[wplug] Linux multipath tools

[Keir Josephson]

Michael,

Unfortunately, I don't have much exposure to the SGI storage products.
However, there are two ways that I've typically handled multi-pathing.

The idea is to have the server be aware of all available paths to a given
LUN but only have one device for that LUN. I've only used methods that are
able to automatically communicate with the storage array's controllers.
That way, you have a better chance that the server is aware of changes to
a LUN path, either from a failover incident or a load balancing scenario,
will function properly and don't confuse the system and generate I/O
errors. This is especially important if the controllers behave in an
Active/Passive mode as they may require signaling from the server in order
to make a LUN available on the secondary controller.

The first method is to utilize software that is available from the storage
vendor that can facilitate the multi-pathing functionality. This usually
involves additional kernel modules that give you this functionality. It
also tends to come with a price tag that may not be worthwhile if you only
need failover.

The other method is to use the firmware on the HBA itself to handle the
multi-pathing. In this scenario, the card itself is correlating the paths,
handling the failover (and in some cases, load balancing), and presenting
a single device to the kernel. This might be you're best approach since
Qlogic has been very aggressive in getting their firmware to support
multi-pathing with the various storage vendors.

-Keir

> Keir Josephson wrote:
>> I have some experience with this. Whose storage array are you using and
>> what distro? Is this on a true dual fabric layout?
>
> The OS is SLES9-SP2 on SGI Altix and the storage is SGI TP9700 (same as
> Engenio 6998). The FC controllers are QLogic 2462 dual port.
>
> The storage has dual controllers with 4 FC host ports/controller. There
> will be 2 luns, 1 owned by each controller. The RAID is configured to
> use automatic volume transfer (AVT). The storage will be direct
> connected to the system using either 4 or 2 paths, whichever makes more
> sense.
>
> The most important feature the customer requires is failover, but
> distributing the luns is important too.  We must be able to manaully
> transfer the path of a lun so we can assure proper lun ownership.
>
> SLES9 SP2 ships with multipath-tools-0.4.4. The customer was planning to
> use LVM, but I'm willing to look at other volume mangers, too. I'm used
> to working with SGI XVM multipath using RDAC mode on the RAID. The
> customer can't use XVM because the application is Oracle and the
> proprietary modules would invalidate their support.
>
> Thanks,
>
> Michael
>
>
>
>>
>> -Keir
>>
>>
>>
>>>Does anyone have experience using the multipath-tools package and can
>>>comment on it's stability? I would be using it to distribute 4 luns of
>>>an LVM volume across 4 fibre channel connections.
>>>
>>>Michael
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