Administrator Guide

Asynchronous Replication
Asynchronous replication copies snapshots from the source volume to the destination volume after they are frozen.
NOTE: By default, data is replicated from the source volume to the lowest storage tier of the destination volume. To
change this default, modify the settings for a replication.
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 dier. 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 modied 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 dier. While deduplication can be resource-intensive, it is useful
when replicating volumes over lower bandwidth WAN links.
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 version The source and destination Storage Centers must meet the
minimum version requirements.
• Synchronous replication: Version 6.3 or later
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Storage Center Replications and Live Volumes