I have read a lot of the articles and hence my queries. For datastores you would enable SIOC in case of the nosy neighbour problem to guarantee that all machines have equal rights to the queues on the HBA.
With RDM's which we are forced to use becuase of MSCS and Linux clustering there is no SIOC and nothing to stop a virtual machine getting given by default more access to the disks the more RDM's it has
Let me make the problem as I see it a bit clearer:-
1 host 5 virtual machines, 4 with one VMDK on one datastore, 1 with one VMDK and 10 RDM's, just the one HBA.
From what you have told me so far the 5 virtual machines are sharing a HBA with queuedepth 32, 32 slots to put scsi commands in for all 5 virtual machines.when accessing the datastore, correct? so each machine has roughly 5 slots in the queue to send out disk requests.
The final machine has 10 RDM disks, each RDM disk has a queue on the HBA of 32, so just for the RDM disks there are 320 slots in the HBA queue for SCSI commands.
This one machine with RDM's has therefore got a total of 325 slots in the various HBA queues available that it can make disk requests to it has 54 times as much potential capacity for disk requests than a machine that just uses a datastore.
It seems to me that the potential for this machine to drag down the whole infrastructure could be a significant problem over which you have no control.
Does this make sense and do you think that this could become a problem