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Volume Number: 23 (2007)
Issue Number: 01
Column Tag: Network Administration

Jumbo Frame Networks

Tap into extra Ethernet speed

By Steve Modica

Approximately one year ago, Apple introduced its first dual Ethernet G5 PowerMac. The system sported an internal Broadcom chip that provided the user with two high-speed Gigabit Ethernet ports.

Why would Apple include a second Ethernet port on their new PowerMac systems? One reason (probably the most obvious) is for Xsan support. To add a system to an Xsan storage network (or almost any clustered filesystem network), the system must be connected to the Fibre Channel based storage, its normal internet connection, and a dedicated "Meta-data" network.

On the previous generation of PowerMacs, this would require the customer to purchase an extra Gigabit Ethernet NIC (network interface card), which uses up one PCIX or PCIE slot and requires the user to open up the machine to install the device. When you consider the extra Fibre Channel card (which may have been ordered directly from Apple), you're now down to one free PCI slot to house your high-speed video capture or other application specific devices.

Given the minimal price premium for going to a dual port Gigabit Chip from a single port, adding that second connector was a no-brainer for Apple.

Xsan and clustered filesystems in general, have a lot of promise. Systems can be attached directly to the shared storage using Fibre Channel, and the Ethernet based "meta-data" network is used as the traffic cop. It assures that multiple systems aren't modifying the same filesystem structures simultaneously.

The problem with these new filesystem technologies is the entry point cost. Purchasing an Apple XRAID is quite economical given their cost per byte ratio (one of the lowest in the industry), but when you add in the cost of a Fibre Channel switch, Fibre Channel adapters for each machine, additional Ethernet ports for the meta-data network and the software licenses, the cost begins to approach $30,000 for even the smallest network. When compared to even high-end NAS storage devices, the choice is easy.

So assuming you are one of the many 5-10 system shops that simply can't justify a $30,000 entry point for storage, but you've paid for all of these great PowerMac and Mac Pro systems with second Gigabit Ethernet ports, what can you do to take advantage of them? There's nothing worse than buying something and leaving it sit idle, especially when shared network performance is one of the most common complaints at small shops.

One very common use of these extra ports is to create a dedicated storage network that uses Gigabit Ethernet's built-in "jumbo frame" technology to speed up the transfer of data, while reducing the amount of cpu overhead created when transferring that data.

A configuration like this can significantly improve network performance for file-transfers, captures and downloads, while freeing the "normal" office network for things like email, web browsing and DHCP services. This setup will also increase the amount of CPU cycles available on the server and the clients for local image processing -- a big win for everyone in the office.

What are Jumbo Frames? Seven years ago, when Gigabit Ethernet was just becoming available, processors were a lot slower than they are today. Vendors weren't crowing about how many Ghz their systems were clocked at. They were still working on hitting the 1Ghz mark! These new "high-speed" Gigabit adapters were expensive and quirky. Finding adapters (which typically ran $1500 or more) that worked with all your platforms and a switch that worked with all your adapters was no small task.

During this period, a typical Gigabit Ethernet adapter would easily eat up an entire CPU just for interrupt processing. It was rare to see a Gigabit adapter perform at "line rate" (IE 1 Gb). More typically, adapters would get between 400 and 600Mbits/sec. This is still plenty good, but it must be remembered that while maintaining these rates, your machine wasn't doing anything else.

Enter the notion of "Jumbo Frames"

Ethernet (and all of its add-on features) are defined as IEEE 802.3. This spec includes things like auto-negotiation, link aggregation, 10MbEthernet, 100Mb/Ethernet and even 10Gb/Ethernet. The spec defines the size of an Ethernet frame as 1500 bytes -- typically somewhere between 1488 and 1514 bytes depending on the platform.

Receiving 1 billion bits of data adds up to 83,333 packets per second. Theoretically, each one of these packets requires the CPU to stop what its doing, receive the packet from the Gigabit card, process the headers, and provide the data to the waiting application (you).

At 83,333 packets per second, most of the CPUs time is spent context switching. It's constantly saving and recalling what it was doing just prior to processing your last packet. No real work is getting done.

Vendors quickly realized that unless they wanted to wait for processor and bus speeds to catch up, they wouldn't sell many cards. They needed a solution that would allow their products to work on the slower processors available at that time. Jumbo frames provided one such solution.

Rather than moving data in 1500 byte chunks, they allowed their cards to use much larger packets. Some vendors supported 16384 bytes, others supported smaller numbers. The "de facto" standard was 9000 bytes.

At 9000 bytes per frame, the packet processing load is reduced by a factor of 6 (13,888 packets per second). The CPU has 1/6th the number of headers to process and theoretically, 1/6th the number of interrupts to handle.

Today, all but the very cheapest switches support jumbo frames and switch ports for Gigabit are well under $100 per port. This makes it extremely easy and economical to create a dedicated jumbo frame network.

Benefits

1. Having a separate network means less bandwidth contention for critical operations like captures or online editing.

2. Jumbo frames allow local systems to have more CPU cycles available for things like Final Cut Pro, improving the users experience running the application.

3. A jumbo frame network can support a larger class of systems including Apple Macbook, Macbook Pros, Mac Minis and iMacs.

4. Jumbo frames can be used to access "storage over Ethernet" devices like iSCSI and AoE RAIDS, such as those designed by companies like 2 Degrees Frost. These products eliminate typical bottlenecks found in client/server networks.

Conclusion

Having extra Gigabit ports on your machines makes using jumbo frames easier than ever and the benefits are tremendous. All small video capture and post houses, audio and pre-press shops should consider this simple, elegant solution to improve their network without breaking the bank.


Steve Modica is the Chief Technical Officer of Small Tree Communications. Mr. Modica brings over 15 years of experience in the field of high performance operating systems to the company. Prior to founding STC, he worked at Silicon Graphics Inc. managing a group of device driver engineers and managing third party vendor relationships. For the four years prior to joining SGI, he owned and operated HPC Consulting; a company specialized in customizing corporate networks. Mr. Modica attended Wayne State University in Detroit on a Merit Scholarship and received his degree in Electrical Engineering in 1992.

 
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