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With Dell UnityOS version 4.2, Asynchronous Replication supports the replication of Read-Only file and block snapshots locally or to a remote site along with the storage resource’s data. Both scheduled snapshots and user created snapshots can be replicated. Snapshot Replication can be enabled on all resources that support Asynchronous Replication, including LUNs, thin clones, consistency groups, file systems, and VMware NFS and VMFS datastores. Snapshot Replication for scheduled snapshots can be enabled or disabled at any time in the lifetime of the replication session. The scheduled and user snapshots can be replicated with a remote retention policy that is longer than that of the source resource. For example, the retention policy for the source snapshots might be set for 2 hours while the retention policy for the replicated snapshot might be set for 1 week.
Dell UnityOS version 4.2 introduced the Replicated attribute for all snapshots. This attribute states the replication state for the snapshot. A snapshot can be in one of the following states:
In Dell UnityOS version 4.4 and later, the Replicated attribute was replaced by two separate attributes to account for the introduction of snapshot replication for file synchronous replication resources. In UnityOS 4.4 and later, these two attributes for snapshots are called “Async Replicated” and “Sync Replicated”. The states referenced above are not affected by the introduction of the two new attributes.
Replicating a manually created snapshot from a system running Dell UnityOS version 4.2 through 4.4 to a destination system running Dell UnityOS version 5.x is not supported. To support this, the source system must be upgraded to Dell UnityOS version 4.5 or later. Upgrading to Dell UnityOS version 4.5 or later is recommended, but not required, if you want to leverage replication without snapshot replication.
For more details in regards operation that can be completed on snapshots, please review the white paper titled Dell Unity: Snapshots and Thin Clones on Dell Technologies Info Hub.
To support snapshot replication, both the source and destination systems must be running Dell UnityOS version 4.2 or later. Snapshots that existed prior to the creation of the replication session can be selected for replication during replication session creation. Snapshots that are older than the last sync (RPO) time can be manually selected for replication and included in the next RPO sync. The Unisphere Management section provides more information regarding the configuration and management of snapshot replication.
Also, with Dell UnityOS 4.2 and Cloud Tiering Appliance (CTA) version 12, the block snapshots can be archived to and restored from the cloud. For more information about Cloud Tiering Appliance (CTA) and how it works with Dell Unity, please review the white paper titled Dell Unity: Cloud Tiering Appliance (CTA) on Dell Technologies Info Hub.
In Dell UnityOS version 5.1 and later, snapshots created on a resource contain a new internal system ID for tracking purposes. This ID is not visible to a user. When a snapshot is replicated, the ID will also be replicated with the snapshot to all destinations, including across all replication sessions within an advanced File remote replication topology. To support this feature both the source and destination systems must be running UnityOS version 5.1 or later. For local replication, the ID is supported if the system is running UnityOS version 5.1 or later.
In UnityOS version 5.1 and later, replicated snapshots can be used to avoid full synchronization of data in certain situations. The snapshot ID allows the system to easily determine if a snapshot on the source and destination are exactly the same. This feature is explained in more detail in section 0 Common Base Snapshots.