Administrator Guide

Replication Types
There are two replication types: asynchronous and synchronous.
Asynchronous replication periodically copies snapshot data to the destination volume after a snapshot is frozen. Synchronous
replication writes data to both the source and destination volumes simultaneously to make sure they are synchronized at all
times.
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 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.
Storage Center Replications and Live Volumes
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