Hi,
I asked a similar question a while ago and I copy in the response. I hope it helps. Thanks to Christophe Varoqui for supplying the answers.
Le lundi 04 février 2008 à 12:50 +0000, Gerrard Geldenhuis a écrit :
> Hi Christophe,
>
> I am a bit confused between the usage of
>
> /dev/mpath
>
> /dev/dm-X and
>
> /dev/mapper/
>
>
>
> I am unsure as to which device I should be using when creating lvm
> volumes. I have asked a consultant from Redhat who gave the following
> response:
>
You can use pvcreate on whatever object you want. The important setup is the "filter" in /etc/lvm/lvm.conf : this filter has to use only one kind of object to avoid pvscan confusion. Here again, whatever naming convention is good.
The /dev/mpath/ contains only multipath-type devmaps, whereas /dev/mapper contains all kind of devmap types (linear for LV)
>
> We ended up having a big discussion about this on IRC yesterday, and
> the outcome was inconclusive. However, the guy who is our oracle/san
> expert says /dev/mpath, so that is what I'd do.
>
Also keep in mind that you can disable user_friendly_names in multipath.conf, which will give you /dev/{mapper,mpath}/6000111122223333
names.
Those are really interesting when you use clusters (like RAC) because the naming is consistent between hosts.
>
>
> You might also find it useful to take a look at the kpartx command,
> and use that after you've added a partition to a LUN. It should see to
> it that the relevant /dev/mpath partition device gets created without
> having to reboot the system
>
Avoid partitioning multipathed device when possible : it will remove considerable complexity to the software stack.
>
>
> I also asked on rhel5 mailinglist where I got the following response:
>
>
>
> Using /dev/mapper is always how I've seen it done. /dev/mpath/* looks
> to be just a symlink to /dev/dm-? device nodes which are, in turn,
> device nodes with identical major/minor numbers as /dev/mapper/*.
>
>
>
> Why /dev/mpath/* is even there, I'm not sure.
>
>
udev rules trigger their creation
Regards,
cvaroqui
> -----Original Message-----
> From: redhat-list-bounces@xxxxxxxxxx [mailto:redhat-list-
> bounces@xxxxxxxxxx] On Behalf Of sunhux G
> Sent: 26 May 2008 10:37
> To: General Red Hat Linux discussion list
> Subject: Hi Hertha/Gerrard/anyone,SAN disk partitions device files changes
> with each reboot
>
> Hi Hertha/Gerrard/Anyone else,
>
>
> Thanks for the previous excellent replies to my questions.
>
> Something new just surfaced with the NetApp SAN disks partitions
> that are presented to our RHES 4.6 :
>
> The current mappings on 1st server is :
> lrwxrwxrwx 1 root root 7 May 22 15:36 mpath0 -> ../dm-2
> lrwxrwxrwx 1 root root 7 May 22 15:36 mpath1 -> ../dm-5
> lrwxrwxrwx 1 root root 7 May 22 15:36 mpath2 -> ../dm-3
> lrwxrwxrwx 1 root root 7 May 22 15:36 mpath3 -> ../dm-4
> lrwxrwxrwx 1 root root 7 May 22 15:36 mpath4 -> ../dm-1
> lrwxrwxrwx 1 root root 7 May 22 15:36 mpath5 -> ../dm-0
>
> & "multipath -ll" gives :
>
> mpath0 (360a98000567244396334493370345055)
> [size=5 GB][features="1 queue_if_no_path"][hwhandler="0"]
> \_ round-robin 0 [active]
> \_ 8:0:2:5 sds 65:32 [active]
> \_ 8:0:3:5 sdy 65:128 [active]
> \_ round-robin 0 [enabled]
> \_ 8:0:1:5 sdm 8:192 [active]
> \_ 8:0:0:5 sdg 8:96 [active]
>
> mpath1 (360a9800056724439633449336c786d69)
> [size=5 GB][features="1 queue_if_no_path"][hwhandler="0"]
> \_ round-robin 0 [active]
> \_ 8:0:2:4 sdr 65:16 [active]
> \_ 8:0:3:4 sdx 65:112 [active]
> \_ round-robin 0 [enabled]
> \_ 8:0:0:4 sdf 8:80 [active]
> \_ 8:0:1:4 sdl 8:176 [active]
>
> On another Linux server (with cluster script /etc/init.d/o2cb_start.sh
> started),
> /dev/mpath/mpath0 or mpath1 or ... mpathx completely maps to different
> minor devices /dev/sd...
>
> So we mounted on server 1 a partition (formatted as ocfs using
> ocfs2console)
> first & create a test file on it & then on server 2, we mount
> mpath0/.../mpathx
> one after another to see which of it has the test file on it to identify
> it.
>
> We then put in vfstab the /dev/mpath/mpathx & mountpoint for each server
> that we have determined the hard way.
>
> Alas, after we rebooted both the servers, all the mappings became
> different
> ie on server 1 where mpath0 ->../dm-2 became mpath0 ->../dm-4 after reboot
> & on server 2 where mpath0 ->../dm-1 became mpath0 ->../dm-3 after reboot.
>
> Oracle told us to use /dev/mapper/mpathx - this appears to be more
> reliable
> (ie it's fixed to a specific partition regardless of how many reboots are
> done).
>
> Can someone explain what's the differences between
> /dev/mpath/mpathx & /dev/dm-x & /dev/mapper/mpathx
>
> Or I've not completely installed all the required stuff from NetApp on our
> Redhat servers yet that triggers this?
>
>
> Thanks
> U
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