Raw device mapping (RDM)
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<yambe:breadcrumb>VMWare_vSphere_or_ESXi|VMWare vSphere or ESXi</yambe:breadcrumb>
Raw device mapping (RDM)
Ideally we can map LUNs to ESXi hosts and create VMFS filesystem. The immediate advantage is that same LUN can be accessed in parallel for read/write from multiple ESXi hosts. On top of this VMFS filesystem we can create .vmdk files or virtual hard-disks which can be used for persistent storage in VMs. However, if it is required to map a hardware device directly with a VM to expose low-level device APIs then we can use Raw Device Mapping (RDM).
Note that RDM is of two types:
- Virtual compatibility mode: In this case RDM acts like a virtual-disk file. Most of the low-level access are masked and managed by ESXi. This mode supports snapshots.
- Physical compatibility mode: Most of the commands are pass-through to the actual device as it is.
There are special considerations while doing vMotion or while expanding RDM devices. Use references below for more information
Refer:
- About RDM, advantages and limitations
- https://docs.vmware.com/en/VMware-vSphere/6.5/com.vmware.vsphere.storage.doc/GUID-9E206B41-4B2D-48F0-85A3-B8715D78E846.html
- Migration VMs with RDM
- https://kb.vmware.com/s/article/1005241
- Expanding size of RDM
- https://kb.vmware.com/s/article/1007021
