With the push for interconnecting SAN islands over WAN due to 9/11 and federal mandates for financial and healthcare firms, everybody is talking about using IP/Ethernet as the transport. SAN applications require guaranteed QoS which IP/Ethernet cannot provide today.
My question is why isn't anyone looking into using ATM as the transport for SAN-WAN-SAN connectivity? ATM provides guaranteed QoS and can meet the latency requirements of the storage applications.
Actually, almost all the major storage vendors support and even recommend ATM as a preferred transport for solutions like StorageWorks' DRM, Hitachi's Truecopy and EMC's SRDF.
I have installed both DRM and Truecopy using ATM and it works just fine.
For shorter distances, the best connection is dark fibre (9u single mode optical). A single fiber cable can be multiplexed into multiple frequencies (DWDM) and can be used for as many as 32 or more sessions over the cable.
When using IP, most of the SAN extenders support "spoofing", some call it buffering or caching, so the storage array thinks every message gets to the other side very quickly. If a retransmission is needed, the extender will handle it. Most replication software that works over IP is aware of packet loss and will retransmit if needed.
The more lossy the network connection, the more latency the solution will experience and total data bandwidth will drop. To solve this, some companies are now looking at SONET as a method for sync replication over longer distances. I know of one customer using SONET to connect SAN islands at around 180 miles with very low latency and HUGE bandwidth per second. They are actually stretching Linux clusters between SAN islands using sync replication between northern and southern California.
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