[dm-devel] what's up with pp_balance_units?

[goggin, edward]

Just wondering why the pp_balance_units path priority callout is not
the default get_priority callout instead of /bin/false which simply
assigns a priority value of 1 to every path.  This seems especially
true for the symmetric (active-active) storage arrays which by their
very definition don't have their own built-in preferential paths for a
logical unit.

Seems like pp_balance_units assigns a higher priority to paths
connected to SCSI targets which it anticipates will be under-utilized
when considering the path accessibility of all mapped devices
accessible on the same storage system from the same host.
It will also yield the same results as /bin/false will (that is, all paths
will have the same priority) in the case where all mapped devices
on a given storage array are accessible from the same set of targets
on that array.

Pp_balance_units does appear to have a deficiency in that it does
not take into account the impact on these storage system targets
from other hosts (admitably difficult to do) accessing these same
(cluster case) or different SCSI logical units.