[linux-lvm] Devices seen twice - how does LVM handle ?

[Markus Baertschi]

Hi Luca,

On Wed, 10 Sep 2003 15:20:11 +0200, Luca Berra wrote:
>>However, does LVM handles this situation (disks visible twice) nicely ?
>does not seem broken to me if you have two hba.
>lvm does not support multipath

What does LVM do if it sees two paths (/dev/sdx1 entries) to the same
virtual disk ?

>>or should I continue to use my workaround (pull a fiber) to make sure
>>the disks are visible only once ?
>it is not a nice workaround.

Yes, but the only one I got to work in the timeframe I had to get the
machines up and running again.

>>- Are there risks of corruption ?
>i don't think so.
Why ?

>>- What happens if the path the LVM happens to use breaks, but the other
>>is still up. Will LVM handle fail-over ?
>lvm does not support multipath
OK.

>at the moment your best chance is setting up an md multipath
>and building your lvm over the md devices. be sure to use mdadm instead
>of raidtools to manage the md.

I'm not familiar with md. The volumes are already there and in production,
If possible I don't want to reconfigure (downtime...).

>i have this setup (2 hbas to same storage with lvm over md multipath) in
>production, and it works fine.
>another option could be if your hba driver handles multipath. i know
>some qlogic boards do.

I'm using qlogic boards with the qla2300 failover driver. This was working
perfectly until someone installed a blade center on unrelated ports
of the same storage box and changed the WWN of the box in the process.
The linux servers were rebooted three months (!) later for an unrelated
purpose and promptly failed to come up (vgscan segfault...). The failover
driver now thinks we have two storage boxes...

In the short run the only solution I had was to remove one path/fiber.

The real fix is to reconfigure the storage box, but this has to be done
when I can stop all servers connected to it (sunday 02:00 AM...).

In the meantime I installed the latest LVM and this fixes the segfault
problem. My main priority is stability right now and I prefer running
on one fiber than in a potentially dangerous configuration. The fiber
can be switched in a hurry (just plug in the other one), software needs
skills the local people don't have (I'm most of the time not on site).

Markus


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