Date: Thu, 10 May 2012 09:36:30 -0700 From: Trent Nelson <trent@snakebite.org> To: Alexander Motin <mav@FreeBSD.org> Cc: "mj@feral.com" <mj@feral.com>, "freebsd-geom@FreeBSD.org" <freebsd-geom@FreeBSD.org> Subject: Re: Teaching gmultipath about path cost/priority Message-ID: <7CED0E39-8C64-4AC1-80BA-D6DE9703E022@snakebite.org> In-Reply-To: <4FABE1A8.4000609@FreeBSD.org> References: <4FABE1A8.4000609@FreeBSD.org>
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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 momen=
t.
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-b884-0011116ae7=
89
GEOM_MULTIPATH: da0 now active path in Fred
GEOM_MULTIPATH: adding da2 to Fred/b631385f-c61c-11db-b884-0011116ae7=
89
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.
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