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Getting VMware terminology straight


How do you choose RDM vs. VMFS formats for virtual machines?

Source:  courtesy of VMware

VMware virtual machines comprise a set of files in typically one of two given formats: virtual machine file system (VMFS) or raw device mapping (RDM). Both formats enable you to access the virtual machine's disk (VMDK), but they differ in approach to storage, and VMware recommends VMFS for the vast majority of VMs. With RDM, VMDK files are only pointers to a VM's disk data, while on VMFS the files also hold the data. VMFS holds disk data from multiple VMs; RDM does not.

VMFS was designed specifically to support virtualization. While RDM is sometimes recommended for I/O-intensive operations, VMware studies show that the performance difference found in VMFS versus RDM formats is negligible.

With VMFS, a storage volume can support one or many VMs. This volume can change without affecting network operations. By sharing storage volumes, VMs are easier to manage and resource utilization remains high. Various ESXi servers can read and write to the file system at once, because it stores information at the block level.

With RDM, the VM directly connects to the storage area network (SAN) via a dedicated storage logical unit number (LUN). The total number of LUNs visible to an ESXi host is capped at 256, with the same LUNs visible across a whole cluster of up to 32 ESXi servers. RDM is recommended in a few specific situations, such as when a virtual machine is SAN-aware.

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Performance is relative. I think Administrators need to look at the bigger picture.
For example, in the course of conducting snapshot and clones in your virtual datacenter, what happens to the performance of my virtual environment?
From what VMware tells me , I can get about the same performance if I use an RDM or VMDK. However did you know, if I start using Snapshots 1/O drops to about 50 percent with just 5 snapshots. This is really bad. This has to do with how snapshots are architected. But this should be of no surprise, just run IO Meter against your cluster while taking snapshots and you will see your performance tank like the titanic.
Actually there is a good video demo of this degradation. You can skip if you like to 23:30 to see the actual demo.
Rather than buy a physical SAN or NAS, you can just install Maxta MxSP as virtual appliance. It aggregates the storage from each of your virtual hosts and presents it as a single NFS Data store eliminating the need for a physical SAN or NAS. The technology out performs VSAN technology by over 50 percent. It is a hundred light years ahead of VSAN with respect features and performance, plus it will support Hyper-V and KVM in the near future.
Maxta is well worth the look, they are backed by Intel.
You friendly storage adviser,

Virtual Ray
Sorry, I have a milestone surveillance system that save camera stream in my SAN. I have Windows 2008 r2 on a vm (on esx)...
I must use VMFS or RDM or iSCSI initiator for this VM storage? My SAN storage has 30 TB.