy 6 is transferred, which becomes the new common Snapshot copy that is used to establish the base, as shown in the following figure:

Example of SnapVault transfer behavior with an out-of-order Snapshot copy transfer

In this example, Data ONTAP created two SnapVault-labeled Snapshot copies, you made a user-created Snapshot copy, and then more SnapVault-labeled Snapshot copies were created. The order of Snapshot copies made would appear as shown in the following figure:

You perform an out-of-order Snapshot transfer using the user-created Snapshot copy, which establishes that Snapshot copy as the new base Snapshot copy, as shown in the following figure:

When the next SnapVault scheduled transfer occurs, only the SnapVault labeled Snapshot copies made after the user-created Snapshot copy are transferred. This occurs because the Snapshot copies created between the previous base Snapshot copy and the current base Snapshot copy are not transferred.