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Appendix J
Remote replication over Metropolitan Area Networks (MAN) and Wide Area Networks (WAN)
has become increasingly widespread with the mass adoption and deployment of iSCSI-based
storage appliances. However, despite its popularity, this activity is not bereft of challenges. One
of the biggest challenges now facing replication over significant distances is the delay incurred
when doing so. Signals transported at nearly the speed of light are saddled with delays, resulting in
latencies that can no longer be ignored, as is the case with LAN data transfers. These round-trip
delays range from a couple of milliseconds for inter-city connections to around 80-100 ms from
coast-to-coast, and as much as 250-300 ms for submarine transmissions across the globe. When
geostationary satellites are used, naturally the distances covered are much greater, resulting in
delays of about 700ms.
This whitepaper will examine the impact of transport distance and latency on the efficiency of
network bandwidth in WAN remote replication situations over TCP/IP. It will then demonstrate
how StorTrends iTX and its powerful WAN acceleration (called WDS) and deduplication
technologies improve bandwidth utilization, through sophisticated compression and data reduction
techniques.
Since it is founded in iSCSI technology, StorTrends relies heavily on the TCP/IP protocol,
optimized for LAN and SAN environments, for serving front-end I/Os and for synchronous
replication operations. StorTrends® iTX harnesses standards-based transport protocols to
minimize the inefficiencies and high latencies of TCP/IP in WAN situations, and greatly improve
and increase data transfer speed, approaching the theoretical maximum of the connection speed.
StorTrends iTX also provides additional performance gains through data reduction technologies
such as compression and data deduplication.
The Effects of Packet Loss and Latency on TCP/IP Efficiency
The core technology behind iSCSI and remote replication is the TCP/IP protocol, which is a
connection-oriented protocol that employs various congestion control algorithms with checks and
balances to ensure guaranteed delivery of packets. Due to the very nature of the protocol, and its
dependence on round-trip acknowledgments and sliding windows, the round trip time (RTT) it
incurs plays a very dominant role.
A second major factor that adds additional challenges is the issue of packet loss. At such
significant transport distances, packets can be dropped due to congestion or bit errors. While
recovering from these hiccups, the TCP protocol gets into a "slow start" mode, where it carries out
more conservative corrective actions, resulting in even more restricted performance. In essence,
the throughput achieved in long distance replication depends on two basic parameters: The link
bandwidth and the transport delays and losses.
WDS and Deduplication in
StorTrends iTX
Appendix J : WDS and Deduplication in StorTrends iTX 303
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