So, I have a FreeNAS VM, and two large disks (1.5TB each) that I want to let the VM have direct access to.

I am assuming I would create a physical RDM?

And the FreeNAS VM would see these as the actual physical disks that they are? (meaning I could take them out and put them into a physical FreeNAS machine and it would be able to use them as is)?

You can go either virtual or physical. With either mode the VM will write directly to the disk so it will be portable

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With ESXi it is possible to create two types of RDMs: virtual and physical. With virtual RDMs the storage is treated much like a regular virtual disk, so features such as snapshots and cloning can be used with these virtual disks. With physical RDMs the VM by-passes storage filtering and is able to issue all commands (except Report LUN) to the physical storage device. With a virtual RDM the VM can only issue read and write operations. In the following example vmkfstools is used to create the RDM mapping file. RDMs are only available when the storage device exports a SCSI serial number for the disk LUN. This is not the case for block and some direct-attach RAID devices, so those LUNs do not appear in the Add Hardware wizard when you edit the settings for a VM to add a new hard disk. In this article the process to create a RDM on a such a device is shown and that same process is used in the following example to create a physical RDM on a Parallel SCSI device.

Hi Scott,

If you use Physical RDM you can read SMART values and you see physical serial numbers; if you use logical RDM you can see only "vmware disk".

Differencies? In both cases you can't hot connect/disconnect the drive, and if one disk fails, your virtual machine will "freeze" (ins't aware about errors). This is the main problem with virtual machines that like to use physical RDM... really, they aren't directly connected to the disk (ESXi allways works as interposer and captures all errors).

I hope that VMware will implements a real physical RDM... more or less like do now with USB devices.

In the future, when USB 3.0 will be supported inside ESXi perhaps will be better to connect the physical disks to the virtual NAS using USB than SATA.

Regards!

OK I think I get it.

So if I implement RAID1 with my two disks set up as physical RDM drives, my VM will still hang if one drive fails? Did not know that.

A somewhat unrelated question, my machine uses ICH9R controller. Am I still unable to use that in a RAID configuration? Or am I just better off letting FreeNAS do my RAID1 for me?

The drivers that come with ESXi don't support software RAID so an ICHxR RAID array will just appear as individual disks.

Unfortunately, if one disk fails, then the virtual machines "freezes" (not hung, only enters in an infinite SCSI wait loop). This is because the ESXi captures all disk errors, and don't elevate them to virtual machines (this is in general a good behaviour because the virtual machines never crash). Nevertheless, in a scenario of physical disks connected to a virtual NAS, this is a big trouble!

I hope VMware will change this behaviour in the future... "scsi0.returnBusyOnNoConnectStatus" in the VMX file don't work at all in ESXi 4.x

Regards.

Hmmm... Interesting.

So, seems like the best model is to actually have two physical devices; one to host iSCSI and the other to host ESXi?

Hi,

Current best model for virtual NAS (a NAS running inside a virtual machine) :

* Option 1: Physical disks over USB 2.0
* Option 2: iSCSI external storage connected to ESXi host
* Option 3: internal DAS storage

Current best model for virtual SAN (a SAN running inside a virtual machine providing storage to other virtual machines and/or ESXi hosts) :

* Option 1: iSCSI external storage connected to virtual machine

Sorry, at time, no more valid options!