rameuniver
Enthusiast
Enthusiast

Difference between RDM and Direct Access to LUN

Hi,

Customer requires the file server as a VM but the main sharing drive should be mapped directly to SAN LUN other than VMDK file.

For this i am thinking two options

1) Mapping a RDM to the file-server as drive

2) Enable iSCSi intiator in Windows Server and map the SAN target directly. In this scenario, ESXi will have any knowledge on this mapping.

Suggest me which one is the best option

8 Replies
zXi_Gamer
Virtuoso
Virtuoso

In first case, the RDM would do good for the VM to have access to the FC Lun. However, there will still be involvement of kernel. While in second case, the vmkernel will play the role of gateway/authentication, blah, blah.

I would recommend to take a look at NPIV to the VM.

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tomtom901
Commander
Commander

Question: Why is the customer demanding the use of something other than VMFS? You should always do VMFS / VMDK's unless there is a damn good reason for it.

vnonymous
Contributor
Contributor

Go with option 1. Map the LUN to the VMware host, and attach to the VM. The MS iSCSI initiator is horrid, plus LUN manageability is much simpler in the vSphere client vs the MS iSCSI initiator. Also consider the RDM compatibility mode when attaching the RDM to the VM. I would suggest the virtual compatibility mode, it give you the access of a raw device but the advantages of a VMFS drive. See notes below from VMware KB 2009226

Physical compatibility mode

  • Physical mode specifies minimal SCSI virtualization of the mapped device, allowing the greatest flexibility for SAN management software.
  • VMkernel passes all SCSI commands to the device, with one exception - The REPORT LUNs command is virtualized, so that the VMkernel can isolate the LUN to the owning virtual machine. Otherwise, all physical characteristics of the underlying hardware are exposed.
  • Physical mode is useful while running SAN management agents or other SCSI target-based software in the virtual machine.
  • Physical mode also allows virtual-to-physical clustering for cost-effective high availability.
  • Virtual Machine Snapshots are not available when the RDM is used in physical compatibility mode.
  • You can use this mode for Physical-to-virtual clustering and cluster-across-boxes.

Virtual compatibility mode

  • Virtual mode specifies full virtualization of the mapped device.
  • VMkernel sends only READ and WRITE to the mapped device. The mapped device appears to the guest operating system exactly the same as a virtual disk file in a VMFS volume.
  • The real hardware characteristics are hidden.
  • If you are using a raw disk in virtual mode, you can realize the benefits of VMFS, such as advanced file locking for data protection and snapshots for streamlining development processes.
  • Virtual mode is more portable across storage hardware than physical mode, presenting the same behavior as a virtual disk file.
  • You can use this mode for both Cluster-in-a-box and cluster-across-boxes.
rameuniver
Enthusiast
Enthusiast

Hi

Customer has the following setup

iSCSi SAN with 12 TB having two LUN's

LUN 1: For ESXi Datastore

LUN 2: Direct Access to Windows Files Server using as H Drive (Not RDM)

Two ESXi Servers

1) ESXi 1 --> 10 VM's

2) ESXi 2 --> 2 VM's

Now we found issues with HP Storage as it has OLD outdated firmware which has many bugs.

So we have decieded to upgrade the firmware and also repartitoning of LUN's

We have bought FC SAN on renatal basis with two FC HBA's connected to two ESXi Servers Each for moving all the VM's and also file server data (which is stored on LUN direclty).

Once storage firmware is upgraded then again we have to move all the VM"s and file server data back to iSCSi SAN.

I am thinking that there will be any challenges for VM migrations, but how to transfer the file server data without any downtime. Is vmdk is suggested for file Server migrations. Customer is very particular about keeping the file Server on direct LUN.

Can i have your suggestion on this?

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rameuniver
Enthusiast
Enthusiast

@tomtom901: Previous engineers are suggested to customer. So customer is unable to take decision now. Can you help me in getting any web link saying that VMDK's are good for file servers.

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tomtom901
Commander
Commander

Is it a single file server or is it clustered of some kind? MSCS?

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rameuniver
Enthusiast
Enthusiast

yes it is a single file server

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vnonymous
Contributor
Contributor

Here is one link from the vmware blogs team.  Note at the bottom the recommendation to use VMDK as a default. In my organization all of our business critical applications web, sql, mail are all using .VMDK drives.

vSphere 5.1 – VMDK versus RDM | VMware vSphere Blog - VMware Blogs