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



On Fri, Aug 07 2009 at  5:07pm -0400,
Akshay Lal <alal cleversafe com> wrote:

> Mike Snitzer wrote:
>> On Fri, Aug 07 2009 at  4:25pm -0400,
>> Akshay Lal <alal 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
>>   


> 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
> }


Please don't top-post.

I'm pretty sure John meant to say "group_by_prio" rather than "failover"
in his initial reply to this thread.  John originally got this insight
(dummy device section et. al. applies to RHEL 5.3) back in April:

https://www.redhat.com/archives/dm-devel/2009-April/msg00157.html

Which multipath/distro are you using?

Mike


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