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Replication and remote recovery
60 Dell EMC SC Series: Best Practices with VMware vSphere | 2060-M-BP-V
15 Replication and remote recovery
SC Series replication in coordination with the vSphere line of products can provide a robust disaster recovery
solution. Because each replication method affects recovery differently, choosing the correct method to meet
business requirements is important. This section provides a brief summary of the different options.
15.1 Synchronous replication
In a synchronous replication, the data is replicated in real time to the destination, and an I/O must be
committed on both systems before an acknowledgment is sent back to the host. Sync replication limits the
type of links that can be used because they need to be highly available with low latencies. High latencies
across the link will slow down access times on the source volume.
For SCOS versions before 6.3, the downside of synchronous replication was that snapshots on the source
volume were not replicated to the destination. Any disruption to the link would force the entire volume to be
re-replicated from scratch. In versions 6.3 and later, the synchronous replication engine was re-written to
remedy these limitations.
In SCOS 6.3 and later, in addition to snapshots being replicated, two synchronous replication modes were
introduced. High availability and high consistency modes control how the source volume behaves when the
destination volume becomes unavailable.
High availability mode: Accepts writes and journals them 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 writes to the source volume when the destination volume is unavailable
to guarantee that the volumes remain identical. However, the source volume cannot be modified during this
time, which can interrupt operations.
Keep in mind that synchronous replication does not make both the source and destination volumes writeable.
That functionality is inherent within the SC Series Live Volume feature.
15.2 Asynchronous replication
In an asynchronous replication, the I/O needs only to be committed and acknowledged to the source system,
and the data can be transferred to the destination in a nonconcurrent timeframe. There are two different
methods to determine when data is transferred to the destination:
Frozen snapshot: The snapshot schedule dictates how often data is sent to the destination. When each
snapshot is taken, the SC Series array determines which blocks have changed since the last snapshot (the
delta changes), and then transfers them to the destination. Depending on the rate of change and the
bandwidth, it is entirely possible for the replications to fall behind. Monitor replications to verify that the
recovery point objective (RPO) can be met.
Replicating the active snapshot: With this method, the data is transferred in near real-time to the
destination, potentially requiring more bandwidth than if the system were only replicating the snapshots. As
each block of data is written on the source volume, it is committed, acknowledged to the host, and then
transferred to the destination as fast as it can. Keep in mind that the replications can still fall behind if the rate
of change exceeds available bandwidth.