How many VM's per datastore

[nathanulph41]

Hi,

I was reading this post regarding how many vm's to store in a datastore How big are your VMFS datastores/how many VMs?

So the 10-15-20 rules seems fairly well tried and tested, so it leads me to wanting to understand what is considered high, medium & low I/O. It seems fairly fundamental to understand this when designing a datastore layout.

Thanks

Nathan

[aherbert23 · Accepted solution]

That whole purpose of limiting the number of VMs per datastore is to reduce the induced latency from queueing. Each ESX host by default has a queue depth of 32 per VMFS datastore. If your storage averaged 5ms latency for all IOs then the maximum theoretical IOPS before inducing latency would be: (1 host) * (32 queue depth) * (1,000 ms / 5ms average latency) = 6,400 IOPS before inducing latency from queueing. So if we had 10 high IO VMs this would equate to about 640 IOPS per VM. Giving each VM the equivalent to a stand alone host with 4 - 15k disk RAID. Thusly having 20VMs per datastore would reduce this to a potential of 320 IOPS per VM. These numbers are per host so as you add hosts you can have more than 10-15-20 per VMFS datastore so long as they are evenly spread across the ESX hosts in the cluster.