![]() The good news is, if you're not limited by any of the reasons above you can deploy on VMDKs with confidence. An RDM is assigned to a single VM (unless using a shared-disk cluster) guaranteeing that the I/O capabilities of the LUN are dedicated to a single VM. Management purposes – Some environments may require greater control over the relationship between LUNs and virtual machines.With database portability only the user objects would need to be updated thus avoiding the time to move mailbox data over the network. This migration could be expedited by swinging the LUNs from the physical mailbox server and attaching them to the Exchange mailbox VM using RDMs. Require the ability to swing a LUN between a native Windows host and a virtual machine – Some deployments may choose to deploy on physical mailbox servers and later migrate to virtual machines. Some reasons for deploying RDMs today might include: This leaves some questioning why we might choose to deploy RDMs for Exchange storage. As ESX and vSphere have evolved the performance difference between RDMs and virtual disks has become almost nonexistent. In early versions of ESX the virtualization overhead associated with deploying virtual disks (VMDK files) was much higher than it is today and why it was considered a best practice to place Exchange data files on physical mode raw-device mappings (RDM). RDMs can be Fibre Channel or iSCSI attached. The RDM file contains metadata used to manage and redirect disk access to the physical device. ![]() Raw-device mappings (RDM) – RDM is a mapping file in a VMFS volume that acts as a proxy for a raw physical device, sometimes called a pass-thru disk.VMFS volumes can be Fibre Channel or iSCSI attached. VMFS offers high I/O capabilities for virtual machines and is optimized for large VMDK files. VMFS based virtual disk (VMDK) – VMFS is a high performance, clustered file system that allows concurrent access by multiple hosts to files on a shared volume.
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