Modern Ethernet is used in ways that the standard's originators never envisioned
One of the things to check on in an iSCSI SAN is that all the equipment is set up to allow the use of jumbo frames. For reasons of backwards compatibility, the gigabit Ethernet standard allows the use of the 1500 byte frames in the original 100 Ethernet specification, as well as "jumbo" frames of up to 9,000 bytes. A lot of gigabit Ethernet is still shipped with a default setting of 1,500 byte frames, apparently on the theory that such a default will allow it to work on any Ethernet network.
And it will work -- but not very well. A piece of an Ethernet network set to small frame sizes will bottleneck the entire network. The smaller frame size requires more frames in order to send the same amount of information relayed by jumbo frames. Since the overhead involved in handling frames is one of the major performance limitations on an iSCSI SAN, this is likely to have a serious impact on SAN speed.
It is important to make sure that every point on an iSCSI SAN has jumbo frames enabled. This includes the ports on the filer and the host interface.
For more information:Solid-state storage for SANs?
SAN School: Lesson 14 -- Troubleshooting your SAN
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 20 years he has been a freelance writer specializing in storage and other computer issues.
This was first published in September 2004