Dell Unity: How asynchronous replication works and the capacity requirements (User Correctable)

Summary: This article explains the asynchronous replication process. It also explains why the Destination Primary LUN should have double the capacity of the Source Primary LUN during initial synchronization for Block Remote Asynchronous Replication. ...

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Instructions

  1. When an Asynchronous (Async) replication session is created on a storage resource, two internal Unity snapshots are created on the Source and Destination storage resources. The snapshots point to the same blocks as the primary, so they are not consuming too much space.
    Source Primary LUN 38 GB
    Source Snapshot-1*  
    Source Snapshot-2*  
    Destination Primary LUN 38 Gb
    Destination Snapshot-1 0 Gb*
    Destination Snapshot-2 0 Gb*
  2. Data is then replicated using Source Snapshot-1 to the Destination Primary LUN. This is the initial synchronization of the Source and Destination storage resources and is a FULL copy of the data.
  3. Once the initial async synchronization is complete, the Destination Snapshot-1 is refreshed and now represents the point-in-time in which the synchronization started. It is here that the allocated capacity is doubled (temporarily). Since the Primary LUN is a Thick LUN (DLU), there is already 38 Gb allocated. After the source writes the 38 GB (initial sync) to the Destination the changes are tracked by SnapShot-1, and another 38 GB is allocated.
    Source Primary LUN 38 GB
    Source Snapshot-1*  
    Source Snapshot-2*  
    Destination Primary LUN 38 GB
    Destination Snapshot-1 38 GB**
    Destination SnapShot-2 0 GB
  4. Over time, the host application writes new data to the Source storage resource.
  5. During the next update, either manually started or by the Asynchronous Replication Recover Point Objective (RPO), Snapshot-2 on the Source is refreshed to reflect the current point-in-time view. All the changes since the last sync session to the Destination are then copied to the Destination.
  6. After the incremental copy is complete, Destination Snapshot-2 is refreshed to reflect the current information located in the destination. Source Snapshot-2 and Destination Snapshot-2 contain the latest information and are the latest common base image for the replication session.
  7. The difference (amount of data) between Destination SnapShot-1 and/or SnapShot-2 is typically small and reflect the minor changes that occurred since the previous sync. When this occurs, the excess space is evacuated to the pool.

Dell Unity: Replication Technologies (February 2024) 

Affected Products

Dell EMC Unity Family |Dell EMC Unity All Flash, Dell EMC Unity Family, Dell EMC Unity Hybrid

Products

VNX/VNXe, VNXe1 Series, VNXe1000 Series, VNXe1600, VNXe2 Series, VNXe3200
Article Properties
Article Number: 000019570
Article Type: How To
Last Modified: 30 May 2025
Version:  3
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