[dm-devel] Multipath Questions

[Mike Anderson]

Joe Thornber [thornber at redhat.com] wrote:
> The idea is that you may have multiple classes of device paths, and
> wish to load balance between one group in priority to another.  I
> don't know if this reflects real life in any way, is there anyone on
> the list with more multipath experience who could tell me if this is a
> sane scenario ?  Originally I planned to put the priority groups in
> the path selectors, but it quickly became apparent that every path
> selector would be duplicating this, so it was lifted into dm-mpath.c.
> I don't have any changes planned for it, so it'll either stay as it is
> or prove worthless and be removed completely at some point.

There are a class of storage devices that have performance penalties if
you load balance IO to two ports vs. using the secondary port for
failover. Depending on the topology of connection between the host
and the storage device the luns can be expose to the OS in a number of
ways.

One example configuration would be a system with 2 FC HBAs (scsi0 and
scsi1) plugged into a switch with a dual ported storage device (port_a
and port_b) connected. The storage could be of a class that you do not
want to load balance between port_a and port_b. For a single lun the
system would see (sda & sdb) as luns off scsi0 and (sdc & sdd) as luns
off scsi1. sda and sdc could represent paths to the primary port. sdb
and sdd would then represent paths to the secondary port.

In the above scenario you would want to load balance over the paths
presented by sda and sdc. Under failure of the primary path you would
want to load balance over the paths presented by sdb and sdc.

-andmike
--
Michael Anderson
andmike at us.ibm.com