Performance Impact With Various Types of Over Subscriptions In Virtual World

Once you start thinking of Virtualization, the first question comes to your mind is that can we leverage stat multiplexing in virtualization world or not, the way we leverage it in Ethernet world. The answer to this question is yes. But there are some oversubscription scenarios which can imapct the performance of system.

Every Virtual Network Function is designed to run in a virtualized environment under the control of a hypervisor. Hypervisor could be of VMware ESXi, Citrix XEN, KVM, etc.). There are various over subscription scenarios which can impact the performance of end to end system in a virtualized environment. The system resource over subscription scenarios in a virtualized environment are totally different from that of a physical environment.

The following resources can be oversubscribed in a virtualized environment & drastically reduce the VNF performance:

1. Memory Over subscription
If the amount of memory allocated to all the VMs provisioned in a host exceeds that of the available physical memory, then the VM performance can drop significantly, as memory is stored on a disk and then swapped in when needed by a VM. There are various memory over subscription techniques are available by various hypervisors but the questions comes does the same is also supported by the VNF or not.

2. I/O Over subscription
The I/O resource cannot be formally oversubscribed as VMs are not allocated I/O bandwidth, but if the expected bandwidth of all the VMs is more than the physical I/O available there will be packet drops. Also QoS performance will be poor if I/O is congested. This also impacts the performance of VNFs.

3. CPU Over subscription
The number of vCPUs allocated to VNF provisioned in a system can exceed the total number of physical CPUs/cores available in the system.
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When To Start Thinking of Virtualization ?

It's always a tough decision when to start thinking of Virtualization. But the best and optimum case of virtualization is when you have high CPU requirement but less IO requirements.






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Which is best method to deploy VxLAN: Multicast or Unicast?

Before deploying VxLAN we have to consider it’s underneath deployment model. It could be multicast or unicast. Now the question arises which one is the best for the network. Should we go with Multicast which is as good as broadcast or unicast?

It should be noted that not all deployment types are supported by all devices both physical and virtual. Despite the available hardware and software the best deployment for a particular scenario often depends on the application that will run within the VXLAN. If the application is based on unicast connectivity, in that case unicast mode is preferable. However if you have very large VXLANS with thousands of VM’s on it with each VM being on a separate physical hypervisor then Multicast mode might be better suited.

If the application within a VXLAN relies heavily on either multicast or broadcast messages to keep hosts in sync then this might be a better use case for multicast mode If multicast heavy application is distributed between two physical hypervisors then multicast mode will see no efficiency benefit and unicast mode would be more efficient than multicast mode
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VXLAN Key Concepts & Basic Operation

This section considers some key concepts to consider deploying VXLAN (Read More About VxLAN Overview). Virtual Tunnel End Point (VTEP). As the name implies, the VTEP corresponds to the IP address that defines the beginning or the end of the overlay tunnel. Depending on the role, the VTEP can reside in many places. Initially, it was defined on the virtual switch, but later on, it found its way to other devices such as physical switches, firewalls, routers and so forth.

Depending on the location of the VTEP, devices can play different roles in the VXLAN construct. At the edge, the virtual switch played the role of encapsulation or de-capsulation device, ingesting the packets from the virtual machine and inserting or stripping of the correct VXLAN header respectively. Examples are the VMware DVS and the Cisco Nexus 1000v. These devices implement the basic VXLAN function.

Below provides an overview of this basic function of VXLAN. A virtual machine sends a packet to another virtual machine on another physical host. The traffic enters the vSwitch untagged and based on the vnic configuration will be assigned a VXLAN segment id and will be encapsulated in a UDP packet with VTEP source and destination IP address. The packet is routed over the layer 3 network and eventually reaches the VTEP tunnel destination IP address where the VXLAN header is stripped off and the packet forwarded to the destination virtual machine.

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VxLAN Overview

Virtual Extensible LAN, VXLAN, is another layer 2 overlay network technology that came into existence to initially address particular problems:
•In massively scaled virtualized data centers, the 4096 VLAN limitation implied through a 12 bit long dot1q tag played a significant role to come up with a more scalable solution.

•The static nature of trunk provisioning is another key driver. One of the most attractive aspects of virtualization is the short deployment timelines brought by the simplified deployment of server workloads, but these were hampered by a slow network deployment time.

•Many of these virtual deployments require layer 2 adjacency of virtual machines, especially in clustered environments. The workload deployments preferably can be placed anywhere in the network irrespective of physical boundaries.

VXLAN initially was conceived as a host overlay technology as the basic function was executed at the hypervisor virtual switch layer.

VXLAN Key Concepts & Basic Operation

some key concepts to consider deploying VXLAN. Virtual Tunnel End Point (VTEP). As the name implies, the VTEP corresponds to the IP address that defines the beginning or the end of the overlay tunnel. Depending on the role, the VTEP can reside in many places. Initially, it was defined on the virtual switch, but later on, it found its way to other devices such as physical switches, firewalls, routers and so forth. Depending on the location of the VTEP, devices can play different roles in the VXLAN construct. At the edge, the virtual switch played the role of encapsulation or de-capsulation device, ingesting the packets from the virtual machine and inserting or stripping of the correct VXLAN header respectively.
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Deep Dive Into Virtualization

Cisco educational website that contains over 57 webcasts on virtualization topics that includes solutions, workspace, branch office, datacenter and networking. All are available free of cost.
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