* Domenico Viggiani
> Red Hat 4.6 defaults for EMC are:
> device {
> vendor "DGC"
> product "*"
> bl_product "LUNZ"
> path_grouping_policy group_by_prio
> getuid_callout "/sbin/scsi_id -g -u -s"
> prio_callout "/sbin/mpath_prio_emc /dev/%n"
> hardware_handler "1 emc"
> features "1 queue_if_no_path"
> path_checker emc_clariion
> failback immediate
> }
> (from /usr/share/doc/device-mapper-multipath-0.4.5/multipath.conf.defaults)
> Why do you use different settings? Are they not "optimal"?
These settings are suitable for PNR mode (failover mode 1, where the
passive paths are unable to process I/O - this will show as large
amounts of I/O errors during boot). When all paths to the currently
active fail, dm-multipath will instruct the CX to move the volume from
the active controller to the passive one. This is bad in cluster
environment, where two cluster nodes might have a differing opinion of
which controller should own the volume and you'll end up having a volume
that constantly moves back and forth between controllers.
My settings are better suited for ALUA mode (failover mode 4, all paths
are able to process I/O), especially if the ALUA-specific support in
dm-multipath isn't available due to old kernels or similar. I sent an
email to the list one hour ago detailing the advantages I see with this
setup.
Unfortunately I have found no way to detect if an array is operating in
ALUA or PNR mode and have dm-multipath automatically apply different
device{} sections based on that. I have some nodes that are connected
to both my CX3 and an old CX200 (which doesn't support ALUA), and due to
this I need to use PNR mode on the CX3 too, wich kinda sucks. Time to
get rid of the CX200 I guess.
Regards,
--
Tore Anderson
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