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Raw device mapping (RDM)
46 Dell EMC SC Series: Best Practices with VMware vSphere | 2060-M-BP-V
10 Raw device mapping (RDM)
A raw device mapping (RDM) is used to map a volume directly to a virtual machine. When an RDM set to
physical compatibility mode is mapped to a virtual machine, the operating system writes directly to the volume
bypassing the VMFS file system. There are several distinct advantages and disadvantages to using RDMs,
but usually, using the VMFS datastores is recommended instead of using RDMs.
Advantages of using RDMs include:
• Virtual mode RDMs (vRDMs) up to 62 TB in size and physical mode RDMs (pRDMs) up to 64 TB in
size can be mapped directly to a guest.
• Before shared VMDKs with vSphere 7.0, a clustered resource (such as Microsoft Cluster Services)
could be created with options including:
- Virtual machine to virtual machine
- Virtual machine to physical machine
• The volume can be remapped to another physical server for recovery.
• Physical machines can be converted to virtual machines more easily because a physical machine
volume can be mapped as an RDM.
• When a VM has special disk performance needs:
- A slight disk performance increase when using an RDM compared to a VMFS virtual disk due to
the lack of contention, no VMFS write penalties, and better queue depth utilization.
- There can be independent disk queues per RDM.
- Certain types of SAN software can be used, such as Dell Replay Manager or the Windows free
space recovery feature.
• A different storage profile can be assigned to each volume. For example, if a database server has its
database and logs separated on to different volumes, each can have a separate storage profile.
• A different snapshot schedule (replay profile) can be used with each volume. For example, a
database and its transaction logs may have different snapshot (replay) intervals and retention periods
for expiration.
• vSphere snapshots are supported with virtual mode RDMs.
Disadvantages of using RDMs include:
• Added administrative overhead due to the number of mappings.
• There are a limited number of volumes that can be mapped to an ESXi host. If all virtual machines
used RDMs for drives, the cluster would have a small maximum number of drives.
• Physical mode RDMs cannot be used with ESXi snapshots. While VMware snapshots are not
available for physical mode RDMs, SC Series snapshots can still be used to recover data
Note: All previous RDM-related tasks can be automated using the Dell EMC SC Series vSphere Client Plug-
in. More information about how to obtain the plug-in is in appendix B.