[CentOS] Help setting up multipathing on CentOS 4.7 to an Equallogic iSCSI target

Sat Mar 14 19:58:55 UTC 2009
nate <centos at linuxpowered.net>

Ross Walker wrote:

> You could ask on the open-iscsi list too.
> I thought if it treated each ip as a separate target portal on the
> initiator you would be able to connect to two "different" targets at
> the same time and let dm-multipath figure out it's the same disk. No?

You can do this, I'm sure of it. But the catch is this doesn't
aggregate the links, even if you use round robin multipathing at
any particular instant in time your only using one link.

>From the docs from device mapper multipath on CentOS 5.2

Path Group:
        A grouping of paths. With DM-MP, only one path group--the
        active path group--receives I/O at any time. Within a path
        group, DM-MP selects which ready path should receive I/O in a
        round robin fashion. Path groups can be in various states (refer to
        "Path Group States").

So as far as I can see you can't aggregate paths in CentOS 5.2
multipath either for a single volume. The only way to use more
than one path is to have more than one volume. You could set it
up as active/passive and have each volume "prefer" a different
path, or use round-robin and just know that at some points
in time both volumes will be going down the same path.

I suppose you could aggregate the volumes themselves using LVM
or software RAID, to present a single file system to the OS that
uses more than one path simultaneously depending on what data is
being accessed.

I think you'll probably find the software iSCSI has more serious
performance bottlenecks before your able to max out a 1Gbps link