RDM - Raw Device Mapping Question

[dkraut]

I seem to be reading conflicting information on what's required to allow multiple VM's to directly access a Raw LUN.  Can this be done within an existing VMFS Datastore or does a new LUN have to be created/presented?  The issue here is that this ESX4i server is using an IBM DS3512 SAS SAN that only came with 1.3TB of storage and thus we configured all available space as a "vmfs" datastore.  Now the project owner wants to connect vm's to dedicated LUN's.  It looks like RDM is the answer, but do I now need to backup all vm's from the existing datastore and rebuild the SAN so that I have available "non-vmfs" space in order to create RDM's or can this be done on the existing vmfs datastore?

[DSTAVERT]

Considering the time and effort / expense in rebuilding the storage, backup etc. it might be more cost effective to add new drives assuming there is space available.

[ThompsG]

Hi,

I know this is going to cause the rolling of eyes and a large sigh but why does the project owner want to do this? To allow multiple VM's (assuming Windows OS here) to connect to a RAW LUN will require you to run MSCS. Are you sure you want to cause this sort of pain to yourself?

Sorry to do this to you but what does the project owner think they mean by dedicated LUNs? Given the fact you have 1.3TB of storage I'm going to assume that you don't have many spindles in the array - max of 12/24 if it is a single enclosure which means dedicating spindles to LUNs is not going to give you the best performance. My feeling is that you have "striped" the data across all available spindles in the array and therefore if you go to the pain of re-organizing your environment these RAW LUNs will still be on the same spindles as the VMFS datastore and therefore NOT dedicated from a physical/contention point of view.

I seem to be reading conflicting information on what's required to allow multiple VM's to directly access a Raw LUN.  Can this be done within an existing VMFS Datastore or does a new LUN have to be created/presented?

New LUN needs to be created and presented to the host/vm

It looks like RDM is the answer, but do I now need to backup all vm's from the existing datastore and rebuild the SAN so that I have available "non-vmfs" space in order to create RDM's or can this be done on the existing vmfs datastore?

Yes, assuming you have no free space on the storage array you will need to do as you suggest. As mentioned by DSTAVERT this is a major and the requirements should justify the effort.

Kind regards.

Message was edited by: ThompsG added DSTAVERTs name

[dkraut]

Thanks for the replies guys.  DStavert, yeah, purchasing additional drives occured to me too, but I have a feeling that they're going to tell me to take the long, painful, "cheaper" route instead.  :smileyplain:

ThompsG, great points... this request is for a development group and they are doing proof of concept testing for an app that will run on Oracle RAC (cluster) so they need the VM's to mimic clustered servers with direct access to a SAN.  The Oracle guy said they require access to raw LUN's from the VM's to get this to work properly.

Thanks, Dave 

[dkraut]

So they decided to purchase a couple more 300GB drives to add to the SAN.  From what I'm reading, the DS3512 has a feature called DAE "Dynamic Array Expansion".  I'm wondering now if I add these drives to the SAN and then expand my current RAID5 array, will it leave the existing VMFS partition alone and then allow me to carve up additional LUN's as needed in the newly acquired empty space?  I realize this is more of an IBM SAN question, but if you guys have any experience with this, every little bit helps.       

[dkraut]

Well, I've hit another hurdle.  I installed the new drives, created an array and LUN's.  I can see them from ESX when I choose to add storage but my option to add Raw Device Mapping from any vm is still greyed out.  any thoughts?  This is an IBM DS3512 SAS SAN. 

[dkraut]

turned out to be some obscure setting in ESXi 4.1 >

•In ESXi 4.1, the option RdmFilter.HbaShared has been selected, this can be located in the following : Configuration > Software > Advanced Settings > RDM Filter

http://kb.vmware.com/selfservice/search.do?cmd=displayKC&docType=kc&externalId=1017704&sliceId=1&doc...

After unchecking that option and rescanning storage, the Raw device mapping was no longer grayed out and I can see the new LUN's I created from the vm's.

Cheers!

[DaPrince]

Thanks - doing the same AND restarting the vSphere Client I was also able to create RAW device mappings on our HP P2000 G3 SAS SAN.