[dm-devel] RE: dm-devel Digest, Vol 18, Issue 2

[Christopher Weis]

On 8/2/05, Christopher Weis <ccweis at gmail.com> wrote:
> 
> On 8/2/05, christophe varoqui <christophe.varoqui at free.fr> wrote:
> > 
> > On mar, 2005-08-02 at 15:07 -0400, goggin, edward wrote:
> > > On Tue, 02 Aug 2005 09:37:14 -0500
> > > "Christopher C. Weis" <ccweis at gmail.com> wrote
> > >
> > > > I have a multipath SAN environment with storage controllers that are 
> > 
> > > > active/active. However, the controllers are not active/active at the
> > > > LUN-level without a performance penalty, meaning if two
> > > > servers want to
> > > > see the same LUN (as in a clustered filesystem environment), they 
> > both 
> > > > need to be using the same controller. I'm trying to figure
> > > > out a way to
> > > > statically "order" the paths so that I can copy a config to all of 
> > the
> > > > nodes using the CFS. 
> > > >
> > > > >From what I've read, in a single-server environment with 
> > controllers
> > > > such as the ones I'm dealing with, the path_grouping_policy should 
> > be
> > > > set to "group_by_serial", which should work fine, but in a clustered 
> > 
> > > > environment, I need to be sure that the path ordering is the same.
> > > >
> > > > Are there any path_selectors, other than round-robin, that might
> > > > accomplish this? Any other ideas?
> > > >
> > >
> > > One was is to configure each multipath to have two groups with one 
> > group
> > > having a higher priority than the other based on whether the path 
> > accesses
> > > the fast path controller. The assignment of the highest priority path 
> > group 
> > > is non deterministic when using the "group_by_serial" path grouping 
> > policy.
> > >
> > > Seems like you want to use the "group_by_priority" path grouping 
> > policy and
> > > create and get_priority executable which when invoked will return a 1 
> > for 
> > > fast path and 0 for slow path. See the code for mpath_prio_emc, the
> > > get_priority executable for the EMC CLARiiON array in
> > > multipath-tools/path_priority/pp_emc/pp_emc.c.
> > >
> > Yes, also note "group_by_priority" path grouping policy may be overkill 
> > for the context. PG produced by "group_by_serial" can be sorted with an
> > adequate prioritizer too.
> > 
> > 
> > 
> Does this mean that "group_by_serial" utilizes a "default_prio_callout" 
> program/script as well, or is there another callout (or something totally 
> different that I'm missing)?


I've verified that by writing my own "default_prio_callout" script I can 
achieve what I want with both the "group_by_priority" and "failover" 
policies. Once I get done testing/tweaking the script, I'll post it in case 
anyone else wants it.

I can't seem to make group_by_serial work, although I'm not sure I 
understand this policy very well. It sounds like it's supposed to make all 
paths that go to a particular controller (serial?) be in the same priority 
group. For me, it's no different than multibus. Probably my 
misunderstanding, so I'm open to any suggestions...

Thanks for all the help everyone.

~Chris
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