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Re: [Linux-cluster] Get qdisk to use /dev/mapper/mpathx devices instead of /dev/dm-x



Hello all,


In my experience, I tend no to use the label in qdisk specification, because it leads qdisk to use the first device it encounters containing that label. Another case is when the device is not found at all, this happened to me before.

I know your question is related to native DM Multipathing, but under EMC storages, using EMC's PowerPath multipath software, there is no way to force qdisk to use the "pseudo device" other than specifying the "mutipath device" instead of the label of the quorum device. Specifically under EMC PowerPath, you may end up with qdisk pointing to one of the devices that corresponds to a single path (/dev/sdXXX), instead of the pseudo device that passes the MP software for failover (/dev/emcpowerXXX under EMC PowerPath).

So I always specify the device I want whenever the multipath software maintains multiple devices to the same LUN. Some MP software (for instance, Linux RDAC)  "hide" the various devices for the same LUN, so you'll end with only one single device per LUN. In this case, you can use the qdisk label safely.

Hope this helps.

Regards, Celso.



----- Original Message ----
From: Carlos Maiolino <cmaiolino redhat com>
To: linux clustering <linux-cluster redhat com>
Sent: Wed, May 5, 2010 5:37:41 PM
Subject: Re: [Linux-cluster] Get qdisk to use /dev/mapper/mpathx devices instead of /dev/dm-x

On Wed, May 05, 2010 at 11:03:21AM -0700, Don Hoover wrote:
> I have recently learned that its dangerous to use /dev/dm-x devices for LVM because they do not function correctly with multipath, and that you should always use the /dev/mapper/mpathx devices.  Something about the fact that udev actually destroys and recreates the dm devices when paths change.
> 
> But, qdisk seems to prefer using dm-x devices.  Is there a way to get it to prefer using /dev/mapper/mpathx devices if they are available?
> 
> I know you can spec the device instead of the label, but I would like to avoid that.
> 
> 
> 
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If I understant correctly your question and what you wrote, You are specifing qdisk label in cluster.conf.   this is sufficient, independent of which device it will use to mount qdisk partition during qdiskd startup.



-- 
---

Best Regards

Carlos Eduardo Maiolino
Software Maintenance Engineer
Red Hat - Global Support Services

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