1 Reply Latest reply: Jul 30, 2015 8:00 AM by Adam Herbert RSS

    How many VM's per datastore

    Nathan Ulph Newbie

      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

        • Re: How many VM's per datastore
          Adam Herbert Adventurer

          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.