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Dell PowerVault MD3600f/MD3620f Remote Replication Functional Guide
Figure 4: Asynchronous Mode with Write Order Consistency
A second feature of AWOC is that if a single replicated pair goes out of synchronization due to a failure
to write to the remote virtual disk, remote write I/Os of the entire consistency group are suspended.
This preserves the consistency as of the last remote write. In turn, this guarantees that the virtual disks
that constitute the database will be able to be recovered for subsequent use by the remote site
applications and servers.
All databases will have this requirement to have inter-virtual disk and intra-virtual disk write I/Os be in
absolute primary sequence when remote replication is used.
Switching Replication Modes
RR provides the ability to dynamically switching replication modes. Synchronous replication can be
switched to Asynchronous Replication and then to Asynchronous Write Order Consistency replication
mode. This order can be reversed and in the specific reverse order. Applications may find this useful to
enhance data protection by switching to synchronous mode or to increase IOPs throughput by
switching away from synchronous replication mode.
An additional requirement if the remote replication communications is bridged to TCP/IP is that all
in-order packet delivery
equipment from one end to the other must have the
option turned on. Due to
the nature of TCP/IP networks being routable, it is entirely possible that two sequential I/Os arrive out-
of-order. In order to eliminate this possibility, the in-order-packet delivery must be turned on for all
bridges, routers, and switches that are configured for RR using AWOC.
With throughput and latency being critical factors in meeting peak period I/O requirements, it is also
jumbo packets
recommended that the
option, if available, be turned on for all equipment end-to-end.
This increases the communications link utilization by using larger Ethernet packets to contain the fibre
channel data payloads.
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