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Date: Wed, 2 Dec 2009 14:39:11 +0100
From: Thomas Quinot <thomas@FreeBSD.ORG>
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Subject: Forcing preferred path in geom_multipath
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I am in the process of setting up access to an iSCSI array with
redundant network connections, and I'm a bit lost as to how to properly
set things up with gmultipath.

The disk array exposes several LUNs, and I have two active iSCSI
sessions, each of which making all LUNs available, and using a distinct
network path.

Thus for example:
  LUN 0 is seen as da0  through session 0, and
                as da24 through session 1
  LUN 2 is seen as da2  through session 0, and
                as da26 through session 1

so I need one multipath geom lun0 with providers da0 and da24,
and another multipath geom lun2 with providers da2 and da26.

Now for load balancing purposes it is desirable to force lun0 to use da0
by default, but lun2 to use da26 by default (da24 and da2 being the
respective fail-overs). However it is not clear to me how this can currently
be achieved, since the preferred path will always be the first tasted
one for each geom, if using automatic configuration.

Any suggestions as to how to achieve the desired ordering of preferred
paths for each multipath geom?

Thanks!
Thomas.