A little post from me since I got an error message while working with large RDM’s. While I twittered out the message it seemed that Duncan Epping from VMware had a posting ready at which he only had to press the “Publish”-button. See his very helpful article over here.
What you might have noticed is that a RDM’s size is displayed as the real size of the physical LUN that it is referring to. So for example, when I publish a 1 TB LUN it will show up as a 1 TB VMDK file even while my actual VMFS volume in which it resides is much smaller (500 GB in this sample).
1 TB RDM VMDK (virtual compatibility mode) on VICL01-15
VICL01-15 showing that it’s actual size is 500 GB
So far so good you might think since the VM accepts that you connect this 1 TB RDM.
The odd thing is the fact that you cannot take a snapshot of this VM, this brings on the error message: File ….. is larger than the Maximum size supported by datastore “[datastore]” (more on this later on in this post)
So lets take a look at the VMFS Block Size as Duncan is also referring to in his article. The default blocksize we used is 2 MB since we figured we can’t make VMDK files bigger than 512 GB (see the table below for sizing’s) on a 500 GB VMFS volume. Exception to this obviously is the RDM VMDK file which can be bigger than a) the datastore size and b) the datastore blocksize.
1 MB = 256 GB
2 MB = 512 GB
4 MB = 1 TB
8 MB = 2 TB (minus 512 bytes) (see Chad Sakac’s article over here)
So now we know that we can connect bigger RDM’s then the original size of the VMFS volume, but what happens if we:
– Snapshot a VM with a 1 TB RDM on VMFS (500 GB, blocksize 2 MB, free space 470 GB) –> Error: File ….. is larger than the Maximum size supported by datastore “[datastore]”
– Snapshot a VM with a 1 TB RDM on VMFS (500 GB, blocksize 8 MB, free space 470 GB) –> Error: File ….. is larger than the Maximum size supported by datastore “[datastore]”
– Snapshot a VM with a 500 GB RDM on VMFS (500 GB, blocksize 8 MB, free space 470 GB) –> Is working
– Snapshot a VM with a 500 GB RDM on VMFS (500 GB, blocksize 2 MB, free space 100 GB) –> Is working
– Snapshot a VM with a 500 GB RDM on VMFS (500 GB, blocksize 1 MB, free space 470 GB) –> Error: File ….. is larger than the Maximum size supported by datastore “[datastore]”
So apparently it’s the block size and the VMFS datastore size (not the free space!) that is important while making a snapshot of a VM with Virtual RDM attached. Keep this in mind while sizing your environment!
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