When will we create parition in a LUN ?

[TonyJK1]

I am new to SAN and would like to know in what circumstances we will create partition in a LUN (In a FC SAN).

Thanks

[Uwe Zessin]

Do you mean partition a container inside a storage array or partition a logical disk by the operating system presented by the storage array?

[TonyJK1]

This is because I am working on VMWare and they say that for a particular storage, it is identified as vmhbaA:T:L:P where

A = is the adapter card
T = is the SCSI Target
L = is the LUN number
P = is the partition number

I just wonder if we use a FC SAN, will there be any partition in a LUN ?

From my understanding, there is no point in creating partition in a LUN. We just have to create another LUN.

Thanks

[Uwe Zessin · Accepted solution]

A VMFS always lives inside a partition and an FC LUN just appears as another SCSI LUN - in fact: FCP is serial SCSI protocol over a Fibre Channel infrastructure.

It is pretty easy in VMware:
- open the Virtual Infrastructure Client
- select a server and goto the 'Configuration' tab
- choose 'Storage (SCSI, SAN, NFS)'
- 'Add Storage...'
and so on.

Make sure you select a Fibre Channel device, but the VI-Client will do all the rest including creating a partition table, setting up the VMFS partition with a nice offset and the correct Id (0xFC). Then it will initialize the VMFS for you.

No need to deal with COS commands like 'fdisk' and 'vmkfstools'.

[Uwe Zessin]

I've just remembered I have a nice example of which I'll attach a screen shot.

vmhba1:0:5:1 is the head partition of a VMFS that is made of 4 extents. Each extent is a partition on the same disk.

[TonyJK1]

Thank you for your advice.

From your Email, my understanding is that every VMFS volume created in a LUN is regarded as a partition. Is it correct ?

And I would like to know instead of creating 1 VMFS volume in a LUN, in what circumstances should we create a large LUN and create a number of small VMFS volumes.

Thanks

[Uwe Zessin]

> every VMFS volume created in a LUN is regarded as a partition

No, sorry. A disk is partitioned. A VMFS partition is marked with the 0xFC ID. Then the filesystem is created _inside_ the partition.
It works the same way like ext2/3 or NTFS or FAT or ... partitions.

> instead of creating 1 VMFS volume in a LUN,
> in what circumstances should we create
> a large LUN and create a number of small VMFS volumes.

If you want get problems ;-)

Seriously. Please understand that each meta-data change on a VMFS (file creation, deletion, VM power on/off and so on) results in a short, LUN-wide SCSI reservation.

If you put multiple, independent VMFS volumes onto a single LUN, there can be heavy SCSI reservation conflicts.

The example I have shown in the previous reply is a spanned VMFS - that is a single VMFS made of multiple partitions residing on the same disk.

It was rather an experiment to test how well array LUN expansion works with VMFS extents. Please note that you _cannot_ expand an existing VMFS partition.

[TonyJK1]

Thank you for your advice.

From your Email, it means that we should not create multiple VMFS volumes in a LUN. On the other hand, we should create LUN so that they can accomodate the VMs we would like to be stored in.

Moreover, for spanned VMFS, it seems that you have created a number of VMFS in different LUNs. And you choose Add Extent to create the spanned VMFS.

From my understanding, the VMFS volume being added (Add Extent) can be located in different RAID Groups (Not necessary on the same disk) ? Is it correct ?

Regards

[Uwe Zessin]

In the screenshot all extents are on the same disk (LUN) = vmhba1:0:5.

But yes, extents can also be drawn from different disks (LUNs). For example, I could extend vmhba1:0:5:1 whith another extent stored on vmhba2:1:3:1.

[TonyJK1]

Thank you for your advice.

I am a little bit confused. Is LUN = Disk ? From my understanding, inside a RAID Group, we can create more than 1 LUN.

Regards