There is no 'perfect' standard solution to your question.  Much depends on the nature and number of VMs per volume.  A single volume much share a single command tag queue (CTQ) on each node.  Typically that CTQ depth is 32, so the max number of outstanding IOs that volume will have on that one ESX node is 32.   If there are more IOs pending, they will be held until the queue is reduced.   So mega volumes with mega VMs can be limited by this queuing.  However, two volumes would double the effective queue, three will triple it, etc..   Making sure that your most active IO VMs are not on the same VMFS volume is another step.  So if you were completely virtualizing exchange, it should be on its own volume.   Also, to take advantage of EQL HIT kits, using the guest OS initiator would be needed to provide the additional benefits they offer.    You also have to deal with iSCSI reservations with ESX v4.1 volumes.  There are times when ESX will reserve and lock out a volume.  (start/stop vm, create/delete/merge vm snapshot, create/delete vm, etc)   While that operation is going on no other node can use that volume.  So more volumes mean that at least IO could be going on to the other volumes.  The reserve/release time is typically short, but if you only have a few volumes and many servers, the issue can get worse.  If you use extents then each volume must be reserved first, then all of them released.  Which can make the situation worse potentially.  Also, losing one extent brings down entire volume.   Once you are at ESXi v5.0 and all the volumes are VMFS v5 things get much better but CTQ considerations still remain.  Regards,   Don 
Hi, Not so much a RAID level question (it's a straight choice between RAID50 and RAID 10 at less capacity but about twice the IOPS), but rather: Is it better to have few max sized volumes (10-15TB depending on RAID level seems to be the option)  or a cluster of smaller volumes, say 5TB?  Obviously, the smaller volumes make for more headaches (though I think I saw that ESX can logically combine them - haven't got that far yet). I just wondered if there was any voodoo in the PS6500E that made one or the other a better choice? I have 3 VMWare hosts...  Many thanks in advance :)  Tim
PS6500E (SATA) recommended layout for VMWare ESXi 4.1