Administrator Guide

Synchronous replication features
14 Dell EMC SC Series: Synchronous Replication and Live Volume | CML1064
threshold, journaled I/O at the source volume is flushed to the destination volume where it will be committed.
During this process, incoming application writes continue to be written to the journal. After all journaled data is
committed to the destination volume, the source and destination will be in sync and the data on both volumes
will be consistent. When the source and destination volumes are in sync, downstream latency will return
within the application at the source volume. Similar to the high consistency mode, application latency and
data consistency are important points to consider in a design that incorporates synchronous replication in high
availability mode.
High availability mode synchronous replication in an out-of-date state
3.1.4 Mode migration
In SCOS 6.5 or newer, replications may be migrated from one mode to another without manually having to
destroy the replication and destination replica volumes, and then rebuild. This includes migrations such as
asynchronous to synchronous high consistency, synchronous high consistency to synchronous high
availability, or synchronous high availability to asynchronous. Leveraging the mode migration feature can
save significant time and replication bandwidth. It also reduces the data availability risk exposure associated
with the time taken to destroy and rebuild a replica volume. Lastly, this method preserves predefined DR
settings in Dell Storage Manager that are tied to restore points and replica volumes. For all of these reasons,
individually or combined, it is recommended to take full advantage of this feature.
Note: This feature is compatible with all replication modes except legacy synchronous replication.
3.2 Minimal recopy
As discussed in section 3.1.3, synchronous replications configured in high availability mode allow write
access to the source volume if the destination volume becomes unavailable or falls behind. While out of date,
a journalizing mechanism shown in Figure 4 tracks the write I/O that makes the source and destination
volumes inconsistent. Prior to SCOS 6.3 with legacy replication, journaling was not performed and if the
destination volume became unavailable and then later available, all data on the source volume needed to be
re-replicated to the destination to get back in sync. However, with the minimal recopy feature, only the
changed data contained in the journal is replicated to the destination volume in order to bring the source and
destination volumes back in sync. This dramatically reduces the recovery time and data inconsistency risk