I must be misunderstanding RDMs. I thought I could create a LUN on my SAN, then VMWare would create a small RDM mapping file on a VMFS store that acted as a pointer to the real LUN. I've created a few test RDMs (200 -400 MB), and VMWare is always creating a "servername-rdmp.vmdk" file that matches the size of the LUN. If this is the case, in order to use a 100 GB RDM, I will need 100 GB on my SAN + 100 GB of free space on my VMFS volume for the RDM mapping file. This can't be right. I'm assuming that the *-rdmp.vmdk files are not actually taking up that much space. How can I find out how much space they really use?
It is misleading - the actual size of the file is small but when viewed through the vi client browser - it will show the RDM as the size of the LUN 0 if you got to the cli you sill be able to use the ls command to see the tru size of the RDM -
I forget how to prove otherwise off the top of my head but the mapping file itself is only a couple of megs if that
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The *-rdm.vmdk mapping file you see on your VMFS volume is just a placeholder and really small even though it shows up as the same size as the RDM. I'm not sure exactly how to see the actual size of the file either.
I did want to add that you do need to make sure you have some space available on the VMFS volume for snapshots though as any snapshot delta files for the RDM will go to the VMFS file where you .vmx is located.