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EMC Fibre Channel Storage System Model FC4700 Configuration Planning Guide
Planning File Systems and LUNs
Multiple Paths to LUNs
A shared storage system includes one or more servers, two Fibre
Channel switches, one or more storage systems, each with two SPs
and the Access Logix option.
With shared storage (switched or direct), there are at least two paths
to each LUN in the storage system. The storage-system Base Software
detects both paths and, using optional Application Transparent
Failover (ATF) software, can automatically switch to the other path,
without disrupting applications, if a device (such as a host-bus
adapter or cable) fails.
With unshared storage (one server direct connection), if the server
has two adapters and the storage system has two SPs, ATF performs
the same function as with shared systems: automatically switches to
the other path if a device (such as host bus adapter or ca ble) fails.
And with two adapters and two SPs (switched or unshared), ATF can
send I/O to each available paths in round-robin sequence (multipath
I/O) for dynamic load sharing and greater throughput.