Sunday, November 29, 2009

Nagle Algorithm

Please take a look at the node where the iSCSI Initiator is running and determine if the following parameter is being set.

[ /kernel/drv/iscsi.conf ] add the following:
# Disable Nagle within the iSCSI Initiator

The tcp-nodelay setting, alters the iSCSI Initiators socket options by setting the well-known option TCP_NODELAY, or more specifically turning off the "Nagle Algorithm". For performance reasons, this setting is hard-coded in the iSCSI Target and also Availability Suite's SNDR, both being TCP/IP based data movers of disk blocks.

For both iSCSI and SNDR, the Nagle algorithm interacts badly when using TCP delayed acknowledgments, of which both iSCSI and SNDR relies upon. These performance issues have been noted by many others, including the author John Nagle. See link at bottom of this page:'s_algorithm

For Windows check the Microsoft ISCSI User Guide to change TCPIP parameter
in registry: "TcpAckFrequency=1"