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That would be pass through.
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RDM, RAW LUN or passthrough, are all the same thing.
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The vm resides in NFS volume , there is no vmdk for this lun in the vm datastore , can this be a RDM?
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It's what is called a RAW LUN, presented to a Guest VM., e.g. it's has a Guest OS e.g. NTFS format on the LUN.<br />
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Similar to when a lun is presented to the ESXi OS, and to the VM, it still has a NTFS format.<br />
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Raw Device mapping is a VMware term.
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If i Understood correctly,<br />
VM call it RDM.<br />
If a physical server or VM &nbsp;attach to a lung via ms initiator/snapdrive then it called RDM too?
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VMware have created the phrase Raw Device Mapped (RDM ), because it's RAW, and Mapped through the VMware Host! (and this was first for Fibre Channel only) to VMs.<br />
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Of course LUNs can be presented to ESXi (VMware), these are formatted with VMFS.<br />
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Microsoft iSCSI Initiator and NetApp Snapdrive just really call it a LUN. (formatted as NTFS).
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Here is my confusion, based on the link below , netapp calls it RDM , but my understanding is that RDM is that you map the lun via esxi host with a proxy file which is in a esxi datastore. When we attach LUN to physical or vm via its initiator then there is no proxy file. Look like netapp call it RDM?<br />
<a href="https://library.netapp.com/ecmdocs/ECMP1189419/html/GUID-CC098BF0-9F08-44C1-8F82-EEC904FA4D4F.html" rel="ugc">https://library.netapp.com/ecmdocs/ECMP1189419/html/GUID-CC098BF0-9F08-44C1-8F82-EEC904FA4D4F.html</a>
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Have a question about RDM, if a virual server connect a &nbsp;LUN using its iscsi initiator(guest os), is that called passthrough lun or <br />
RDM ?
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Have a question about RDM, if a virual server connect a  LUN using its iscsi initiator(guest os), is that called passthrough lun or 
RDM ?

RDM vs Passthrough lun

VMware, a software company founded in 1998,  was one of the first commercially successful companies to offer x86 virtualization. The storage company EMC purchased VMware in 1994. Dell Technologies acquired EMC in 2016. VMware’s parent company is now Dell Technologies.

VMware has many software products that run on desktops, Microsoft Windows, Linux, and macOS, which allows the virtualizing of the x86 architecture.  Its enterprise software hypervisor for servers, VMware vSphere Hypervisor (ESXi), is a bare-metal hypervisor that runs directly on the server hardware and does not require an additional underlying operating system.