Managing Superdome Complexes: A Guide for HP-UX System Administrators

Planning Superdome Configurations
Rules and Guidelines for Configuring a Complex
Appendix A 329
• If a partition comprises four cells or fewer, all the cells should
be connected to the same crossbar.
For example, two four-cell partitions should be configured such that
the cells of each plug into a single crossbar.
Explanation: There are two crossbars to a CPU cabinet, and a
maximum of four cells can plug into each. The crossbars are linked by
means of their crossbar controllers (XBCs) but traffic that does not
cross an XBC link is faster than traffic that does. In addition, a
partition that depends on only one XBC is less vulnerable to
hardware failure than a partition that depends on two.
Exception: A 64-way-capable system containing a large partition
may need to break this rule. For example, if you have a system with
an eight-cell partition and two four-cell partitions, one of the four-cell
partitions is probably spread across two crossbars, one in each
cabinet. In this large configuration the goal is to spread the load for
the complex as a whole across the available links, getting good
performance for all partitions rather than optimal for some and much
worse for others. See the Exception under “Building a Complex from
Scratch” on page 326.
• Do not configure partitions such that two or more partitions
share the same crossbar link.
Explanation: A crossbar link is the link between the two crossbar
controllers (XBCs) in a CPU cabinet. A partition comprising more
than four cells must use a crossbar link, since there is one XBC per
crossbar and a maximum of four cells can plug into one crossbar. A
six-cell partition, for example, must use acrossbar link. When loading
an empty cabinet,you shouldconfigure a six-cellpartition such that it
occupies all the slots (0-3) on the left crossbar and two of the slots on
the right crossbar; you could then use the remaining two slots on the
right crossbar for a two-cell partition. But you should not configure a
six-cell partition such that it occupies three slots on each crossbar and
then configure a second, two-cell partition from the remaining slots,
because both partitions would then be using the same crossbar link.
That could result in slower traffic and the loss of both partitions if the
link fails.