Administrator Guide

For asynchronous replication, you can enable the following options:
• Replicate Active Snapshot: Attempts to keep the Active Snapshots (current, unfrozen volume data) of the source and destination
volumes synchronized, which could require more bandwidth. Data that is written to the source volume is queued for delivery to the
destination volume. If the local Storage Center or site fails before the write is delivered, it is possible that writes will not be delivered to
the destination volume. When this feature is disabled, snapshots are copied to the destination after they are frozen.
• Deduplication: Reduces the amount of data required to transfer snapshots to the destination Storage Center by copying only the
changed portions of the snapshot history. This is accomplished by comparing the changed data in the snapshot being replicated with
the previous data block by block, and transmitting only blocks that differ. While deduplication can be resource-intensive, it is useful
when replicating volumes over lower bandwidth WAN links.
Synchronous Replication
Synchronous replication makes sure that both the source volume and the destination volume are fully synchronized and there is no data
loss in the event of a failure on the source Storage Center.
Synchronization of the source and destination volumes is achieved by making sure that each write is successfully written to both the
source volume and the destination volume before responding to the server. Because writes are written to both the source and destination
volume, write performance is limited by the speed of the connection to the remote Storage Center.
Synchronous replication copies the volume Active Snapshot (current, unfrozen volume data) and any snapshots to the destination
Storage Center. When the source and destination volume are synchronized, new snapshots are created by pausing IO and creating
snapshots for both the source volume and the destination volume, and then resuming IO.
Synchronous Replication Modes
The synchronous replication mode controls how the source volume behaves when the destination volume is unavailable.
There are two synchronous replication modes:
• High Availability Mode: Accepts IO requests to the source volume when the destination volume is unavailable (or when latency is
too high) to avoid interrupting service. However, if writes are accepted to the source volume, the destination volume data becomes
stale.
• High Consistency Mode: Prevents IO requests to the source volume when the destination volume is unavailable to make sure that
the volumes remain identical. However, the source volume cannot be modified during this time, which can interrupt operations.
When the destination volume comes back online, both modes resume transferring snapshots and Active Snapshot data from the source
volume.
Deduplication for Synchronous Replication
Deduplication reduces the amount of data required to transfer snapshots to the destination Storage Center by copying only the changed
portions of the snapshot history. This is accomplished by comparing the changed data in the snapshot being replicated with the previous
data block by block, and transmitting only blocks that differ. While deduplication can be resource-intensive, it is useful when replicating
volumes over lower bandwidth WAN links.
NOTE:
When you enable replication deduplication, the Storage Center creates a secondary 'Delta' volume. This
secondary volume adds to the overall volume memory usage and therefore will reduce the amount of configurable
volume space that can be deployed. The additional volume memory usage affects the overall System Scalability
Guidelines that are documented in the Storage Center Release Notes.
Replication Requirements
To replicate a volume from one Storage Center to another Storage Center, the requirements listed in the following table must be met.
Requirement
Description
Storage Center license The source and destination Storage Centers must be licensed for Remote Instant Replay
(Snapshot).
Storage Manager configuration The source and destination storage system must be added to Storage Manager Data
Collector.
NOTE: Replications cannot be created or managed when Unisphere is
connected directly to a Storage Center.
Storage Center communication The storage systems must be connected using Fibre Channel or iSCSI, and each storage
system must be defined on the other storage system.
Storage Center Replications and Live Volumes 477