The “proxy file” points to a raw physical Storage Area Network (SAN) Logical Unit Number (LUN) storage device. vmdk extension but is a “proxy” file containing only mapping information. These files are stored on VMware Datastores - storage of various supported types such as SAN or NFS volumes - that are presented to the ESXi hosts and formatted using the VMFS filesystem.
This storage consists of virtual disk or “VMDK” (or “.vmdk”) files.
Typically, VMs use storage that the VMware ESXi host virtualizes. To help you understand the pros and cons of VMware RDM, in this article, we’ll take a closer look at what it is, how it works, and popular use cases. However, there are some disadvantages too, and it is not an ideal choice for all VMware environments. There are a variety of scenarios where using RDM devices is advantageous or even required. Many of these raw SCSI features are not available with virtual disk files (VMDKs). It provides all the performance and flexibility you require to virtualize your most critical business applications.VMware Raw Device Mapping (RDM) is a method of presenting storage to a virtual machine (VM) in VMware vSphere that enables many features of raw Small Computer Systems Interface (SCSI) devices. The decision between VMDK and RDM is now one of architecture or functional requirement and without a special need, VMDK should be used by default. +/- 1% is insignificant in today’s infrastructure and can often be attributed to noise. What you should takeaway from this vSphere 5.1 data, as well as all previously published data, is that there is really no performance difference between VMDK and RDM and this holds true for all versions of our platform. (Special thanks to Razvan Cheveresan for his efforts collecting this data) This dispels the notion that an RDM is significantly more efficient. Here we can see that the CPU cost of an RDM is actually slightly higher (though less than 1%) than that of a VMDK for both tests. These graphs outline the cycles required per transaction which can be viewed a measure of IO cost per transaction (note: lower is better). Here the application performance achieved is nearly identical (approximately +/- 1%) between VMDK and RDM. Using these graphs we can take a closer look at the difference between VMDK and RDM using the same OPM measure. The 4VM test demonstrates near linear scaling for both technologies. In fact, OPM throughput on VMDK was approximately 1% fast than RDM using a single DVD Store instance. You can see that OPM are nearly identical between VMDK and RDM. These graphs outlines the scaling capabilities of both VMDK and RDM by measuring orders per minute (OPM). Additionally, a number of client virtual machines were required to simulate the high numbers of transactions. The scale out tests below were configured such that a number of DVD Store virtual machines were placed side by side on the Dell R910 (2x E5620) vSphere host. This data was generated on vSphere 5.1 using the DVD Store test application running mySQL 5.5 which simulates an online e-commerce store. Let’s take a look at some new internal data for vSphere 5.1 that continues to validate that VMDK is the right default choice. Definitely not worth giving up the value of encapsulating a VM. The VMware team has published a lot of evidence in the past that the difference is very minor, in fact difficult to measure accurately and probably unperceivable to customers. It seems the debate between using a VMDK on VMFS or an RDM still rages when it comes to the question which one is better for performance.
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