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What is Raw Device Mapping (RDM)?
Raw Device Mapping (RDM) is a technology commonly used in virtualization environments, such as VMware, to enable direct access to underlying physical storage devices. In simpler terms, it allows a virtual machine (VM) to use a physical storage device, such as a hard drive or Solid-State Drive (SSD), as if it were directly connected to it.
The Basics of RDM
RDM essentially creates a mapping file that acts as a proxy between a VM and a physical storage device. This mapping file, known as an RDM descriptor file, contains the metadata and pointers required for the VM to communicate with the underlying storage. By utilizing RDM, VMs can access storage resources with minimal overhead and take advantage of the features provided by the underlying storage device.
There are two types of RDM:
1. Virtual Compatibility (vRDM): Also known as “mapped virtual mode,” vRDM presents the physical storage device to the VM as a virtual disk file (VMDK). The VM interacts with the storage through the hypervisor, which handles the data transfers and manages the disk I/O.
2. Physical Compatibility (pRDM): In this mode, pRDM allows the VM to have direct access to the underlying physical storage device. The VM interacts directly with the storage without the intervention of the hypervisor. This mode offers the highest level of performance but comes with the drawback that the VM is tightly coupled to a specific physical storage device.
The Advantages of RDM
Using RDM provides several benefits in virtualized environments:
1. Improved Performance: By bypassing the virtual disk layer, RDM allows for lower latency and higher data throughput, especially when using pRDM. This is crucial for applications that have high-performance requirements, such as databases or applications dealing with large data sets.
2. Support for Advanced Storage Features: RDM enables VMs to leverage advanced storage features provided by the underlying storage, such as snapshots, thin provisioning, or replication. This can enhance data protection and simplify the management of storage resources.
3. Flexibility and Compatibility: RDM provides the flexibility to use different operating systems and filesystems that may not be directly supported by the hypervisor. It allows VMs to take advantage of specific storage-related features that may be unavailable when using virtual disks.
Real-World Use Cases for RDM
RDM finds application in various scenarios, including:
1. Virtualizing Databases: RDM allows virtualized database servers to directly access the underlying storage, eliminating the potential performance bottlenecks incurred by the virtual disk layer. This ensures optimal performance and scalability for database workloads.
2. Physical-to-Virtual (P2V) Migrations: RDM can be particularly useful in P2V migrations, wherein physical machines are converted into virtual machines. By utilizing pRDM, the VM can maintain direct access to the original physical storage, enabling a seamless migration without the need for data migration or modifications.
In conclusion, Raw Device Mapping (RDM) is a key technology in virtualization, providing virtual machines with direct access to physical storage devices. By understanding its basics, advantages, and use cases, you can make informed decisions when implementing RDM in your virtualized environment.
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