Dell Compellent Series 40 - Manual (Page 337)

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Synchronous and Asynchronous Replications
Synchronous and Asynchronous Replications
Synchronous Replication
Synchronous replication makes sure that a write is successfully written to the remote
system before returning a Successful Completion command to the server IO request. The
Storage Center does not acknowledge completion of the write-back to the Server until both
the write IO to the local volume and the IO sent to the remote system are complete. This
means both the replicating volume and the replicated volume are fully synchronized - there
is no data loss in the event of a failure on the source system. Replays, including Data
Instant Replays and Remote Instant Replays that are taken on the Replicating System are
not copied to the Remote system. Typically Synchronous replication is used only to load
storage from other vendors, or to enable immediate remote volume availability during
Disaster Recovery.
If connectivity is lost between the Replicating System and the Remote Connection system,
the entire data volume must be re-copied to ensure all data is present and accounted for in
both locations. This also means historical instant Replay information will not be available
from the replicated volume.
Asynchronous Replication
Asynchronous replication acknowledges a write IO back to the server as soon as it has
been completed on the source system. The write IO is also queued for delivery to the
Remote system. This allows for more efficient link utilization and data transfer optimization.
It also means that in the event of a local failure, writes present on the source system may
not be present on the remote system.
Note When doing Async replication, you have the option to Replicate the active Replay. If
you do not specify this option and no Replays have been taken, replication does not
begin to replicate data until the first Replay is taken. (Until that time all data resides
in the Active Replay.) Not selecting to replicate active volumes is appropriate for
volumes that have little change activity and are not mission critical.
Asynchronous Replication and Data Instant Replay
Asynchronous replication uses Data Instant Replay to create checkpoints between the
source volume and the destination volume. A Replay created on the Replicating System is
sent intact to the Remote Connection system.
Replay checkpoints serve as re-synchronization points, reducing the amount of data that
needs to be transferred from the source system to the destination system in the event of a
communication failure between the Replicating System and the Remote Connection
system.
Replay checkpoints copied to the Remote Connection system also serve as remote
recovery points in the event the data must be recovered from the Remote Connection
system.
Replays are scheduled regularly on the Replicating (local) System as described in Data
Instant Replay on page 283 . Specifying Replay schedules on the Remote Connection
system is not recommended; they are provided by the schedule on the Replicating System.
Before you replicate a volume, make sure of the following: