NetApp
SnapMirror Synchronous and Asynchronous
When the replication is synchronous, the source host sends a
write request to the source storage system, which is the source of the
replication. Then, the source storage system sends a replication request, and
it also sends the write to the destination storage system.
The target destination storage sends an acknowledgement back
to the source storage, and the source storage sends the acknowledgement back to
the client. With synchronous replication, the data is written to the source and
target storages before an acknowledgement is sent back to the client.
Therefore, you can't have too much delay in the data being written to both
locations.
When you use asynchronous replication, the source host sends
in a write request to its source storage system, and the source storage system
immediately returns an acknowledgement to the client.
Then, based on a predetermined schedule that you decide, for
example, once every 10 minutes, the source sends all of the data written to it
in the previous 10 minutes to the target storage. The target storage then sends
an acknowledgement back to the source storage system.
The asynchronous replication breaks it down into two
separate operations. With synchronous, the write goes to both the source and
the target storage before the acknowledgement returns. With asynchronous
replication, the write comes into the source storage and immediately sends the
acknowledgement back to the client.
Later on, on the schedule in a separate operation, all of
the writes will be written to the target storage, and the target storage will
return an acknowledgement.
With asynchronous replication, the source storage sends an
acknowledgement immediately back to the client host system, so there's no time
and distance limitation. The application will not time out in the source
because the acknowledgement is sent back immediately.
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