Teaching gmultipath about path cost/priority

[Trent Nelson]

On Thu, May 10, 2012 at 08:41:28AM -0700, Alexander Motin wrote:
> Hi.
>
>  > It'd be ideal if there was a way of teaching gmultipath about a path
>  > cost/priority, so that it can make an informed decision about which
>  > path to choose a) when first initializing, and b) during failover.
>
> The problem with this is how to store that in metadata. As soon as all
> paths have exactly the same metadata and provider names are not
> mentioned there, separate paths properties are not possible at this moment.

    Good point.  In my environment, I use all sorts of convoluted ways
    to figure out which /dev/da[n] disk represents the optimal path I
    want to use.  At the very least, I need to probe things from isp
    like port id, wwnn and wwpn.

    It's all very driver/HBA/environment specific.

> As dirty workaround, you may periodically run some script to enforce
> active path on your specific preference.

    Ah!  Yeah, that was another approach I was thinking of suggesting;
    an ability to (gracefully) tell gmultipath to start using another
    path.  Ignore my path cost/priority suggestion and pretend I asked
    that instead ;-)

    Extending the example in gmultipath(8):

      # gmultipath label -v FRED /dev/da0 /dev/da2
      # disklabel -Brw /dev/multipath/FRED auto
      # newfs /dev/multipath/FREDa
      # mount /dev/multipath/FREDa /mnt....
      GEOM_MULTIPATH: adding da0 to Fred/b631385f-c61c-11db-b884-0011116ae789
      GEOM_MULTIPATH: da0 now active path in Fred
      GEOM_MULTIPATH: adding da2 to Fred/b631385f-c61c-11db-b884-0011116ae789

    Something along the lines of:

    + # gmultipath active FRED /dev/da2
    + GEOM_MULTIPATH: da2 now active path in Fred

    ....would be perfect!  Don't care what it's called; active, failover,
    switch, select, activate.

    (I presume gmultipath could affect the path switch in a cleaner fashion
     than forcing the disk to fail?  i.e. make sure all outstanding I/O is
     finished first, etc.)


> Alexander Motin


	Trent.