On 26 Jan 2010, at 1:24 pm, Tim Cutts wrote:
2) Raw device maps (where you pass a LUN straight through to a
single virtual machine, rather than carving the disk out of a
datastore) reduce contention and increase performance somewhat, at
the cost of using up device minor numbers on ESX quite quickly;
because ESX is basically Linux, you're limited to 256 (I think - it
might be 128) LUNs presented to each host, and probably to each
cluster, since VMs need to be able to migrate. I basically use RDMs
for database applications where the storage requirements are greater
than about 500 GB. For less than that I use datastores.
It's been pointed out to me that of course Linux supports a lot more
than 256 devices presented. Nevertheless, for some reason ESX does
not - presumably it's only using a single major number or something.
Tim
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