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Chapter 4 StorEdge File Replicator 4-3
4.1.1 Real-time Mirroring
Real-time mirroring is the simplest to describe, and the most difficult and expensive
to implement. The requirement and guarantee of real-time mirroring is that data is
committed in a persistent manner on both the Master and Mirror prior to reflecting
transaction complete to the client. If the mirroring is remote, e.g., over a WAN, the
expense can be quite great because the user must ensure that the link between the
systems is very fast and of exceptional quality, or risk serious reductions in the
quality of service locally due to the latencies associated with remote communication.
Real-time mirroring systems typically provide for extensive parameter control to
enable the user to define policies that manage the mirror link. For instance, the user
may want to automatically break the link if serious local QOS issues arise due to
telecommunications issues. If the link were broken, the systems would function in
pseudo real-time mode until the mirroring system 'caught up' with the mirrored
system, at which time the real-time link would be automatically reinstated.
Real-time mirroring is frequently referred to as synchronous mirroring because of
the requirement to commit the transaction both locally and remotely prior to
reflecting transaction complete to the client.
4.1.2 Pseudo Real-time Mirroring
Pseudo real-time mirroring provides mirroring capabilities approximating those of
real-time mirroring, but does not require that transaction complete be received from
the mirroring system before reflecting back transaction complete to the client.
Pseudo real-time mirroring would typically be implemented where greater than
Checkpoint mirroring protection were required, but factors such as economics,
infrastructure etc. precluded the need for or feasibility of real-time mirroring.
As with Checkpoint mirroring, pseudo real-time mirroring is an asynchronous
operation.
4.1.3 StorEdge File Replicator
The StorEdge File Replicator employs a Pseudo Real-time mirroring approach. A
diagram of the lifecycle of a transaction follows: