On Thu, 16 May 2002, Don Mulvey wrote:
> I have heard that multipath i/o is supported on s390 and was curious how
> this is accomplished. If anyone has any information I'd appreciate a
> reference or a reply to this post.
>
> On other architectures, multipath i/o is handled in the scsi layer, using
> the scsi id found on newer drives. However, on a 390 system I haven't a
> clue as to how multipath would be handled ... unless the dasd driver is
> handling it ... but I don't think it does. Can anyone shed some light on
> this?
Veering slightly OT, just out of curiosity since I know little
about SCSI other than how to set LUNs and the mundanity of cabling SCSI
devices together -
How do you have "multipath i/o" on SCSI when most devices have
only one parallel cable? (MF devices, particularly large ones often
have two or more slots for "Bus and Tag" connector pairs.
To answer just a little bit of your question (though Jim Elliot
pointed you to the manuals), the OS doesn't need to know about the
multiple paths. When you add hardware you tell the IOCDS (IO
configuration data set) what channels and controllers reach it; your
engine (CPU) launches an ssch command and the IO subsystem takes over
from there (well you do need to do some setup before the ssch).
Alternately if you're running under VM, your guest may know nothing about
multiple paths to a device but the VM control program (CP) very well may
(or not, leaving it completely to the IO system) - so when CP intercepts
the IO request it may well "think": "H'mm I already have an outstanding
operation through this controller on this channel, but I have another
channel open to this device here so...".
-- TWZ
