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



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 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


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