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I inherited an environment where there was no LUN masking or proper zoning. With that said, I need to identify the correct LUNs that are presented to a particular server. I see 6 LUNs if I run "fdisk -l" One of these LUNs is the "new" one I added but of course I can't tell which one it is since the sizes are the same. I guess the easiest thing to do is to remove the LUN and see which one disappeared but who knows if the order of the partition changes (i.e. from sde to sdf?).

Another question, i noticed the server has /dev/sdg and /dev/sdf and they are identical, I know because I was able to mount both and show identical data. Why is that?

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Regarding the multiple identical LUNs, I'm assuming that is due to multipathing. If so, it's also pretty likely that your six entries via fdisk are actually 3 LUNs in total.

Try looking into DM-Multipath a bit, and see if that's being used: http://www.redhat.com/docs/manuals/enterprise/RHEL-5-manual/en-US/RHEL510/DM_Multipath/

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yep. 3 LUNs that's correct. Does it matter whihc one I mount? I see /dev/sdc and /dev/sdg are the exact same LUN. –  luckytaxi Feb 11 '10 at 22:17
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Run "multipath -ll", it'll show you a little more visually how they group together. (edit: those are L's) –  cagenut Feb 11 '10 at 22:56
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DM-Multipath is the way to go unless your storage is presented by a vendor (like EMC) that provide their own multipathing solution (Powerpath in EMC's case).

It wouldn't be a problem which devices you mount if you can be sure that they wont change, but you can't. If a path fails or more storage is presented the LUN behind /dev/sdc may end up on /dev/sdd or something else if you reboot. It's OK to mount and work on for a bit but it isn't safe to leave it unattended for long.

At a minimum you could use udev to ensure you have a persistent device naming across reboots and changes in the environment which but it's no substitute a proper multipath mapper which will give you path failover and load balancing as well.

You really should work on the zoning and masking on the SAN too - performance will probably be suffering if zoning isn't right and the risk of accidentally destroying data if masking isn't correct is significant.

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