LUN question

I'm new to LUN concepts.  I read the documetns and still hard to visualize the concepts. Is it he partion number?
mokkanAsked:
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svgmucCommented:
It's the unit number. Additionally to the SCSI-ID, you can have one or more LUNs assigned to each device. Every LUN has to be unique for the same SCSI ID.
Usually, devices with multiple media assign a different LUN per media, eg. a CD changer used to have as many LUNs as CDs could be inserted.

The partition number is a different concept.
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SAM1Commented:
LUN ---  Logical Unit Number
more here. http://en.wikipedia.org/wiki/Logical_Unit_Number

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turnbulldCommented:
It's difficult because the term is diluted; lots of people use it for similar but slightly different things.  

The three most common uses for lun in my opinion are:

- a multi-device peripheral.  A tape changer with a robot will have a LUN for the robot and one LUN each for each tape drive installed.  For example, the robot might be called /dev/rmt/c4t0d0 and the first tape drive /dev/rmt/c4t0d1, the next tape drive /dev/rmt/c4t0d2, and so forth.

- some RAID cards represent storage as LUNs.  For example, a Sun ST array might have 12 disks.  These might be configured in the array as three RAID5 stripes each with four disks.  In Solaris, these devices might be called something like c1t0d0345F567AD78C01, c1t0d0345F567AD78C02, c1t0d0345F567AD78C03.  The value past the d in the logical device name is the WWN given to each raid set by the array and used as the LUN identifier

- in large arrays like EMC and in some RAID cards like Dell's PERC card, HP P410, and LSI's 9260-8i, a LUN is the division of a raid group represented as a 'disk' to the OS.  This is similar to the previous example except that this is another division of storage beyond the set of disks in a RAID group.  For example, suppose you have an EMC Clariion array with 150 disks.  It is divided into RAID10 groups of 10 disks each.  Each of these groups has 2, 500GB LUNs defined.  When the array is connected to a server and configured to allow the server to use the array, the server would see 30 'disks' from the array each of which can be partitioned.  

Think of this last type in this way:  RAID groups are sets of disks combined into Redundant Arrays of Inexpensive Disks.  Each RAID group has a certain amount of storage based on the size of each disk, the number of those disks, and the RAID standard used.  LUNs are units of space that are generally a portion of the space from a RAID group (it can be different by why be confusing?).  LUNs are what operating systems see as disks and those disks are available for partitioning.  

SO, for this example, RAID groups >= LUNs >= partitions.
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mokkanAuthor Commented:
Thank you. If I understand correctly if I have raid5 array with 5 disks and then if I create 10 partitions, then I will have 10 logical unit numbers right? Initiator(my host) will access the target(raid5 storage) using lun number. If I'm wrong correct me
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turnbulldCommented:
Well, the term partition will trip you up.  A partition is a very specific thing that is limited by the disk technology used.  For example, SCSI disks have that ability to have up to 8 partitions whereas ATAPI disks can only have four partitions.

With that said, I think you are on the right track.  If, in your RAID device, you create a raid5 array with 5 disks and create 10 divisions of that device, you have created 10 LUNs.  Your initiator will see those LUNs as 10 disks each identified in some way with the LUN number.

Where this gets hairy is each OS and RAID device handles these issues differently.  For example, a 3ware RAID controller  presenting LUNs to a Solaris system would appear to be disks with different target numbers (c0t1d0, c0t2d0, c0t3d0, etc..).  The same controller presenting LUNs to a Redhat system shows up as disks with a different name (sda, sdb, sdc, etc..).  In Winders, these LUNs appear as D:\, E:\, F:\, and so on.

The same Solaris system would see a Sun RAID array as I described before with each LUN's identifer being used as the d value in the logical disk name (c1t0d[World Wide Number]).  

On an HP system with a P410 card running redhat, the LUNs appear as c0d0, c0d1, c0d2, etc..

You need to know enough about the combination of stuff to know its idiosyncrasies.
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mokkanAuthor Commented:
turnbulld,

Thanks a lot for detail information. We already bought NS20 storage from EMC and now we are in the processes of buying more disks(FC and SATA). After that we are going to use navisphere  tool to manage our storage. Did you use navisphere?
Also, what is the advantage of using METALUN? Can you please provide more info on METALUN with example? Thanks a lot.
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turnbulldCommented:
I do use navisphere but I do a lot of scripting and use naviseccli more often than not.  For a while, navisphere is better but the command line utility is more precise and can do lots of repetitive work (like building raid groups, luns, and so forth) faster since you can script the same task in a loop varying only portions of the command.

Anyway, metaluns are what I had in mind when I wrote:

LUNs are units of space that are generally a portion of the space from a RAID group (it can be different by why be confusing?)

Metaluns are what can be different.  Basically, a metalun can be used to combine different raid groups into one lun.  This is backwards in my way of thinking as I am accustomed to carving LUNs out of a raid group.  Instead of a one-to-many relationship between RAID groups and LUNs, metaluns allow a many-to-many relationship.

I've not had a lot of use for metaluns personally.  Generally, I need a lun to be the same size or smaller than the overall raid group.  There is a disadvantage to that however.  As you carve luns out of raid groups, the physical space used for the lun digs deeper into the disks in the raid group affecting performance.  

For example, suppose you have a raid5 raid group of 10 disks totaling 9TB of space.  You carve a 1TB lun from that group and it will be laid out on the fastest 110GB (the outer cylinders) of each disk in the group.  The next 1TB lun will be offset by that original 110GB to the next 110GB on each disk and so on.  The last lun will be on the inner-most cylinders of each disk.  On a sata disk, the outer cylinders might run at 100MB/s while the inner ones are accessed at 50MB/s; a not insignificant difference.  This can create a real perf imbalance between lun 0 and lun 8.

Metaluns resolve this by allowing you to recombine luns across raid groups and stripe them together.  If I take the previous example and make it 9, 1TB luns from each of four raid groups, I can create nine metaluns that mix up fast and slow luns from different raid groups into one big metalun.  This evens out performance making the metaluns perform all the same.  They are slower than the fastest luns but faster than the slowest as an average of I/O.

With all that said, I find it is better for my workloads (sequential data warehouse apps and dbs) to just plan my raid groups such that I have one or two luns in each and then balance my data across the luns to get the same effect. I've been not real happy with metalun performance and it adds a level of complexity I prefer to avoid.

But, your mileage may vary; it's instructional to experiment at the very least.
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mokkanAuthor Commented:
Thanks a lot and very informative. When you are combining LUN from different RAID groups, those RAID groups have to be same type? I mean if one group is RAID5, other group can be RAID10?
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turnbulldCommented:
Th raid groups can be of any type you want.  The luns need to be similarly sized or you waste space though.
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mokkanAuthor Commented:
Thank you very much. We are planning to get FC disks and SATA disk more, If I want to configure METALUN,  can I use from FC disk aray and SATA disks array?
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turnbulldCommented:
Yes, metaluns don't care about the disk technology.  But you can't mix FC and SATA disks in the same DAE (tray).
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