How much network bandwidth is enough for virtual data centers?

Network bandwidth is a precious commodity in virtual data centers. But a 10 GbE network may be overkill in certain situations.

IT pros often cite virtualization and backups as reasons why they need more network bandwidth, but you don't necessarily need a 10 GbE network to maintain a high level of performance.

On the surface, it makes sense that a virtual infrastructure needs plenty of network bandwidth. Let's say that an organization just consolidated 20 physical servers, each with two Gigabit Ethernet (GbE) ports, into one virtual host. Surely that means the host needs more than a few GbE ports?

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The reality is that almost all of those physical hosts didn't use their full network bandwidth, apart from tiny bursts. So sharing a GbE port among a dozen virtual machines (VMs) won't be a problem. Virtualization tends to increase the average utilization of these ports from less than 1% to 5% or 10%. The VMs just don't need a lot of network bandwidth.

That said, the virtualization hosts require fast ports, mostly for transferring VMs between hosts. Moving 16 GB worth of a VM’s contents during a powered-on live migration will saturate a GbE port for a few minutes. The issue is exacerbated when migrations involve a huge amount of RAM.

If my virtual host with 128 GB of RAM is filled to capacity, it may take a half an hour or more to migrate all of the VMs using a single GbE port. If I'm migrating these VMs because of an impending physical failure, it will feel a lot longer. (Just imagine that feeling when a host with terabytes of memory is about to fail.)  But emptying the same 128 GB host with 10 GbE connection will take about five minutes, reducing the risk of a VM outage because of a virtual host failure.

The importance of network bandwidth for storage

Storage networks with plenty of bandwidth are also a valuable asset in virtual infrastructures. The required amount of storage bandwidth depends not only on the number of transactions but also on the transaction size.  Windows File Servers, for example, tend to use tiny transactions to access the storage, and database servers use medium-size transactions.

In both cases, these workloads will likely be limited by the storage's transaction rate, long before network bandwidth becomes a factor. For low-utilization, easy-to-virtualize VMs, the storage network won't be limiting, even at GbE speeds. For the more critical, resource-intensive VMs, however, you'd better make sure you have dozens of spindles or a fair amount of solid-state drives before you start demanding faster storage networks.

Backups: The network bandwidth killer

As we've moved from a 9-to-5 working day to the always-on Internet age, the working day overlaps with the backup window -- which is when organizations back up their servers, usually during offpeak times. Now companies need full-speed performance during a backup, so the network better not be saturated.

Backups involve large transactions and can quickly fill a network. As such, it's common sense to have a nice, fat pipe to carry the data. If that's the case, the storage disks will be the limiting factor, rather than the link itself.

More specifically, if your backup tool uses agents inside the VMs, then you want fat pipes into the virtual hosts to avoid a blowout during the backup windows. (Also make sure your hosts have ample CPU and RAM to cope with this load spike.) Hopefully, your backups are offloaded from your VMs and virtual hosts, however. Therefore, connecting your backup server to the storage array, using the fattest pipe you have, is definitely a good idea. If you have a fast network between your backup server and your main storage, does it make sense to have a slower network for your hosts? If you're buying new equipment, then probably not. The incremental cost of fast network ports won't be much, and over time, the demand for bandwidth will likely increase.

On the other hand, if you're simply solving the problem of backup speed, you will probably buy faster ports just for the backup servers and storage.

In the end, as you buy new networking equipment, you will put in faster network ports. Over time, you'll wonder how you ever managed with those slower networks.

This was first published in July 2012

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