Administrator Guide

Synchronous replication use cases
22 Dell EMC SC Series: Synchronous Replication and Live Volume | CML1064
A replication link or destination volume issue in St. Paul results in database outage in Minneapolis
To summarize, there are high consistency use cases that can be integrated with virtualization as well as
database platforms. The key benefit being provided is data consistency and zero transaction loss. Keep in
mind that the infrastructure supporting synchronous replication between sites must perform sufficiently. In the
case of high consistency, the supporting infrastructure must be highly redundant and immune to outages for
slowness or an outage of the replication link or the destination site is reflected equally at the source site
where the end user applications are running.
An important factor when considering the type of replication to be implemented is that the infrastructure
required to keep two sites well connected, particularly at greater distances, often comes at a premium.
Stakeholders may be skeptical about implementing a design where a failure at the secondary site or a failure
of the connection between sites can have such a large impact on application availability and favor
asynchronous replication over synchronous replication. However, with the high availability synchronous
replication offered with SC series storage, customers have additional flexibility compared to legacy
synchronous replication.
4.3 High availability
Many organizations prefer asynchronous replication for its cost effectiveness and its significant reduction in
risk of an application outage, should the destination storage become unavailable. The high availability
synchronous replication mode in SC Series arrays provide data consistency throughout normal uptime
periods. However, if unexpected circumstances arise resulting in a degradation or outage of the replication
link or destination storage, latency or loss of production application connectivity at the replication source is not
at risk. While this deviates slightly from the industry-recognized definition of synchronous replication, it adds
flexibility that is not found in high consistency synchronous by blending desirable features of both
synchronous and asynchronous replication. In addition, SC Series storage automatically adapts to shifting
destination replica availability. Refer to section 3 for more detail on high availability synchronous replication
operational characteristics.
4.3.1 VMware and Hyper-V
Encapsulated virtual machines are replicated in a data consistent manner as they are when using high
consistency mode replication. The difference of behavior comes into play if the replication link (or the
destination replica volume) becomes burdened with excess latency or is unavailable. Instead of failing writes
from the hypervisor, the writes are committed and journaled at the source volume, allowing applications to
continue functioning but at the expense of a temporary lack of data consistency while the destination volume
is unavailable.
In the following examples, note that using high availability mode in place of high consistency mode does not
automatically allow the design to be stretched over further distances without consideration to application
latency. High availability mode is still a form of synchronous replication and should not be confused with