VMXNET3 RX Ring Buffer Exhaustion and Packet Loss

ESXi is generally very efficient when it comes to basic network I/O processing. Guests are able to make good use of the physical networking resources of the hypervisor and it isn’t unreasonable to expect close to 10Gbps of throughput from a VM on modern hardware. Dealing with very network heavy guests, however, does sometimes require some tweaking.

I’ll quite often get questions from customers who observe TCP re-transmissions and other signs of packet loss when doing VM packet captures. The loss may not be significant enough to cause a real application problem, but may have some performance impact during peak times and during heavy load.

After doing some searching online, customers will quite often land on VMware KB 2039495 and KB 1010071 but there isn’t a lot of context and background to go with these directions. Today I hope to take an in-depth look at VMXNET3 RX buffer exhaustion and not only show how to increase buffers, but to also to determine if it’s even necessary.

Rx Buffering

Not unlike physical network cards and switches, virtual NICs must have buffers to temporarily store incoming network frames for processing. During periods of very heavy load, the guest may not have the cycles to handle all the incoming frames and the buffer is used to temporarily queue up these frames. If that buffer fills more quickly than it is emptied, the vNIC driver has no choice but to drop additional incoming frames. This is what is known as buffer or ring exhaustion.

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Beacon Probing Deep Dive

Today I’ll be looking at a feature I’ve wanted to examine for some time – Beacon Probing. I hope to take a fresh look at this often misunderstood feature, explore the pros, cons, quirks and take a bit of a technical deep-dive into its inner workings.

According to the vSphere Networking Guide, we see that Beacon Probing is one of two available NIC failure detection mechanisms. Whenever we’re dealing with a team of two or more NICs, ESXi must be able to tell when a network link is no longer functional so that it can fail-over all VMs or kernel ports to the remaining NICs in the team.

Beacon Probing

Beacon probing takes network failure detection to the next level. As you’ve probably already guessed, it does not rely on NIC link-state to detect a failure. Let’s have a look at the definition of Beacon Probing in the vSphere 6.0 Network guide on page 92:

“[Beacon Probing] sends out and listens for beacon probes on all NICs in the team and uses this information, in addition to link status, to determine link failure.”

This statement sums up the feature very succinctly, but obviously there is a lot more going on behind the scenes. How do these beacons work? How often are they sent out? Are they broadcast or unicast frames? What do they look like? How do they work when multiple VLANs are trunked across a single link? What are the potential problems when using beacon probing?

Today, we’re going to answer these questions and hopefully give you a much better look at how beacon probing actually works.

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Suppressing ESXi Shell and SSH Warnings

Are you tired of seeing SSH and Shell warnings on your ESXi hosts? If you are at all like me, it’s maddening to see yellow warnings and banners on hosts in the vCenter Server inventory – especially when it’s for something as simple as the ESXi Shell and SSH service being enabled.

Granted, what’s a minor annoyance in a lab environment might be a warning that’s taken seriously in a locked down production environment. In these sorts of environments, administrators will need to enable/disable SSH and Shell access on an as-needed basis. Without the alarms and banners, services may be left turned on accidentally.

 

ESXi Shell and SSH warning banners
Nobody likes warning banners on the summary page 😦

If you are using vSphere 6.0 or later, there is a nifty new ‘Suppress Warning’ option in the vSphere Web client. It can be found on the summary page of an ESXi host with an ESXi Shell or SSH warning currently triggered.

As you can see in the above screenshot, there are separate alerts for both the ESXi Shell and for SSH as well as an option to ‘Suppress Warning’ on each. Although it may appear that each can be suppressed independently, clicking one of the ‘Suppress Warning’ links will disable both ESXi Shell and SSH warnings on the host.

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