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LUN Presentation

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Operating Systems Linux Red Hat LUN Presentation
# 1  
LUN Presentation


I have presented a LUN from my EVA 4400 to a RHEL server.

The server sees 4 paths to the LUN which I can see from fdisk.

If I want to add this LUN to my OS i.e.

# fdisk /dev/sd???

....which actual device do I run fdisk against?

I am running multipath so is there another command (set of commands that I need to run?).

# 2  
What is the output of
fdisk -l

# 3  

The following example is of 2 paths, but which one do create on the OS?

> Disk /dev/sdg: 214.7 GB, 214748364800 bytes
> 255 heads, 63 sectors/track, 26108 cylinders
> Units = cylinders of 16065 * 512 = 8225280 bytes
> Disk /dev/sdh: 214.7 GB, 214748364800 bytes
> 255 heads, 63 sectors/track, 26108 cylinders
> Units = cylinders of 16065 * 512 = 8225280 bytes

Are there multipath command alternatives to the fdisk approach?
# 4  
You need to find out what the multipath device name is.

Depending on what multipathing software you are using, it could be something like /dev/emcpower<letter>

or /dev/mapper/dm-0,1,2,3 etc

That is the device you use, otherwise you are NOT using multipathing. You are only allowing multipath to run, but you are choosing a single path, and losing performance and reliability.
This User Gave Thanks to mark54g For This Post:
# 5  
Once I have the /dev/dm-<number> can you advise what commands I need to use in order to present the paths (LUN) to the OS?
# 6  
If you have a /dev/dm device, you HAVE presented the LUN. This is the multipath daemon's way of abstracting the multiple paths as one device.

That way, assuming you had /dev/sdb, sdc, sdd, sde as 4 paths to the same disk, you would use /dev/dm-0 as the path you choose, and if you lose /dev/sdc then you still have 3 active paths and no loss in access, but perhaps performance

(Besides, an EVA only allows access via one controller at a time until a failure causes it to switch controllers. It's really the "Speak and Spell" of SAN technology)

In the Command View EVA tool you present the device to the WWN(s) of the host (You did properly zone the switch, correct?)

After that, you should follow the instructions on this page:

Scan and Configure New LUNS on Redhat Linux (RHEL)

If you are using native multipathing, run multipath -ll

Make sure the system sees all the paths. You can also create a disk lable with e2label (That way, when you run blkid, you can see which devices correspond to which path, etc without multipath -ll)

Tutorial: e2label, fdisk, /etc/fstab, mount, linux rescue, rescue disk, CentOS should help you there.
# 7  
Thanks Mark54g,

Now once I have scanned in the new LUN on the OS how do I create a new FS from this space?

Can I use "fdisk" against the dm-<no> to present all paths to the OS?

.....or can I just pvcreate the /dev/dm-<no> and take it from there?


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