Administrator Guide

Data replication primer
9 Dell EMC SC Series: Synchronous Replication and Live Volume | CML1064
2 Data replication primer
Data replication is one of many options that exist to provide data protection and availability. The practice of
replication evolved out of a necessity to address a number of matters such as substantial data growth,
shrinking backup windows, more resilient and efficient disaster recovery solutions, high availability, mobility,
globalization, cloud, and regulatory requirements. The common requirement is to maintain multiple copies of
data and make them highly available and easily accessible. Traditional backup methods satisfied early data
protection requirements, but this feasibility diminished as data sets and other availability constraints grew.
Vanishing backup windows, ecommerce, and exponential growth of transactions brought about the need for
continuous data protection (CDP). Replicas are typically used to provide disaster recovery or high availability
for applications and data, to minimize or eliminate loss of transactions, to provide application and data locality,
or to provide a disposable data set that can be internally developed or tested. At a higher level, data
protection translates to guarding the reputation of an organization by protecting end-user data.
2.1 Replication methods
There are a number of replication approaches, but two methods stand out as highly recognized today:
asynchronous and synchronous. SC Series arrays support a flexible variety of replication methods that fall in
the category of asynchronous or synchronous.
2.1.1 Synchronous
Synchronous replication guarantees data consistency (zero data loss) between the replication source and
destination. This is achieved by ensuring write I/O commitments at the replication source and destination
before a successful write acknowledgement is sent back to the storage host and the requesting application. If
the write I/O cannot be committed at the source or destination, the write will not be committed at either
location to ensure consistency. Furthermore, a write failure is sent back to the storage host and its
application. Application error handling will then determine the next appropriate step for the pending
transaction. By itself, synchronous replication provides CDP. Coupled with hardware redundancy, application
clustering, and failover resiliency, continuous availability for applications and data can be achieved.
Because of the method used in synchronous replication to ensure data consistency, any issues impacting the
source or destination storage, or the replication link in-between, will adversely impact applications in terms of
latency (slowness) and availability. This applies to Live Volumes built on top of synchronous replications as
well. For this reason, appropriate performance sizing is paramount for the source and destination storage, as
well as the replication bandwidth and any other upstream infrastructure that the storage is dependent on.