Tom,
As Rick states, it depends on both software and hardware. From our side,
z/VM's EDEVice support is entirely multipath capable. An EDEVice created
and brought online with a single path can be modified via command to have
multiple paths. Those new paths will immediately be usable for I/O
directed to the device, presuming it was a valid connection.
> There is then the question of what VM does with those four paths.
> Does it perform some kind of rotation, spreading the I/O load? Or do
> the other paths serve only for fail-over?
z/VM does a round-robin on all available (accessible at VARY ON time) and
healthy (no SAN failures) paths across an EDEV. Paths that are deemed
"sick" for some reason (SAN events, I/O errors, etc.) are temporarily
removed from the path selection criterion while they undergo the
appropriate error recovery. The exception to this rule is the ATTRibute
sets for the DS6800 (ATTR 1750) and SAN Volume Controller (ATTR 2145),
where we round-robin on the healthy paths connected to the preferred half
of those boxes and use the non-preferred paths for failover purposes.
Regards,
Eric
Eric Farman
z/VM I/O Development
IBM Endicott, NY
