IBM 2145-4F2, Installation Manual

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All I/O operations that are managed by the nodes in an I/O group are cached on
both nodes. Each virtual volume is defined to an I/O group. To avoid any single
point of failure, the nodes of an I/O group are protected by independent
uninterruptible power supplies (UPSs). There are two different UPSs. The UPSs are
called the 2145 uninterruptible power supply-1U (2145 UPS-1U) or 2145
uninterruptible power supply (2145 UPS) units.
A SAN Volume Controller I/O group takes the storage that is presented to the
SAN by the storage subsystems as MDisks and translates that storage into logical
disks, known as VDisks, that are used by applications on the hosts. Each node
must reside in only one I/O group and provide access to the VDisks in that I/O
group.
Figure 7. SAN Volume Controller 2145-4F2 node
svc00074
Figure 8. SAN Volume Controller 2145-8F2 and SAN Volume Controller 2145-8F4 node
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IBM System Storage SAN Volume Controller: Installation Guide