Understanding LUN

Posted on 2006-11-29
Medium Priority
Last Modified: 2010-01-13
on a 16 SCSI Disk array block, that have a mixture of RAID 5 and RAID 0 (striping)
what is the necessity of creating LUNs?

Question by:jskfan
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LVL 18

Assisted Solution

simsjrg earned 200 total points
ID: 18042565
In computer storage, a logical unit number or LUN is an address for an individual disk drive and by extension, the disk device itself. The term originated in the SCSI protocol as a way to differentiate individual disk drives within a common SCSI target device like a disk array.

The term has become common in storage area networks (SAN) and other enterprise storage fields. Today, LUNs are normally not entire disk drives but rather virtual partitions (or volumes) of a RAID set.

In SCSI, LUNs are addressed in conjunction with the controller ID of the host bus adapter, the target ID of the storage array, and an optional (and no longer common) slice ID. In the UNIX family of operating systems, these IDs are often combined into a single "word". For example, "c1t2d3s4" would refer to controller 1, target 2, disk 3, slice 4. Only Sun's Solaris operating system continues to use LUN slices, and IBM's AIX has abandoned the "ctd" nomenclature in favor of more familiar names.

Source: http://en.wikipedia.org/wiki/LUN


Author Comment

ID: 18045728
an example would be better.

let's say I have 16 disk array block.
on the disk array utility, I configure :
Disk 1 to disk 3 as RAID5
Disk 4 to disk 6 as RAID5
Disk 7 to Disk 10 as RAID5
Disk 11 to diak 16 as RAID 0

In the disk management utility:

I will format and name :

Disk 1 to disk 3 as RAID5 as disk G
Disk 4 to disk 6 as RAID5 as disk H
Disk 7 to Disk 10 as RAID5 as disk I
Disk 11 to diak 16 as RAID 0 as disk K

now what's the point of creating LUNs ???



Assisted Solution

durindil earned 200 total points
ID: 18046779
the SCSI array will present the entire array as one disk.  Therefore, it has to be carved into Logical units, or LUNs.  "disk 1 to disk 3 as RAID5" can not be done without presenting the resulting "disk" as a LUN to the host.  When you format and name, you are doing that to the LUN, not the physical disk.
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LVL 57

Expert Comment

ID: 18048599
My answer from your original question:

Do you know what a LUN is?


You need to define a LUN so that when the I/O command is sent down the bus the drives know which ones (either real or virtual) the I/O command is to.

It sort of like putting a person's name on the outside of a envelope when you send them a letter.    If there are 8 people living in a house, they all  have the same address ("bus").  So you need to put their name ("LUN") on the outside of the envelope also so they know exactly who in the house the letter is for.

Better yet, it is like a TCP/UDP port on a sever.  You don't send a packet just to an IP address, you send it to a port and address.

To expand based on your arrary example.  Windows knows about the "G", "H", "I", and "K" drives.  The RAID arrary  and the SCSI controller don't.  They know about LUN's.  The G drive may map to LUN 01 and the H to 02 and so on.  When the I/O is issued at the SCSI level, it is issued to LUN 01, not to "G" drive.  

Author Comment

ID: 18050510

If I understand well LUNs refer to individual SCSI disks.


G,H,I,K disks are setup through disk management in windows
RAID5 and RAID0 are set up through Disk Arrray Utility(SmartStart disk)
What about LUNs which utility is used to set up LUNS?

LVL 57

Expert Comment

ID: 18050810
No, LUN reference the logical units, not the physical units.

A single LUN could referer to all 16 physical devices on a a single SCSI controller.  The host sees a LUN as a single logical drive.

This may help some more www.storageheaven.com/downloads/raid-techsummary.pdf


Author Comment

ID: 18051252
if you look at the SAN storage management through w2003 server R2 ,
There are subsystems, I guess this refers to each block of SCSI disks that are connected to each Host adapter. By the way I don't know how many host adpaters we need for 16 SCSI disks.

when you create a LUN in a certain subsystem you gave it a name and size and then you can assign a drive letter to the same LUN and format it and assign volume name.

seeing all these  actions, does it mean that you don't need to do anything from the Windows Disk Management console?

because Assigning drive letters and formating used to be done through Disk Management.


Author Comment

ID: 18051343
and if you extend the LUN, would the volume assigned to the LUN get extended automatically?

LVL 57

Accepted Solution

giltjr earned 800 total points
ID: 18051361
I have never used Windows to manage a SAN, so I am not sure.  

I am not sure, but I think you are getting SCSI controller card and Host adapters mixed up.  A single SCSI controller card can support up to 8 or 16 SCSI drives.  Host bus adapters are what connect a host to the SAN, through which you can access all drives through.  You may not get the greatest performance, but it can be done.

That is you could have a computer with a single attachment to a SAN through a host bus adapter.  Then in the SAN you could have 4 SCSI controller cards with 16 drives each (64 drives).  The computer can access all 64 drives.

--> when you create a LUN in a certain subsystem you gave it a name and size and then you can assign a drive letter to the same LUN and format it and assign volume name.

From a Windows point of view yep that is it.  If you wanted you could just create the LUN, name it an size it and then go to Windows Disk Managment console and assign the letter and format it.
LVL 57

Expert Comment

ID: 18051438
--> and if you extend the LUN, would the volume assigned to the LUN get extended automatically?

I would not get extended automatically.  You MIGHT be able to extend it manually.   Umm, this I would have to read up, I have never tried, nor have I thought about doing it.  My initial thought is that you would need some special utility, like Partition Magic, as you are changing the size of a "partition."

I know that under Linux/Unix you can have logically volumes that are mapped to one or more paritions.  You could map a logical volume to one or more LUN's.  Then you could create a new LUN, add that to the logical volume and increase the size of a logically volume.  I am not 100% sure, but I think that Windows supports logical volumes and so you should be ablt to do the same thing.

Author Comment

ID: 18053076
thank you all for your help

Author Comment

ID: 18053085
I may come back later for another question in the same topic LUN

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