Why rdm vmware

Raw device mapping is one of two schemes that VMware servers can use to access storage. Under RDM, a logical unit manager can be directly attached to a virtual machine. Connecting a device directly can improve performance, especially when virtual machines are clustered.

By: Justin Stoltzfus Contributor, Reviewer. By: Satish Balakrishnan. Dictionary Dictionary Term of the Day. Natural Language Processing. Because an RDM provides the ability for a virtual machine to access the underlying storage directly it is often thought that configuration within the VM itself was required.

Luckily things are not that complicated and the configuration is no different than that of a VMFS datastore. All of the available paths are abstracted from the VM and the RDM is presented as a disk with a single path. All of the multipathing logic is handled in the lower levels of the ESXi host. When utilizing RDMs people often have questions regarding the "Sharing" option while adding the RDM to the virtual machine illustrated below.

Since RDMs are most often used for situations like clustering this becomes a concern on whether or not this value should be set. There is a fear that if left unspecified which defaults to "No Sharing" corruption of come kind can happen on the disk.

This is a good mindset to have as protecting data should always be the number one goal. So if your environment is utilizing physical RDMs then you do not need to worry about this setting. If you utilizing virtual RDMs then there is a possibility that setting this option would be required, specifically if you are utilizing Oracle RAC on your virtual machines. As of this writing this is the only scenario in which multi-writer is known to be required with virtual RDMs on a Pure Storage FlashArray.

VMware has provided additional information on this in their Enabling or disabling simultaneous write protection provided by VMFS using the multi-writer flag KB. If there are questions around this topic please open a case with Pure Storage Technical Support for additional information. An additional benefit to utilizing a Raw Device Mapping is that each RDM has its own queue depth limit, which may in some cases provide increased performance.

Aside from the potentially shared queue depth on the virtual machine SCSI controller, each RDM has its own dedicated queue depth and works under the same rules as a datastore would.

This means that if you have a Raw Device Mapping presented to a single virtual machine the queue depth for that RDM would be whatever the HBA queue depth was configured to. The mapping file is presented to the management software as a normal disk file. RDMs have following benefits:. To configure an RDM for a virtual machine is a super easy task for the server administrators.

This scenario comes in the picture if you want that your virtual machine should use the same LUN and storage unit to read and write the purpose for multiple VMs.

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