[dm-devel] Re: User specific priorities on for iscsi paths doesn't seem to work

[Akshay Lal]

Mike:

It seems that if I were to set the path_grouping_policy to 
"group_by_prio", then it seems to be working similar to a multibus 
configuration. What I would like is for within a single multipath group, 
(say mpath1), specify a primary path and an alternate/failover path. If 
I can make this configurable via user land tool that'd be great. In this 
vein, I had considered priorities with hope that if I can set the 
priority of a certain path within a group then the path with the highest 
priority will always be chosen and the other path (with the lower 
priority) will only come into play when the primary goes down.

Below is the output of the multipath -ll & conf file when setting the 
path_grouping_policy to group_by_prio.


multipath -ll:
--------------
mpath2 (244534e3833623961) dm-1 DSNET,Dispersed Store
[size=47G][features=1 queue_if_no_path][hwhandler=0][rw]
\_ round-robin 0 [prio=15][enabled]
 \_ 85:0:0:0 sdc 8:32  [active][ready]
 \_ 87:0:0:0 sde 8:64  [active][ready]
mpath1 (244534e3266616134) dm-0 DSNET,Dispersed Store
[size=47G][features=1 queue_if_no_path][hwhandler=0][rw]
\_ round-robin 0 [prio=15][active]
 \_ 84:0:0:0 sdb 8:16  [active][ready]
 \_ 86:0:0:0 sdd 8:48  [active][ready]


/etc/multipath.conf:
--------------------
defaults {
        udev_dir                /dev
        polling_interval        1
        selector                "round-robin 0"
        path_grouping_policy    group_by_prio
        getuid_callout          "/sbin/scsi_id -g -u -s /block/%n"
        prio_callout            "/bin/bash 
/root/MultipathScripts/mpath_prio_alt %n"
        path_checker            tur
        rr_min_io               128
        max_fds                 8192
        rr_weight               priorities
        failback                immediate
        no_path_retry           queue
        user_friendly_names     yes
}


Mike Snitzer wrote:
> On Fri, Aug 07 2009 at  4:25pm -0400,
> Akshay Lal <alal at cleversafe.com> wrote:
>
>   
>> I'm having a few issues with path priorities. It seems that the choice  
>> of path to use during I/O is independent of the user defined priorities  
>> for each path.
>>
>> I am setting the priorities by executing writing a script that is used  
>> by prio_callout. This seems to work when I execute multipath -ll since  
>> all the specified priorities show up correctly. (the  
>> path_grouping_policy being used is  failover)
>>     
>
> ...
>
>   
>> Is there something I'm doing wrong? I would like to be able to define  
>> the priorities per device, and ensure that data only traverses on the  
>> lower priority path when
>> a) a failure to the first path (path with a higher priority) occures
>> b) no other path with a higher priority exists
>>     
>
> Do things behave as you'd like if you change path_grouping_policy to
> 'group_by_prio'?
>
> Mike
>