What you will learn from this tip: Options for increasing performance of iSCSI storage, ranging from free and simple to costly and complex. Need a downloadable copy? Go here.
Five ways to boost iSCSI performance
1. Use jumbo frames
For compatibility reasons, most Ethernet equipment defaults to a frame carrying 1500 bytes of information. However, modern Ethernet gear also offers the option of using jumbo frames with 9000 bytes of information per frame. Packing six times as much information in each frame improves performance considerably. If any pieces of your iSCSI network are still using regular frames, a few software commands can give you a big boost at no cost.
Note that this change has to be applied to everything on the network path. A single piece of equipment set to 1500-byte frames can make the entire network default to the smaller frames.
2. Set your network to full duplex
Generally speaking, full duplex gives better performance than half duplex in storage networks, because it allows data to flow to and from storage simultaneously. Some networks are set to half duplex for reliability or other reasons. Check your settings.
3. Use ISCSI HBAs with built-in TOES
While you can set up an iSCSI SAN with nothing but regular Ethernet NICs, you run the risk of paying a substantial price in performance. Forcing the server to perform "unwrap" each iSCSI packet takes a good deal of CPU power. While you can get away with it on lightly loaded servers, you will probably see a significant performance improvement if you go to interface cards with built-in TCP Offload Engines (TOEs). These are a good deal more expensive than regular NICs, costing about $600 each or more, but they can make an enormous difference in performance.
4. Use a separate storage network
It's not uncommon for iSCSI SANs to run over the enterprise's regular Ethernet LAN for reasons of economy. Mix SAN and regular traffic is certainly cheaper, but it can quickly lead to network congestion and sub-optimal performance.
Analyze your traffic and consider building a separate Ethernet network to support your iSCSI SAN.
5. Go with Gigabit
Gigabit Ethernet is still a premium product, but it will give you the best possible performance (this month anyway) for your iSCSI SAN. If you're installing a network just for the SAN, Gigabit Ethernet is worth serious consideration.
For more information:
Tip: Pros and cons of iSCSI arrays
Tip: Pound-foolish iSCSI -- Should you use NICs in SANs?
Tip: How to tweak Ethernet for iSCSI SANs
About the author: Rick Cook has been writing about mass storage since the days when the term meant an 80 K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.
This was first published in January 2005