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Questions :
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1- Does that mean we have 1 LUN with multiple paths to the storage ?
2- Is it mandatory to format the LUN before using it with VG/FS?
Two paths each for two channels for four devices that connect you to a single LUN. If one fails, the storage does not fail. IF one channel drops, the storage does not. Normal pattern for critical SAN storage.
I do not believe it is mandatory, but would have to check my documentation for the right way to make use of it. I think I remember that if you pre-format it then dynamic resizing becomes difficult.
You use it by the mpath device or mapper paths, not by the individual device paths. I have only used those when maintaining the LUN: resizing storage.
Only refer to your DM-multipath device via its /dev/mapper/mpath* name (or whatever you've aliased it to in multipath.conf or the bindings file). Do not use the /dev/sd* devices, as those would only give you one path to the LUN, as mentioned. Also, do not use the /dev/dm-* devices, as those can change across reboots.
What do you mean by "format the LUN"? You can partition the LUN prior to adding it to a volume group (vgcreate will automatically mark it as a physical volume if it's not already). You format LVs, not PVs.
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