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| When to use Raw Device Mapping and Symantec Storage Foundation | ||
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| Prev | How Symantec Storage Foundation and High Availability Solutions work in a VMware environment | Next |
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| When to use Raw Device Mapping and Symantec Storage Foundation | ||
|---|---|---|
| Prev | How Symantec Storage Foundation and High Availability Solutions work in a VMware environment | Next |
Raw Device Mapping (RDM) enables a virtual machine to have direct access to the storage rather than going through VMFS. RDM is configured per physical storage device, i.e. a disk or LUN is assigned to one or more virtual machines. It is not possible to assign a part of a physical storage device to a virtual machine. Different types of storage (local SCSI disks, iSCSI disks, Fibre Channel disks) can be used with raw device mapping; Veritas Volume Manager supports all three types of disks.
Note: |
The Storage Foundation products work well with the iSCSI disks mapped directly to the Virtual Machines. |
VMware provides two different modes for raw device mapping:
Logical mode offers the same functionality and compatibility as a Virtual Disk with respect to VMware ESXi features.
Physical mode is the most similar method to storage access in a non-virtual environment. Only one SCSI command, REPORT_LUNS, is virtualized as it is required to enable vMotion and a few other features in VMware.
With Storage Foundation, physical mode is recommended as it enables maximum functionality of Veritas Volume Manager in a VMware environment.
The different modes affect the functionality and behavior of Storage Foundation. It is important to use the correct mode for the desired functionality. The benefit of each storage access method is dependent on the workload in the virtual machine. It is easy to get started with one way of deploying storage without considering the long-term implications because of the ease of use of the virtual environment.
For applications with little to no storage need, using raw device mapping is overkill and not recommended. Also, if your environment depends on VMware snapshots, using Raw Device Mapping in physical mode is not possible as it is not supported by VMware.
Raw Device Mapping is a great fit for:
Applications with large storage needs
Applications that need predictable and measurable performance
Multi-node clusters using disk quorums
Applications with storage that is currently managed by Storage Foundation but is moving into a virtual environment
Applications that require direct access to storage, such as storage management applications