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StorNext File System Tuning
The Distributed LAN (Disk Proxy) Networks
StorNext File System Tuning Guide 21
anticipated traffic between all Distributed LAN clients and servers
connected to them.
A network switch that is dropping packets will cause TCP
retransmissions. This can be easily observed on both Linux and Windows
platforms by using the netstat -s command while Distributed LAN is in
progress. Reducing the TCP window size used by Distributed LAN might
also help with an oversubscribed network switch. The Windows client
Distributed LAN tab and the Linux dpserver file contain the tuning
parameter for the TCP window size. Note that Distributed LAN server
remounts are required after changing this parameter.
It is best practice to have all SNFS Distributed LAN clients and servers
directly attached to the same network switch. A router between a
Distributed LAN client and server could be easily overwhelmed by the
data rates required.
It is critical to ensure that speed/duplex settings are correct, as this will
severely impact performance. Most of the time auto-detect is the correct
setting. Some managed switches allow setting speed/duplex, such as
1000Mb/full, which disables auto-detect and requires the host to be set
exactly the same. However, performance is severely impacted if the
settings do not match between switch and host. For example, if the switch
is set to auto-detect but the host is set to 1000Mb/full, you will observe a
high error rate and extremely poor performance. On Linux the ethtool
command can be very useful to investigate and adjust speed/duplex
settings.
In some cases, TCP offload seems to cause problems with Distributed
LAN by miscalculating checksums under heavy loads. This is indicated
by bad segments indicated in the output of netstat -s. On Linux, the TCP
offload state can be queried by running ethtool -k, and modified by
running ethtool -K. On Windows it is configured through the Advanced
tab of the configuration properties for a network interface.
The internal bus bandwidth of a Distributed LAN client or server can also
place a limit on performance. A basic PCI- or PCI-X-based workstation
might not have enough bus bandwidth to run multiple Gigabit Ethernet
NICs at full speed; PCI Express is recommended but not required.
Similarly, the performance characteristics of NICs can vary widely and
ultimately limit the performance of Distributed LAN. For example, some
NICs might be able to transmit or receive each packet at Gigabit speeds,
but not be able to sustain the maximum needed packet rate. An
inexpensive 32-bit NIC plugged into a 64-bit PCI-X slot is incapable of
fully utilizing the host's bus bandwidth.