Tune up your iSCSI storage router

Cisco's SN 5400 storage router is a popular choice for iSCSI under Windows 2000. However by tuning the iSCSI driver you can improve its performance.

One thing that needs to be looked at is the TCP window size. There is a mismatch between the size of a TCP window on the 5400, which is 256 kbytes, and the default size under Windows, 8760 bytes. The appropriate TCP window size is the bandwidth delay, which can be determined by multiplying the bandwidth by the round trip delay. A properly sized TCP window will make best use of the channel between the router and host.

The TCP window size is fixed on the SN 5400 with the iSCSI driver. However it can be adjusted in the Windows Registry. Cisco recommends resetting the TCP window to 64 kbytes (0xFFFF) for most networks.

Another thing that needs to be looked at for best iSCSI performance is flow control. Make sure flow control is enabled on both the NIC card in the host and all the switches on the Ethernet network path between the host and the router. Many NICs ship with flow control disabled, but in a Gigabit Ethernet environment, flow control can significantly reduce the number of dropped frames and increase overall performance.

Cisco discusses these and other issues relating to the 5400 and its iSCSI driver in a tech note titled "Troubleshooting the iSCSI Driver for Windows 2000" available on its web site at

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Rick Cook has been writing about mass storage since the days when the term meant an 80K 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 July 2003

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