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I am using XenServer and wondering if there are any performance benefits to bonding two "virtual" network adapters on a virtual machine? I am certain it would benefit a bare metal server (make one inbound, and one outbound).

The actual server only has one adapter active at this time.

Kladskull
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Performance-wise, you may have a penalty using a vNICs bond. The physical NIC may have to change mode, while your Guest OS changes vNIC. However, from availability point of view, there may be some small benefit: A vNIC can be blocked somehow, in OS level and then a second vNIC may take over - a very low probability case.

Then, if you physical switch supports it, you may set tag-based VLANs and push it to the GuestOS (linux supports it). This will allow finer-grain control and shaping of your traffic. Alternative routes made by the switch and any router may help in very specific cases or improve HA.

In other cases there may be a benefit, depending on the setup. If your bare-metal has 6 NIC and you directly assign 2 of them to the VM, then, of course, the bonding of VM NIC will be practically a bond of the physical NIC.

MiGrieves
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The only performance benefit I can think of is if your physical adapter speed is bigger than the single virtual adapters.

If for example your server has a single 10GbE (10Gbit/s) adapter and your VMs have Gigabit (1 Gbit/s) virtual adapters, you can bond two of those for a theoretical 2Gbit/s throughput.

Daniele Santi
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    It's been my experience that virtual adapters are not limited on their real throughput. The adapter may say it is 1Gb/s but if it's on a 10Gb/s real connection you'll see the higher throughput. – doneal24 Sep 04 '18 at 15:57