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© Rakhesh Sasidharan

Misc ESXI/ vSphere stuff

Just some notes to myself so I can refer to this later.

  • You can only have a maximum of 256 VMFS datastores per ESXI host. (This is one reason why you wouldn’t want to create a LUN/ datastore per VM. Wouldn’t work if you have a lot of VMs!)
    • Other maximums (for vSphere 5.5) can be found at this link.
  • When you create distributed switch port group there are 3 port binding options:
    • Static Binding (the default): VM NICs are connected to the port group at VM creation and remain so until the VM is removed from the port group. Power off a VM or disconnecting the NIC from the port group does not remove it from the port group – the port is still kept aside for the VM. What this means is that once you connect a VM to a port it stays with that forever.
      • Since the ports are assigned at VM creation, even if vCenter is down when the VM later powers on/ connects to the port group, it will continue to have network connectivity. (Note the emphasis on “later”. If the VM were already running and vCenter were to go down network traffic isn’t affected in either of the binding options).
    • Dynamic Binding (deprecated): VM NICs are connected to the port group only when the VM is powered on and its NIC connected to the port group. Power off the VM or disconnect the NIC and it is not longer connected to the same port when it comes back on or is reconnected.
      • Since the port binding happens only when the VM is powered on or connected, and the port group resides with vCenter, what this means is that you can only power on / off such VMs via vCenter. If vCenter is off / unreachable when the VM powers on / connects, it will not have network connectivity as it won’t have a port in the port group. (As above, note that this doesn’t affect VMs that are already running).
      • Dynamic Binding is deprecated but is useful when the number of VMs is larger than the number of ports in the port group and not all VMs will be on / connected at the same time.
    • Ephemeral Binding: Similar to Distributed Binding, VM NICs are connected to the port group only when the VM is powered on and its NIC connected to the port group. Powering off the VM or disconnecting it results in the port being removed from the port group. 
      • Although Dynamic and Ephemeral Bindings seem similar, they don’t have similar limitations. Thus while VMs with Dynamic Binding port groups won’t have network connectivity if they are powered on / connected when vCenter is off / unreachable, VMs with Ephemeral Binding have no such limitation. They don’t get a proper port number from the port group, but get a temporary one like h-1 which changes to a proper port number whenever connectivity with vCenter is restored.
      • Below screenshot shows the port numbers of three VMs, each connected to a port group of different binding (Ephemeral, Dynamic, Standard from top to bottom) and powered on when the vCenter was unreachable. Bindings
      • Although the NIC is unable to get a port – like Dynamic Binding – with an Ephemeral Binding port group the host creates a fake port and connects the VM anyway. 
      • I don’t understand why Dynamic Binding even exists as an option – unless it’s for backward compatibility? Ephemeral Binding seems to have the advantage of Dynamic Binding – ports are created at VM connection / powering on and so you can oversubscribe to a port group – but doesn’t have the disadvantage of lost connectivity when vCenter is off / unreachable. (I assume Ephemeral port groups too can be used for over subscribing, though the official KB articles don’t say anything like this so I could be wrong).
      • Dynamically creating / removing ports from the port group is an expensive operation so Dynamic and Ephemeral Binding port groups have a performance overhead. Static Binding is the preferred one.
      • Also, Ephemeral Binding port groups lose their history and security controls across host reboots. Apparently Dynamic Binding port groups don’t do this as I don’t see any mention of this as a Dynamic Binding limitation anywhere.

That’s all for now!

 

Misc ESXI/ vSphere stuff by rakhesh is licensed under a Creative Commons Attribution 4.0 International License.